Late Jurassic–Holocene, 160–0Ma
|A composite image showing the diversity of birds; 18 biological orders are depicted in this image (from top, left to right): Cuculiformes, Ciconiiformes, Phaethontiformes, Accipitriformes, Gruiformes, Galliformes, Anseriformes, Trochiliformes, Charadriiformes, Casuariiformes, Psittaciformes, Phoenicopteriformes, Sphenisciformes, Pelecaniformes, Suliformes, Coraciiformes, Strigiformes, Piciformes.|
And see text
Birds (class Aves or clade Avialae) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates. All present species belong to the subclass Neornithes, and inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds emerged within theropod dinosaurs during the Jurassic period, around Birds are the only members of the clade originating with the earliest dinosaurs to have survived the Cretaceous–Paleogene extinction event
Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings; the most recent species without wings was the moa, which is generally considered to have become extinct in the 16th century. Wings are evolved forelimbs, and most bird species can fly. Flightless birds include ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.
Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours, including cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.
Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertiliser. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them.
Evolution and classification
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.
Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the modern bird groups, the crown group. This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Gauthier (page 34) identified four conflicting ways of defining the term "Aves", which is a problem because the same biological name is being used four different ways. Gauthier proposed a solution, number 4 below, which is to reserve the term Aves only for the crown group, the last common ancestor of all living birds and all of its descendants. He assigned other names to the other groups.
|The birds' phylogenetic relationships to major living reptile groups.|
- Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
- Aves can mean those that fly (alternately Avialae)
- Aves can mean all reptiles closer to birds than to crocodiles (alternately Avemetatarsalia [=Panaves])
- Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group"). (alternately Neornithes)
Under the fourth definition Archaeopteryx is an avialan, and not a member of Aves. Gauthier's proposals have been adopted by many researchers in the field of paleontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.
Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus." Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.
Dinosaurs and the origin of birds
|Cladogram following a phylogenetic study by Godefroit et al., 2013.|
Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists have discovered more nonavian theropods closely related to birds, the previously clear distinction between nonbirds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod dinosaurs had feathers, contribute to this ambiguity.
The consensus view in contemporary paleontology is that the birds, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these form a group called Paraves. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Unlike Archaeopteryx and the feathered dinosaurs, who primarily ate meat, recent studies suggest that the first birds were herbivores.
The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, though it is possibly closely related to the real ancestor.
Alternative scientific theories and controversies
Early disagreements on the origins of birds included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp, there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors. Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently. In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae.
A small minority of researchers, such as paleornithologist Alan Feduccia of the University of North Carolina, challenge the majority view, contending that birds are not dinosaurs, but evolved from early archosaurs like Longisquama.
Early evolution of birds
|Cladogram following the results of a phylogenetic study by Godefroit et al., 2013.|
The earliest known bird (avialan) fossils currently known hail from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui. The well-known early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features which may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.
Avialans diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved better olfactory senses.
|Mesozoic bird phylogeny simplified after O’Connor et al., 2013.|
The first large, diverse lineage of short-tailed birds to evolve were the enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of land birds during the Cretaceous period, enantiornithes became extinct along with many other dinosaur groups at the end of the of the Mesozoic era.
Many species of the second major bird lineage to diversify, the Euornithes ("true birds", including the ancestors of modern birds), were semi-aquatic and specialized in eating fish and other small aquatic organisms. Unlike the enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked perching adaptations and seem to have included shorebirds, waders, and swimming and diving species. The later included the superficially gull-like genus Ichthyornis (fish birds), the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws). Euornithes also included the first birds to develop true pygostyle and a fully mobile fan of tail feathers, which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.
Diversification of modern birds
|Basal divergences of modern birds
based on Sibley-Ahlquist taxonomy
All modern birds lie within the crown group Neornithes (alternately Aves), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds. These two subdivisions are often given the rank of superorder, although Livezey and Zusi assigned them "cohort" rank. Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050.
Due largely to the discovery of Vegavis, a late Cretaceous neognath member of the duck lineage, Neornithes is now known to have split into several modern lineages by the end of the Mesozoic era. The earliest divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. The Neornithes are agreed to have evolved in the Cretaceous, and the split between the Galloanseri from other Neognathes occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs. This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.
Classification of modern bird orders
The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds, although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate, but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.
This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This list uses the traditional classification (the so-called Clements order), revised by the Sibley-Monroe classification. The list of birds gives a more detailed summary of the orders, including families.
The name of the superorder is derived from paleognath, the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. The Palaeognathae consists of two orders which comprise 49 existing species.
The superorder Neognathae comprises 27 orders which have a total of nearly ten thousand species. The Neognathae have undergone adaptive radiation to produce the staggering diversity of form (especially of the bill and feet), function, and behaviour that are seen today.
The orders comprising the Neognathae are:
- Charadriiformes—gulls, button-quails, plovers and allies
- Procellariiformes—albatrosses, petrels, and allies
- Pelecaniformes—pelicans and allies
- Ciconiiformes—storks and allies
- Cathartiformes—New World vultures
- Falconiformes—falcons, eagles, hawks and allies
- Gruiformes—cranes and allies
- Columbiformes—doves and pigeons
- Psittaciformes—parrots and allies
- Cuculiformes—cuckoos and turacos
- Caprimulgiformes—nightjars and allies
- Apodiformes—swifts and hummingbirds
- Coraciiformes—kingfishers and allies
- Piciformes—woodpeckers and allies
- Passeriformes—passerines, the songbirds or perching birds
The radically different Sibley-Monroe classification (Sibley-Ahlquist taxonomy), based on molecular data, found widespread adoption in a few aspects, as recent molecular, fossil, and anatomical evidence supported the Galloanserae.
Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the Snow Petrel's breeding colonies up to 440 kilometres (270 mi) inland in Antarctica. The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics, however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics. Several families of birds have adapted to life both on the world's oceans and in them, with some seabird species coming ashore only to breed and some penguins have been recorded diving up to 300 metres (980 ft).
Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the Ring-necked Pheasant, for example, has been introduced around the world as a game bird. Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity. Some species, including Cattle Egret, Yellow-headed Caracara and Galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.
Anatomy and physiology
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the respiratory system. The skull bones in adults are fused and do not show cranial sutures. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings.
Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestine. Birds do not have a urinary bladder or external urethral opening and (with exception of the Ostrich) uric acid is excreted along with feces as a semisolid waste. However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. They also excrete creatine, rather than creatinine like mammals. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets. Males within Palaeognathae (with the exception of the kiwis), the Anseriformes (with the exception of screamers), and in rudimentary forms in Galliformes (but fully developed in Cracidae) possess a penis, which is never present in Neoaves. The length is thought to be related to sperm competition. When not copulating, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth. Most birds are highly adapted for rapid digestion to aid with flight. Some migratory birds have adapted to use protein from many parts of their bodies, including protein from the intestines, as additional energy during migration.
Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Sound production is achieved using the syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea; the trachea being elongated in some species, increasing the volume of vocalizations and the perception of the bird's size. The bird's heart has four chambers like a mammalian heart. In birds the main arteries taking blood away from the heart originate from the right aortic arch (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the aorta. The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating red blood cells in birds retain their nucleus.
The nervous system is large relative to the bird's size. The most developed part of the brain is the one that controls the flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. Most birds have a poor sense of smell with notable exceptions including kiwis, New World vultures and tubenoses. The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Some species also have dual fovea. Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones. This allows them to perceive ultraviolet light, which is involved in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Male Blue Tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers. Ultraviolet light is also used in foraging—kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents. The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds. The bird retina has a fan shaped blood supply system called the pecten. Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field. The avian ear lacks external pinnae but is covered by feathers, although in some birds, such as the Asio, Bubo and Otus owls, these feathers form tufts which resemble ears. The inner ear has a cochlea, but it is not spiral as in mammals.
A few species are able to use chemical defenses against predators; some Procellariiformes can eject an unpleasant oil against an aggressor, and some species of pitohuis from New Guinea have a powerful neurotoxin in their skin and feathers.
Birds have two sexes: male and female. The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).
In nearly all species of birds, an individual's sex is determined at fertilization. However, one recent study demonstrated temperature-dependent sex determination among Australian Brush-turkeys, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio.
Feathers, plumage, and scales
Feathers are a feature characteristic of birds (though also present in some dinosaurs not currently considered to be true birds). They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signaling. There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex.
Plumage is regularly moulted; the standard plumage of a bird that has moulted after breeding is known as the "non-breeding" plumage, or—in the Humphrey-Parkes terminology—"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey-Parkes system as "alternate" plumages. Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, flight feathers are replaced one at a time with the innermost primary being the first. When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop. After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary coverts are moulted in synchrony with the primary that they overlap. A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. As a general rule, the tail feathers are moulted and replaced starting with the innermost pair. Centripetal moults of tail feathers are however seen in the Phasianidae. The centrifugal moult is modified in the tail feathers of woodpeckers and treecreepers, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail. The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation.
Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland; these secretions protect the feathers' flexibility and act as an antimicrobial agent, inhibiting the growth of feather-degrading bacteria. This may be supplemented with the secretions of formic acid from ants, which birds receive through a behaviour known as anting, to remove feather parasites.
The scales of birds are composed of the same keratin as beaks, claws, and spurs. They are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers. The scales of birds are thought to be homologous to those of reptiles and mammals.
Most birds can fly, which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for breeding, feeding, and predator avoidance and escape. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb (wing) that serves as an aerofoil. Wing shape and size generally determine a bird species' type of flight; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are flightless, as were many extinct birds. Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators. Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.
Most birds are diurnal, but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.
Diet and feeding
Birds' diets are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds. Because birds have no teeth, their digestive system is adapted to process unmasticated food items that are swallowed whole.
Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists. Birds' feeding strategies vary by species. Many birds glean for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek pest insects are considered beneficial 'biological control agents' and their presence encouraged in biological pest control programs. Nectar feeders such as hummingbirds, sunbirds, lories, and lorikeets amongst others have specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers. Kiwis and shorebirds with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of ecological niches. Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. Flamingos, three species of prion, and some ducks are filter feeders. Geese and dabbling ducks are primarily grazers.
Some species, including frigatebirds, gulls, and skuas, engage in kleptoparasitism, stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet; a study of Great Frigatebirds stealing from Masked Boobies estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%. Other birds are scavengers; some of these, like vultures, are specialised carrion eaters, while others, like gulls, corvids, or other birds of prey, are opportunists.
Water and drinking
Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands. Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting. Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.
Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the pigeon, finch, mousebird, button-quail and bustard families are capable of sucking up water without the need to tilt back their heads. Some desert birds depend on water sources and sandgrouse are particularly well known for their daily congregations at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers. Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called crop milk that they provide to their chicks.
Feathers being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, ectoparasitic feather mites and birdlice. The physical condition of feathers are maintained by preening often with the application of secretions from the preen gland. Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as anting in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.
Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, shorebirds, and waterbirds undertake annual long distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the temperate or arctic/antarctic regions and a non-breeding season in the tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs. Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi), although the Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km (6,300 mi). Seabirds also undertake long migrations, the longest annual migration being those of Sooty Shearwaters, which nest in New Zealand and Chile and spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km (39,800 mi). Other seabirds disperse after breeding, travelling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability. Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates. Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory. Altitudinal migration is a form of short distance migration in which birds spend the breeding season at higher altitudes elevations and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories also become inhospitable due to lack of food. Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.
The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s a Manx Shearwater released in Boston returned to its colony in Skomer, Wales, within 13 days, a distance of 5,150 km (3,200 mi). Birds navigate during migration using a variety of methods. For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an internal clock. Orientation with the stellar compass depends on the position of the constellations surrounding Polaris. These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors.
Birds communicate using primarily visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).
Birds sometimes use plumage to assess and assert social dominance, to display breeding condition in sexually selected species, or to make threatening displays, as in the Sunbittern's mimicry of a large predator to ward off hawks and protect young chicks. Variation in plumage also allows for the identification of birds, particularly between species. Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds. The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements; males' breeding success may depend on the quality of such displays.
Bird calls and songs, which are produced in the syrinx, are the major means by which birds communicate with sound. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs. Calls are used for a variety of purposes, including mate attraction, evaluation of potential mates, bond formation, the claiming and maintenance of territories, the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season), and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat. Some birds also use mechanical sounds for auditory communication. The Coenocorypha snipes of New Zealand drive air through their feathers, woodpeckers drum territorially, and Palm Cockatoos use tools to drum.
Flocking and other associations
While some birds are essentially territorial or live in small family groups, other birds may form large flocks. The principal benefits of flocking are safety in numbers and increased foraging efficiency. Defence against predators is particularly important in closed habitats like forests, where ambush predation is common and multiple eyes can provide a valuable early warning system. This has led to the development of many mixed-species feeding flocks, which are usually composed of small numbers of many species; these flocks provide safety in numbers but increase potential competition for resources. Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.
Birds sometimes also form associations with non-avian species. Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface. Hornbills have a mutualistic relationship with Dwarf Mongooses, in which they forage together and warn each other of nearby birds of prey and other predators.
Resting and roosting
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats. Swifts are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight. It has been suggested that there may be certain kinds of sleep which are possible even when in flight. Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by viewing the outer margins of the flock. This adaptation is also known from marine mammals. Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators. Roosting sites are often chosen with regard to thermoregulation and safety.
Many sleeping birds bend their heads over their backs and tuck their bills in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. Perching birds have a tendon locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus Loriculus roost hanging upside down. Some hummingbirds go into a nightly state of torpor accompanied with a reduction of their metabolic rates. This physiological adaptation shows in nearly a hundred other species, including owlet-nightjars, nightjars, and woodswallows. One species, the Common Poorwill, even enters a state of hibernation. Birds do not have sweat glands, but they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or by using special behaviours like urohidrosis to cool themselves.
Ninety-five percent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate. Monogamy allows for biparental care, which is especially important for species in which females require males' assistance for successful brood-rearing. Among many socially monogamous species, extra-pair copulation (infidelity) is common. Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of forced copulation in ducks and other anatids. For females, possible benefits of extra-pair copulation include getting better genes for her offspring and insuring against the possibility of infertility in her mate. Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.
Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur. Polygamous breeding systems arise when females are able to raise broods without the help of males. Some species may use more than one system depending on the circumstances.
Breeding usually involves some form of courtship display, typically performed by the male. Most displays are rather simple and involve some type of song. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Females are generally the ones that drive partner selection, although in the polyandrous phalaropes, this is reversed: plainer males choose brightly coloured females. Courtship feeding, billing and allopreening are commonly performed between partners, generally after the birds have paired and mated.
Territories, nesting and incubation
Many birds actively defend a territory from others of the same species during the breeding season; maintenance of territories protects the food source for their chicks. Species that are unable to defend feeding territories, such as seabirds and swifts, often breed in colonies instead; this is thought to offer protection from predators. Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense.
All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay camouflaged eggs. There are many exceptions to this pattern, however; the ground-nesting nightjars have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of brood parasites have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts.
Bird eggs are usually laid in a nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows. Some bird nests, however, are extremely primitive; albatross nests are no more than a scrape on the ground. Most birds build nests in sheltered, hidden areas to avoid predation, but large or colonial birds—which are more capable of defence—may build more open nests. During nest construction, some species seek out plant matter from plants with parasite-reducing toxins to improve chick survival, and feathers are often used for nest insulation. Some bird species have no nests; the cliff-nesting Common Guillemot lays its eggs on bare rock, and male Emperor Penguins keep eggs between their body and feet. The absence of nests is especially prevalent in ground-nesting species where the newly hatched young are precocial.
Incubation, which optimises temperature for chick development, usually begins after the last egg has been laid. In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Warmth from parents passes to the eggs through brood patches, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process; adult albatrosses, for instance, lose as much as 83 grams (2.9 oz) of body weight per day of incubation. The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or volcanic sources. Incubation periods range from 10 days (in woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).
Parental care and fledging
At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed altricial, and tend to be born small, blind, immobile and naked; chicks that are mobile and feathered upon hatching are termed precocial. Altricial chicks need help thermoregulating and must be brooded for longer than precocial chicks. Chicks at neither of these extremes can be semi-precocial or semi-altricial.
The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in megapodes ends at hatching; the newly hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately. At the other extreme, many seabirds have extended periods of parental care, the longest being that of the Great Frigatebird, whose chicks take up to six months to fledge and are fed by the parents for up to an additional 14 months. The chick guard stage describes the period of breeding during which one of the adult birds is permanently present at the nest after chicks have hatched. The main purpose of the guard stage is to aid offspring to thermoregulate and protect them from predation.
In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of only one sex. In some species, other members of the same species—usually close relatives of the breeding pair, such as offspring from previous broods—will help with the raising of the young. Such alloparenting is particularly common among the Corvida, which includes such birds as the true crows, Australian Magpie and Fairy-wrens, but has been observed in species as different as the Rifleman and Red Kite. Among most groups of animals, male parental care is rare. In birds, however, it is quite common—more so than in any other vertebrate class. Though territory and nest site defence, incubation, and chick feeding are often shared tasks, there is sometimes a division of labour in which one mate undertakes all or most of a particular duty.
The point at which chicks fledge varies dramatically. The chicks of the Synthliboramphus murrelets, like the Ancient Murrelet, leave the nest the night after they hatch, following their parents out to sea, where they are raised away from terrestrial predators. Some other species, such as ducks, move their chicks away from the nest at an early age. In most species, chicks leave the nest just before, or soon after, they are able to fly. The amount of parental care after fledging varies; albatross chicks leave the nest on their own and receive no further help, while other species continue some supplementary feeding after fledging. Chicks may also follow their parents during their first migration.
Brood parasitism, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism. After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either obligate brood parasites, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or non-obligate brood parasites, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young. One hundred bird species, including honeyguides, icterids, estrildid finches and ducks, are obligate parasites, though the most famous are the cuckoos. Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks; this ensures that all food brought to the nest will be fed to the parasitic chicks.
Birds occupy a wide range of ecological positions. While some birds are generalists, others are highly specialised in their habitat or food requirements. Even within a single habitat, such as a forest, the niches occupied by different species of birds vary, with some species feeding in the forest canopy, others beneath the canopy, and still others on the forest floor. Forest birds may be insectivores, frugivores, and nectarivores. Aquatic birds generally feed by fishing, plant eating, and piracy or kleptoparasitism. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised scavengers. Avivores are animals that are specialized at predating birds.
Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal. Plants and pollinating birds often coevolve, and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.
Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfill ecological roles typically played by larger animals. For example, in New Zealand the moas were important browsers, as are the Kereru and Kokako today. Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa. Nesting seabirds may also affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of guano, which may enrich the local soil and the surrounding seas.
A wide variety of Avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.
Relationship with humans
Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man. Sometimes, these relationships are mutualistic, like the cooperative honey-gathering among honeyguides and African peoples such as the Borana. Other times, they may be commensal, as when species such as the House Sparrow have benefited from human activities. Several bird species have become commercially significant agricultural pests, and some pose an aviation hazard. Human activities can also be detrimental, and have threatened numerous bird species with extinction (hunting, avian lead poisoning, pesticides, roadkill, and predation by pet cats and dogs are common sources of death for birds).
Birds can act as vectors for spreading diseases such as psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis, and cryptosporidiosis over long distances. Some of these are zoonotic diseases that can also be transmitted to humans.
Domesticated birds raised for meat and eggs, called poultry, are the largest source of animal protein eaten by humans; in 2003, 76 million tons of poultry and 61 million tons of eggs were produced worldwide. Chickens account for much of human poultry consumption, though turkeys, ducks, and geese are also relatively common. Many species of birds are also hunted for meat. Bird hunting is primarily a recreational activity except in extremely undeveloped areas. The most important birds hunted in North and South America are waterfowl; other widely hunted birds include pheasants, wild turkeys, quail, doves, partridge, grouse, snipe, and woodcock. Muttonbirding is also popular in Australia and New Zealand. Though some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species.
Other commercially valuable products from birds include feathers (especially the down of geese and ducks), which are used as insulation in clothing and bedding, and seabird feces (guano), which is a valuable source of phosphorus and nitrogen. The War of the Pacific, sometimes called the Guano War, was fought in part over the control of guano deposits.
Birds have been domesticated by humans both as pets and for practical purposes. Colourful birds, such as parrots and mynas, are bred in captivity or kept as pets, a practice that has led to the illegal trafficking of some endangered species. Falcons and cormorants have long been used for hunting and fishing, respectively. Messenger pigeons, used since at least 1 AD, remained important as recently as World War II. Today, such activities are more common either as hobbies, for entertainment and tourism, or for sports such as pigeon racing.
Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions. Many homeowners erect bird feeders near their homes to attract various species. Bird feeding has grown into a multimillion dollar industry; for example, an estimated 75% of households in Britain provide food for birds at some point during the winter.
Religion, folklore and culture
Birds play prominent and diverse roles in folklore, religion, and popular culture. In religion, birds may serve as either messengers or priests and leaders for a deity, such as in the Cult of Makemake, in which the Tangata manu of Easter Island served as chiefs; or as the rooster(cock) serves as a tangible vessel of Christ as in the gospel of –Matthew, Mark and Luke in the New Testament with Christ speaking through the cock; or as attendants, as in the case of Hugin and Munin, two Common Ravens who whispered news into the ears of the Norse god Odin. In several civilizations of ancient Italy, particularly Etruscan and Roman religion, priests were involved in augury, or interpreting the words of birds while the "auspex" (from which the word "auspicious" is derived) watched their activities to foretell events. They may also serve as religious symbols, as when Jonah (Hebrew: יוֹנָה, dove) embodied the fright, passivity, mourning, and beauty traditionally associated with doves. Birds have themselves been deified, as in the case of the Common Peacock, which is perceived as Mother Earth by the Dravidians of India. Some birds have also been perceived as monsters, including the mythological Roc and the Māori's legendary Pouākai, a giant bird capable of snatching humans.
Birds have been featured in culture and art since prehistoric times, when they were represented in early cave paintings. Birds were later used in religious or symbolic art and design, such as the magnificent Peacock Throne of the Mughal and Persian emperors. With the advent of scientific interest in birds, many paintings of birds were commissioned for books. Among the most famous of these bird artists was John James Audubon, whose paintings of North American birds were a great commercial success in Europe and who later lent his name to the National Audubon Society. Birds are also important figures in poetry; for example, Homer incorporated Nightingales into his Odyssey, and Catullus used a sparrow as an erotic symbol in his Catullus 2. The relationship between an albatross and a sailor is the central theme of Samuel Taylor Coleridge's The Rime of the Ancient Mariner, which led to the use of the term as a metaphor for a 'burden'. Other English metaphors derive from birds; vulture funds and vulture investors, for instance, take their name from the scavenging vulture.
Perceptions of various bird species often vary across cultures. Owls are associated with bad luck, witchcraft, and death in parts of Africa, but are regarded as wise across much of Europe. Hoopoes were considered sacred in Ancient Egypt and symbols of virtue in Persia, but were thought of as thieves across much of Europe and harbingers of war in Scandinavia.
Though human activities have allowed the expansion of a few species, such as the Barn Swallow and European Starling, they have caused population decreases or extinction in many other species. Over a hundred bird species have gone extinct in historical times, although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of Melanesian, Polynesian, and Micronesian islands. Many bird populations are declining worldwide, with 1,227 species listed as threatened by Birdlife International and the IUCN in 2009.
The most commonly cited human threat to birds is habitat loss. Other threats include overhunting, accidental mortality due to structural collisions or long-line fishing bycatch, pollution (including oil spills and pesticide use), competition and predation from nonnative invasive species, and climate change.
Governments and conservation groups work to protect birds, either by passing laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the California Condor and Norfolk Parakeet.
- Pascal Godefroit, Andrea Cau, Hu Dong-Yu, François Escuillié, Wu Wenhao and Gareth Dyke (2013). "A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds". Nature. in press. doi:10.1038/nature12168.
- Brands, Sheila (14 August 2008). "Systema Naturae 2000 / Classification, Class Aves". Project: The Taxonomicon. Retrieved 11 June 2012.
- del Hoyo, Josep; Andy Elliott and Jordi Sargatal (1992). Handbook of Birds of the World, Volume 1: Ostrich to Ducks. Barcelona: Lynx Edicions. ISBN 84-87334-10-5.
- (Latin) Linnaeus, Carolus (1758). Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Holmiae. (Laurentii Salvii). p. 824.
- Livezey, Bradley C.; Zusi, RL (January 2007). "Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion". Zoological Journal of the Linnean Society 149 (1): 1–95. doi:10.1111/j.1096-3642.2006.00293.x. PMC 2517308. PMID 18784798.
- Padian, Kevin; L.M. Chiappe Chiappe LM (1997). "Bird Origins". In Philip J. Currie and Kevin Padian (eds.). Encyclopedia of Dinosaurs. San Diego: Academic Press. pp. 41–96. ISBN 0-12-226810-5.
- Gauthier, Jacques (1986). "Saurischian Monophyly and the origin of birds". In Kevin Padian. The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Science 8. San Francisco, CA: Published by California Academy of Sciences. pp. 1–55. ISBN 0-940228-14-9.
- Gauthier, J., and de Queiroz, K. (2001). "Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves." Pp. 7-41 in New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom (J. A. Gauthier and L. F. Gall, eds.). Peabody Museum of Natural History, Yale University, New Haven, Connecticut, U.S.A.
- Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.) (2004). The Dinosauria, Second Edition. University of California Press., 861 pp.
- Senter, P. (2007). "A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda)." Journal of Systematic Palaeontology, (doi:10.1017/S1477201907002143).
- Gauthier, J. (1986). "Saurischian monophyly and the origin of birds." In: K. Padian, ed. The origin of birds and the evolution of flight. San Francisco: California, Acad.Sci. pp.1–55. (Mem.Calif.Acad.Sci.8.)
- Prum, Richard O. Prum (2008). "Who's Your Daddy". Science 322 (5909): 1799–1800. doi:10.1126/science.1168808. PMID 19095929.
- Paul, Gregory S. (2002). "Looking for the True Bird Ancestor". Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press. pp. 171–224. ISBN 0-8018-6763-0.
- Norell, Mark; Mick Ellison (2005). Unearthing the Dragon: The Great Feathered Dinosaur Discovery. New York: Pi Press. ISBN 0-13-186266-9.
- Xing Xu, Hailu You, Kai Du and Fenglu Han (28 July 2011). "An Archaeopteryx-like theropod from China and the origin of Avialae". Nature 475 (7357): 465–470. doi:10.1038/nature10288. PMID 21796204.
- Turner, Alan H.; Pol, D; Clarke, JA; Erickson, GM; Norell, MA (2007). "A basal dromaeosaurid and size evolution preceding avian flight" (PDF). Science 317 (5843): 1378–1381. doi:10.1126/science.1144066. PMID 17823350.
- Xu, X; Zhou, Z; Wang, X; Kuang, X; Zhang, F; Du, X (2003). "Four-winged dinosaurs from China". Nature 421 (6921): 335–340. doi:10.1038/nature01342. PMID 12540892.
- Luiggi, Christina (July 2011). "On the Origin of Birds". The Scientist. Retrieved 11 June 2012.
- Mayr, G.; Phol, B.; Hartman, S.; Peters, D.S. (2007). "The tenth skeletal specimen of Archaeopteryx". Zoological Journal of the Linnean Society 149: 97–116. doi:10.1111/j.1096-3642.2006.00245.x.
- Heilmann, Gerhard (1927). The Origin of Birds. New York: Dover Publications.
- Rasskin-Gutman, Diego; Buscalioni, Angela D. (March 2001). "Theoretical morphology of the Archosaur (Reptilia: Diapsida) pelvic girdle". Paleobiology 27 (1): 59–78. doi:10.1666/0094-8373(2001)027<0059:TMOTAR>2.0.CO;2.
- Feduccia, Alan; Lingham-Soliar, T; Hinchliffe, JR (November 2005). "Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence". Journal of Morphology 266 (2): 125–66. doi:10.1002/jmor.10382. PMID 16217748.
- This theory is contested by most paleontologists. Prum, Richard O. (April 2003). "Are Current Critiques Of The Theropod Origin Of Birds Science? Rebuttal To Feduccia 2002". The Auk 120 (2): 550–61. doi:10.1642/0004-8038(2003)120[0550:ACCOTT]2.0.CO;2. JSTOR 4090212.
- Zheng X., Zhou Z., Wang X., Zhang F., Zhang X., Wang Y., Wei G., Wang S., and Xu X. (2013). "Hind Wings in Basal Birds and the Evolution of Leg Feathers." Science, 339(6125): 1309-1312. doi:10.1126/science.1228753
- Chiappe, Luis M. (2007). Glorified Dinosaurs: The Origin and Early Evolution of Birds. Sydney: University of New South Wales Press. ISBN 978-0-86840-413-4.
- Agency France-Presse (April 2011). "Birds survived dino extinction with keen senses". Cosmos Magazine. Retrieved 11 June 2012.
- o’Connor, J. K.; Zhang, Y.; Chiappe, L. M.; Meng, Q.; Quanguo, L.; Di, L. (2013). "A new enantiornithine from the Yixian Formation with the first recognized avian enamel specialization". Journal of Vertebrate Paleontology 33: 1. doi:10.1080/02724634.2012.719176.
- Clarke, Julia A. (2004). "Morphology, Phylogenetic Taxonomy, and Systematics of Ichthyornis and Apatornis (Avialae: Ornithurae)" (PDF). Bulletin of the American Museum of Natural History 286: 1–179. doi:10.1206/0003-0090(2004)286<0001:MPTASO>2.0.CO;2.
- Louchart, A., and Viriot, L. (2011). "From snout to beak: the loss of teeth in birds." Trends In Ecology & Evolution, 26(12): 663-673.
- Clarke, J. A., Zhou, Z., and Zhang, F. (2006). "Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of Yixianornis grabaui." Journal of Anatomy, 208(3): 287-308.
- Ritchison, Gary. "Bird biogeography". Avian Biology. Eastern Kentucky University. Retrieved 10 April 2008.
- Clements, James F. (2007). The Clements Checklist of Birds of the World (6th ed.). Ithaca: Cornell University Press. ISBN 978-0-8014-4501-9.
- Gill, Frank (2006). Birds of the World: Recommended English Names. Princeton: Princeton University Press. ISBN 978-0-691-12827-6.
- Clarke, Julia A.; Tambussi, CP; Noriega, JI; Erickson, GM; Ketcham, RA (2005). "Definitive fossil evidence for the extant avian radiation in the Cretaceous" (PDF). Nature 433 (7023): 305–308. doi:10.1038/nature03150. PMID 15662422. Nature.com, Supporting information
- Ericson, Per G.P.; Anderson, CL; Britton, T; Elzanowski, A; Johansson, US; Källersjö, M; Ohlson, JI; Parsons, TJ et al. (2006). "Diversification of Neoaves: integration of molecular sequence data and fossils" (PDF). Biology Letters 2 (4): 543–547. doi:10.1098/rsbl.2006.0523. PMC 1834003. PMID 17148284.
- Brown, Joseph W.; Payne, RB; Mindell, DP (June 2007). "Nuclear DNA does not reconcile 'rocks' and 'clocks' in Neoaves: a comment on Ericson et al". Biology Letters 3 (3): 257–259. doi:10.1098/rsbl.2006.0611. PMC 2464679. PMID 17389215.
- Mindell, David P.; Brown, Joseph W.; Harshman, John (June 2008). "Neoaves". Tree of Life Project. tolweb.org. Retrieved 11 June 2012.
- Sibley, Charles; Jon Edward Ahlquist (1990). Phylogeny and classification of birds. New Haven: Yale University Press. ISBN 0-300-04085-7.
- Mayr, Ernst; Short, Lester L. (1970). Species Taxa of North American Birds: A Contribution to Comparative Systematics. Publications of the Nuttall Ornithological Club, no. 9. Cambridge, Mass.: Nuttall Ornithological Club. OCLC 517185.
- Newton, Ian (2003). The Speciation and Biogeography of Birds. Amsterdam: Academic Press. p. 463. ISBN 0-12-517375-X.
- Brooke, Michael (2004). Albatrosses And Petrels Across The World. Oxford: Oxford University Press. ISBN 0-19-850125-0.
- Weir, Jason T.; Schluter, D (2007). "The Latitudinal Gradient in Recent Speciation and Extinction Rates of Birds and Mammals". Science 315 (5818): 1574–76. doi:10.1126/science.1135590. PMID 17363673.
- Schreiber, Elizabeth Anne; Joanna Burger (2001). Biology of Marine Birds. Boca Raton: CRC Press. ISBN 0-8493-9882-7.
- Sato, Katsufumi; N; K; N; W; C; B; H et al. (1 May 2002). "Buoyancy and maximal diving depth in penguins: do they control inhaling air volume?". Journal of Experimental Biology 205 (9): 1189–1197. PMID 11948196.
- Hill, David; Peter Robertson (1988). The Pheasant: Ecology, Management, and Conservation. Oxford: BSP Professional. ISBN 0-632-02011-3.
- Spreyer, Mark F.; Enrique H. Bucher (1998). "Monk Parakeet (Myiopsitta monachus)". The Birds of North America. Cornell Lab of Ornithology. doi:10.2173/bna.322.
- Arendt, Wayne J. (1 January 1988). "Range Expansion of the Cattle Egret, (Bubulcus ibis) in the Greater Caribbean Basin". Colonial Waterbirds 11 (2): 252–62. doi:10.2307/1521007. JSTOR 1521007.
- Bierregaard, R.O. (1994). "Yellow-headed Caracara". In Josep del Hoyo, Andrew Elliott and Jordi Sargatal (eds.). Handbook of the Birds of the World. Volume 2; New World Vultures to Guineafowl. Barcelona: Lynx Edicions. ISBN 84-87334-15-6.
- Juniper, Tony; Mike Parr (1998). Parrots: A Guide to the Parrots of the World. London: Christopher Helm. ISBN 0-7136-6933-0.
- Ehrlich, Paul R.; David S. Dobkin, and Darryl Wheye (1988). "Adaptations for Flight". Birds of Stanford. Stanford University. Retrieved 13 December 2007. Based on The Birder's Handbook (Paul Ehrlich, David Dobkin, and Darryl Wheye. 1988. Simon and Schuster, New York.)
- Gill, Frank (1995). Ornithology. New York: WH Freeman and Co. ISBN 0-7167-2415-4.
- Noll, Paul. "The Avian Skeleton". paulnoll.com. Retrieved 13 December 2007.
- "Skeleton of a typical bird". Fernbank Science Center's Ornithology Web. Retrieved 13 December 2007.
- Ehrlich, Paul R.; David S. Dobkin, and Darryl Wheye (1988). "Drinking". Birds of Stanford. Standford University. Retrieved 13 December 2007.
- Tsahar, Ella; Martínez Del Rio, C; Izhaki, I; Arad, Z (2005). "Can birds be ammonotelic? Nitrogen balance and excretion in two frugivores". Journal of Experimental Biology 208 (6): 1025–34. doi:10.1242/jeb.01495. PMID 15767304.
- Skadhauge, E; Erlwanger, KH; Ruziwa, SD; Dantzer, V; Elbrønd, VS; Chamunorwa, JP (2003). "Does the ostrich (Struthio camelus) coprodeum have the electrophysiological properties and microstructure of other birds?". Comparative biochemistry and physiology. Part A, Molecular & integrative physiology 134 (4): 749–755. doi:10.1016/S1095-6433(03)00006-0. PMID 12814783.
- Preest, Marion R.; Beuchat, Carol A. (April 1997). "Ammonia excretion by hummingbirds". Nature 386 (6625): 561–62. doi:10.1038/386561a0.
- Mora, J.; Martuscelli, J; Ortiz Pineda, J; Soberon, G (1965). "The regulation of urea-biosynthesis enzymes in vertebrates" (PDF). Biochemical Journal 96 (1): 28–35. PMC 1206904. PMID 14343146.
- Packard, Gary C. (1966). "The Influence of Ambient Temperature and Aridity on Modes of Reproduction and Excretion of Amniote Vertebrates". The American Naturalist 100 (916): 667–82. doi:10.1086/282459. JSTOR 2459303.
- Balgooyen, Thomas G. (1 October 1971). "Pellet Regurgitation by Captive Sparrow Hawks (Falco sparverius)" (PDF). Condor 73 (3): 382–85. doi:10.2307/1365774. JSTOR 1365774.
- Phenomena: Not Exactly Rocket Science How Chickens Lost Their Penises (And Ducks Kept Theirs)
- Ornithology, 3rd Edition - Waterfowl: Order Anseriformes
- McCracken, KG (2000). "The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata)". The Auk 117 (3): 820–825. doi:10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2.
- Gionfriddo, James P.; Best (1 February 1995). "Grit Use by House Sparrows: Effects of Diet and Grit Size" (PDF). Condor 97 (1): 57–67. doi:10.2307/1368983.
- Attenborough, David (1998). The Life of Birds. Princeton: Princeton University Press. ISBN 0-691-01633-X.
- Battley, Phil F.; Piersma, T; Dietz, MW; Tang, S; Dekinga, A; Hulsman, K (January 2000). "Empirical evidence for differential organ reductions during trans-oceanic bird flight". Proceedings of the Royal Society B 267 (1439): 191–5. doi:10.1098/rspb.2000.0986. PMC 1690512. PMID 10687826. (Erratum in Proceedings of the Royal Society B 267(1461):2567.)
- Maina, John N. (November 2006). "Development, structure, and function of a novel respiratory organ, the lung-air sac system of birds: to go where no other vertebrate has gone". Biological Reviews 81 (4): 545–79. doi:10.1017/S1464793106007111. PMID 17038201.
- Suthers, Roderick A.; Sue Anne Zollinger (2004). "Producing song: the vocal apparatus". In H. Philip Zeigler and Peter Marler (eds.). Behavioral Neurobiology of Birdsong. Annals of the New York Academy of Sciences 1016. New York: New York Academy of Sciences. pp. 109–129. doi:10.1196/annals.1298.041. ISBN 1-57331-473-0. PMID 15313772
- Fitch, W. T. (1999). "Acoustic exaggeration of size in birds via tracheal elongation: comparative and theoretical analyses". Journal of Zoology 248: 31–48. doi:10.1017/S095283699900504X.
- Scott, Robert B. (March 1966). "Comparative hematology: The phylogeny of the erythrocyte". Annals of Hematology 12 (6): 340–51. doi:10.1007/BF01632827. PMID 5325853.
- Sales, James (2005). "The endangered kiwi: a review" (PDF). Folia Zoologica 54 (1–2): 1–20.
- Ehrlich, Paul R.; David S. Dobkin, and Darryl Wheye (1988). "The Avian Sense of Smell". Birds of Stanford. Standford University. Retrieved 13 December 2007.
- Lequette, Benoit; Verheyden; Jouventin (1 August 1989). "Olfaction in Subantarctic seabirds: Its phylogenetic and ecological significance" (PDF). The Condor 91 (3): 732–35. doi:10.2307/1368131.
- Wilkie, Susan E.; Vissers, PM; Das, D; Degrip, WJ; Bowmaker, JK; Hunt, DM (February 1998). "The molecular basis for UV vision in birds: spectral characteristics, cDNA sequence and retinal localization of the UV-sensitive visual pigment of the budgerigar (Melopsittacus undulatus)". Biochemical Journal 330 (Pt 1): 541–47. PMC 1219171. PMID 9461554.
- Andersson, S.; J. Ornborg and M. Andersson (1998). "Ultraviolet sexual dimorphism and assortative mating in blue tits". Proceeding of the Royal Society B 265 (1395): 445–50. doi:10.1098/rspb.1998.0315.
- Viitala, Jussi; Korplmäki, Erkki; Palokangas, Pälvl; Koivula, Minna (1995). "Attraction of kestrels to vole scent marks visible in ultraviolet light". Nature 373 (6513): 425–27. doi:10.1038/373425a0.
- Williams, David L.; Flach, E (March 2003). "Symblepharon with aberrant protrusion of the nictitating membrane in the snowy owl (Nyctea scandiaca)". Veterinary Ophthalmology 6 (1): 11–13. doi:10.1046/j.1463-5224.2003.00250.x. PMID 12641836.
- White, Craig R.; Day, N; Butler, PJ; Martin, GR; Bennett, Peter (July 2007). "Vision and Foraging in Cormorants: More like Herons than Hawks?". In Bennett, Peter. PLoS ONE 2 (7): e639. doi:10.1371/journal.pone.0000639. PMC 1919429. PMID 17653266.
- Martin, Graham R.; Katzir, G (1999). "Visual fields in Short-toed Eagles, Circaetus gallicus (Accipitridae), and the function of binocularity in birds". Brain, Behaviour and Evolution 53 (2): 55–66. doi:10.1159/000006582. PMID 9933782.
- Saito, Nozomu (1978). "Physiology and anatomy of avian ear". The Journal of the Acoustical Society of America 64 (S1): S3. doi:10.1121/1.2004193.
- Warham, John (1 May 1977). "The Incidence, Function and ecological significance of petrel stomach oils" (PDF). Proceedings of the New Zealand Ecological Society 24 (3): 84–93.
- Dumbacher, J.P.; Beehler, BM; Spande, TF; Garraffo, HM; Daly, JW (October 1992). "Homobatrachotoxin in the genus Pitohui: chemical defense in birds?". Science 258 (5083): 799–801. doi:10.1126/science.1439786. PMID 1439786.
- Göth, Anne (2007). "Incubation temperatures and sex ratios in Australian brush-turkey (Alectura lathami) mounds". Austral Ecology 32 (4): 278–85. doi:10.1111/j.1442-9993.2007.01709.x.
- Belthoff, James R.; Dufty,; Gauthreaux, (1 August 1994). "Plumage Variation, Plasma Steroids and Social Dominance in Male House Finches". The Condor 96 (3): 614–25. doi:10.2307/1369464.
- Guthrie, R. Dale. "How We Use and Show Our Social Organs". Body Hot Spots: The Anatomy of Human Social Organs and Behavior. Archived from the original on 21 June 2007. Retrieved 19 October 2007.
- Humphrey, Philip S.; Parkes, K. C. (1 June 1959). "An approach to the study of molts and plumages" (PDF). The Auk 76 (1): 1–31. doi:10.2307/4081839. JSTOR 4081839.
- Pettingill Jr. OS (1970). Ornithology in Laboratory and Field. Burgess Publishing Co. ISBN 0125524552.
- de Beer SJ, Lockwood GM, Raijmakers JHFS, Raijmakers JMH, Scott WA, Oschadleus HD, Underhill LG (2001). Web.uct.ac.za SAFRING Bird Ringing Manual.
- Gargallo, Gabriel (1 June 1994). "Flight Feather Moult in the Red-Necked Nightjar Caprimulgus ruficollis". Journal of Avian Biology 25 (2): 119–24. doi:10.2307/3677029. JSTOR 3677029.
- Mayr, Ernst (1954). "The tail molt of small owls" (PDF). The Auk 71 (2): 172–78. doi:10.2307/4081571.
- Payne, Robert B. "Birds of the World, Biology 532". Bird Division, University of Michigan Museum of Zoology. Retrieved 20 October 2007.
- Turner, J. Scott (1997). "On the thermal capacity of a bird's egg warmed by a brood patch". Physiological Zoology 70 (4): 470–80. doi:10.1086/515854. PMID 9237308.
- Walther, Bruno A. (2005). "Elaborate ornaments are costly to maintain: evidence for high maintenance handicaps". Behavioural Ecology 16 (1): 89–95. doi:10.1093/beheco/arh135.
- Shawkey, Matthew D.; Pillai, Shreekumar R.; Hill, Geoffrey E. (2003). "Chemical warfare? Effects of uropygial oil on feather-degrading bacteria". Journal of Avian Biology 34 (4): 345–49. doi:10.1111/j.0908-8857.2003.03193.x.
- Ehrlich, Paul R. (1986). "The Adaptive Significance of Anting" (PDF). The Auk 103 (4): 835.
- Lucas, Alfred M. (1972). Avian Anatomy—integument. East Lansing, Michigan, US: USDA Avian Anatomy Project, Michigan State University. pp. 67, 344, 394–601.
- Roots, Clive (2006). Flightless Birds. Westport: Greenwood Press. ISBN 978-0-313-33545-7.
- McNab, Brian K. (October 1994). "Energy Conservation and the Evolution of Flightlessness in Birds". The American Naturalist 144 (4): 628–42. doi:10.1086/285697. JSTOR 2462941.
- Kovacs, Christopher E.; Meyers, RA (2000). "Anatomy and histochemistry of flight muscles in a wing-propelled diving bird, the Atlantic Puffin, Fratercula arctica". Journal of Morphology 244 (2): 109–25. doi:10.1002/(SICI)1097-4687(200005)244:2<109::AID-JMOR2>3.0.CO;2-0. PMID 10761049.
- Robert, Michel; McNeil, Raymond; Leduc, Alain (January 1989). "Conditions and significance of night feeding in shorebirds and other water birds in a tropical lagoon" (PDF). The Auk 106 (1): 94–101. doi:10.2307/4087761.
- N Reid (2006). "Birds on New England wool properties – A woolgrower guide" (PDF). Land, Water & Wool Northern Tablelands Property Fact Sheet. Australian Government – Land and Water Australia. Retrieved 17 July 2010.
- Paton, D. C.; Collins, B. G. (1 April 1989). "Bills and tongues of nectar-feeding birds: A review of morphology, function, and performance, with intercontinental comparisons". Australian Journal of Ecology 14 (4): 473–506. doi:10.1111/j.1442-9993.1989.tb01457.x.
- Baker, Myron Charles; Baker, Ann Eileen Miller (1 April 1973). "Niche Relationships Among Six Species of Shorebirds on Their Wintering and Breeding Ranges". Ecological Monographs 43 (2): 193–212. doi:10.2307/1942194. JSTOR 1942194.
- Cherel, Yves; Bocher, P; De Broyer, C; Hobson, KA (2002). "Food and feeding ecology of the sympatric thin-billed Pachyptila belcheri and Antarctic P. desolata prions at Iles Kerguelen, Southern Indian Ocean". Marine Ecology Progress Series 228: 263–81. doi:10.3354/meps228263.
- Jenkin, Penelope M. (1957). "The Filter-Feeding and Food of Flamingoes (Phoenicopteri)". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 240 (674): 401–93. doi:10.1098/rstb.1957.0004. JSTOR 92549.
- Miyazaki, Masamine; Kuroki, M.; Niizuma, Y.; Watanuki, Y. (1 July 1996). "Vegetation cover, kleptoparasitism by diurnal gulls and timing of arrival of nocturnal Rhinoceros Auklets" (PDF). The Auk 113 (3): 698–702. doi:10.2307/3677021. JSTOR 3677021.
- Bélisle, Marc; Giroux (1 August 1995). "Predation and kleptoparasitism by migrating Parasitic Jaegers" (PDF). The Condor 97 (3): 771–781. doi:10.2307/1369185.
- Vickery, J. A.; Brooke, M. De L. (1 May 1994). "The Kleptoparasitic Interactions between Great Frigatebirds and Masked Boobies on Henderson Island, South Pacific" (PDF). The Condor 96 (2): 331–40. doi:10.2307/1369318. JSTOR 1369318.
- Hiraldo, F.C.; Blanco, J. C.; Bustamante, J. (1991). "Unspecialized exploitation of small carcasses by birds". Bird Studies 38 (3): 200–07. doi:10.1080/00063659109477089.
- Engel, Sophia Barbara (2005). Racing the wind: Water economy and energy expenditure in avian endurance flight. University of Groningen. ISBN 90-367-2378-7.
- Tieleman, B.I.; Williams, JB (1999). "The role of hyperthermia in the water economy of desert birds". Physiol. Biochem. Zool. 72 (1): 87–100. doi:10.1086/316640. PMID 9882607.
- Schmidt-Nielsen, Knut (1 May 1960). "The Salt-Secreting Gland of Marine Birds". Circulation 21 (5): 955–967. doi:10.1161/01.CIR.21.5.955.
- Hallager, Sara L. (1994). "Drinking methods in two species of bustards". Wilson Bull. 106 (4): 763–764. hdl:10088/4338.
- MacLean, Gordon L. (1 June 1983). "Water Transport by Sandgrouse". BioScience 33 (6): 365–369. doi:10.2307/1309104. JSTOR 1309104.
- Eraud C; Dorie A; Jacquet A & Faivre B (2008). "The crop milk: a potential new route for carotenoid-mediated parental effects". Journal of Avian Biology 39 (2): 247–251. doi:10.1111/j.0908-8857.2008.04053.x.
- Mario, Principato; Federica, Lisi; Iolanda, Moretta; Nada, Samra; Francesco, Puccetti (2005). "The alterations of plumage of parasitic origin". Italian Journal of Animal Science 4: 296–299.
- Revis, Hannah C.; Waller, Deborah A. (2004). "Bactericidal and fungicidal activity of ant chemicals on feather parasites: an evaluation of anting behavior as a method of self-medication in songbirds". The Auk 121 (4): 1262–1268. doi:10.1642/0004-8038(2004)121[1262:BAFAOA]2.0.CO;2.
- Clayton, Dale H.; Koop, Jennifer A. H.; Harbison, Christopher W.; Moyer, Brett R.; Bush, Sarah E. (2010). "How Birds Combat Ectoparasites". The Open Ornithology Journal 3: 41–71.
- Klaassen, Marc (1 January 1996). "Metabolic constraints on long-distance migration in birds". Journal of Experimental Biology 199 (1): 57–64. PMID 9317335.
- Gill, Frank (1995). Ornithology (2nd ed.). New York: W.H. Freeman. ISBN 0-7167-2415-4.
- "Long-distance Godwit sets new record". BirdLife International. 4 May 2007. Retrieved 13 December 2007.
- Shaffer, Scott A.; Tremblay, Y; Weimerskirch, H; Scott, D; Thompson, DR; Sagar, PM; Moller, H; Taylor, GA et al. (2006). "Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer". Proceedings of the National Academy of Sciences 103 (34): 12799–802. doi:10.1073/pnas.0603715103. PMC 1568927. PMID 16908846.
- Croxall, John P.; Silk, JR; Phillips, RA; Afanasyev, V; Briggs, DR (2005). "Global Circumnavigations: Tracking year-round ranges of nonbreeding Albatrosses". Science 307 (5707): 249–50. doi:10.1126/science.1106042. PMID 15653503.
- Wilson, W. Herbert, Jr. (1999). "Bird feeding and irruptions of northern finches:are migrations short stopped?" (PDF). North America Bird Bander 24 (4): 113–21.
- Nilsson, Anna L. K.; Alerstam, Thomas; Nilsson, Jan-Åke (2006). "Do partial and regular migrants differ in their responses to weather?". The Auk 123 (2): 537–47. doi:10.1642/0004-8038(2006)123[537:DPARMD]2.0.CO;2.
- Chan, Ken (2001). "Partial migration in Australian landbirds: a review". Emu 101 (4): 281–92. doi:10.1071/MU00034.
- Rabenold, Kerry N. (1985). "Variation in Altitudinal Migration, Winter Segregation, and Site Tenacity in two subspecies of Dark-eyed Juncos in the southern Appalachians" (PDF). The Auk 102 (4): 805–19.
- Collar, Nigel J. (1997). "Family Psittacidae (Parrots)". In Josep del Hoyo, Andrew Elliott and Jordi Sargatal (eds.). Handbook of the Birds of the World, Volume 4: Sandgrouse to Cuckoos. Barcelona: Lynx Edicions. ISBN 84-87334-22-9.
- Matthews, G. V. T. (1 September 1953). "Navigation in the Manx Shearwater". Journal of Experimental Biology 30 (2): 370–96.
- Mouritsen, Henrik; L (15 November 2001). "Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass". Journal of Experimental Biology 204 (8): 3855–65. PMID 11807103.
- Deutschlander, Mark E.; P; B (15 April 1999). "The case for light-dependent magnetic orientation in animals". Journal of Experimental Biology 202 (8): 891–908. PMID 10085262.
- Möller, Anders Pape (1988). "Badge size in the house sparrow Passer domesticus". Behavioral Ecology and Sociobiology 22 (5): 373–78.
- Thomas, Betsy Trent; Strahl (1 August 1990). "Nesting Behavior of Sunbitterns (Eurypyga helias) in Venezuela" (PDF). The Condor 92 (3): 576–81. doi:10.2307/1368675.
- Pickering, S. P. C. (2001). "Courtship behaviour of the Wandering Albatross Diomedea exulans at Bird Island, South Georgia" (PDF). Marine Ornithology 29 (1): 29–37.
- Pruett-Jones, S. G.; Pruett-Jones (1 May 1990). "Sexual Selection Through Female Choice in Lawes' Parotia, A Lek-Mating Bird of Paradise". Evolution 44 (3): 486–501. doi:10.2307/2409431.
- Genevois, F.; Bretagnolle, V. (1994). "Male Blue Petrels reveal their body mass when calling". Ethology Ecology and Evolution 6 (3): 377–83. doi:10.1080/08927014.1994.9522988.
- Jouventin, Pierre; Aubin, T; Lengagne, T (June 1999). "Finding a parent in a king penguin colony: the acoustic system of individual recognition". Animal Behaviour 57 (6): 1175–83. doi:10.1006/anbe.1999.1086. PMID 10373249.
- Templeton, Christopher N.; Greene, E; Davis, K (2005). "Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size". Science 308 (5730): 1934–37. doi:10.1126/science.1108841. PMID 15976305.
- Miskelly, C. M. (July 1987). "The identity of the hakawai" (PDF). Notornis 34 (2): 95–116.
- Murphy, Stephen; Legge, Sarah; Heinsohn, Robert (2003). "The breeding biology of palm cockatoos (Probosciger aterrimus): a case of a slow life history". Journal of Zoology 261 (4): 327–39. doi:10.1017/S0952836903004175.
- Sekercioglu, Cagan Hakki (2006). "Foreword". In Josep del Hoyo, Andrew Elliott and David Christie (eds.). Handbook of the Birds of the World, Volume 11: Old World Flycatchers to Old World Warblers. Barcelona: Lynx Edicions. p. 48. ISBN 84-96553-06-X.
- Terborgh, John (2005). "Mixed flocks and polyspecific associations: Costs and benefits of mixed groups to birds and monkeys". American Journal of Primatology 21 (2): 87–100. doi:10.1002/ajp.1350210203.
- Hutto, Richard L. (1 January 988). "Foraging Behavior Patterns Suggest a Possible Cost Associated with Participation in Mixed-Species Bird Flocks". Oikos 51 (1): 79–83. doi:10.2307/3565809. JSTOR 3565809.
- Au, David W. K.; Pitman (1 August 1986). "Seabird interactions with Dolphins and Tuna in the Eastern Tropical Pacific" (PDF). The Condor 88 (3): 304–17. doi:10.2307/1368877.
- Anne, O.; Rasa, E. (June 1983). "Dwarf mongoose and hornbill mutualism in the Taru desert, Kenya". Behavioral Ecology and Sociobiology 12 (3): 181–90. doi:10.1007/BF00290770.
- Gauthier-Clerc, Michael; Tamisier, Alain; Cézilly, Frank (2000). "Sleep-Vigilance Trade-off in Gadwall during the Winter Period" (PDF). The Condor 102 (2): 307–13. doi:10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2. JSTOR 1369642.
- Bäckman, Johan; A (1 April 2002). "Harmonic oscillatory orientation relative to the wind in nocturnal roosting flights of the swift Apus apus". The Journal of Experimental Biology 205 (7): 905–910. PMID 11916987.
- Rattenborg, Niels C. (2006). "Do birds sleep in flight?". Die Naturwissenschaften 93 (9): 413–25. doi:10.1007/s00114-006-0120-3. PMID 16688436.
- Milius, S. (6 February 1999). "Half-asleep birds choose which half dozes". Science News Online 155 (6): 86. doi:10.2307/4011301. JSTOR 4011301.
- Beauchamp, Guy (1999). "The evolution of communal roosting in birds: origin and secondary losses". Behavioural Ecology 10 (6): 675–87. doi:10.1093/beheco/10.6.675.
- Buttemer, William A. (1985). "Energy relations of winter roost-site utilization by American goldfinches (Carduelis tristis)" (PDF). Oecologia 68 (1): 126–32. doi:10.1007/BF00379484.
- Buckley, F. G.; Buckley (1 January 1968). "Upside-down Resting by Young Green-Rumped Parrotlets (Forpus passerinus)". The Condor 70 (1): 89. doi:10.2307/1366517.
- Carpenter, F. Lynn (1974). "Torpor in an Andean Hummingbird: Its Ecological Significance". Science 183 (4124): 545–47. doi:10.1126/science.183.4124.545. PMID 17773043.
- McKechnie, Andrew E.; Ashdown, Robert A. M.; Christian, Murray B.; Brigham, R. Mark (2007). "Torpor in an African caprimulgid, the freckled nightjar Caprimulgus tristigma". Journal of Avian Biology 38 (3): 261–66. doi:10.1111/j.2007.0908-8857.04116.x.
- Frith, C.B (1981). "Displays of Count Raggi's Bird-of-Paradise Paradisaea raggiana and congeneric species". Emu 81 (4): 193–201. doi:10.1071/MU9810193.
- Freed, Leonard A. (1987). "The Long-Term Pair Bond of Tropical House Wrens: Advantage or Constraint?". The American Naturalist 130 (4): 507–25. doi:10.1086/284728.
- Gowaty, Patricia A. (1983). "Male Parental Care and Apparent Monogamy among Eastern Bluebirds (Sialia sialis)". The American Naturalist 121 (2): 149–60. doi:10.1086/284047.
- Westneat, David F.; Stewart, Ian R.K. (2003). "Extra-pair paternity in birds: Causes, correlates, and conflict". Annual Review of Ecology, Evolution, and Systematics 34: 365–96. doi:10.1146/annurev.ecolsys.34.011802.132439.
- Gowaty, Patricia A.; Buschhaus, Nancy (1998). "Ultimate causation of aggressive and forced copulation in birds: Female resistance, the CODE hypothesis, and social monogamy". American Zoologist 38 (1): 207–25. doi:10.1093/icb/38.1.207.
- Sheldon, B (1994). "Male Phenotype, Fertility, and the Pursuit of Extra-Pair Copulations by Female Birds". Proceedings: Biological Sciences 257 (1348): 25–30. doi:10.1098/rspb.1994.0089.
- Wei, G; Zuo-Hua, Yin; Fu-Min, Lei (2005). "Copulations and mate guarding of the Chinese Egret". Waterbirds 28 (4): 527–30. doi:10.1675/1524-4695(2005)28[527:CAMGOT]2.0.CO;2.
- Short, Lester L. (1993). Birds of the World and their Behavior. New York: Henry Holt and Co. ISBN 0-8050-1952-9.
- Burton, R (1985). Bird Behavior. Alfred A. Knopf, Inc. ISBN 0-394-53957-5.
- Schamel, D; Tracy, Diane M.; Lank, David B.; Westneat, David F. (2004). "Mate guarding, copulation strategies and paternity in the sex-role reversed, socially polyandrous red-necked phalarope Phalaropus lobatus" (PDF). Behaviour Ecology and Sociobiology 57 (2): 110–18. doi:10.1007/s00265-004-0825-2.
- Bagemihl, Bruce. Biological exuberance: Animal homosexuality and natural diversity. New York: St. Martin's, 1999. pp. 479–655. One hundred species are described in detail.
- Kokko, H; Harris, M; Wanless, S (2004). "Competition for breeding sites and site-dependent population regulation in a highly colonial seabird, the common guillemot Uria aalge". Journal of Animal Ecology 73 (2): 367–76. doi:10.1111/j.0021-8790.2004.00813.x.
- Booker, L; Booker, M (1991). "Why Are Cuckoos Host Specific?". Oikos 57 (3): 301–09. doi:10.2307/3565958. JSTOR 3565958.
- Hansell M (2000). Bird Nests and Construction Behaviour. University of Cambridge Press ISBN 0-521-46038-7
- Lafuma, L; Lambrechts, M; Raymond, M (2001). "Aromatic plants in bird nests as a protection against blood-sucking flying insects?". Behavioural Processes 56 (2): 113–20. doi:10.1016/S0376-6357(01)00191-7.
- Warham, J. (1990) The Petrels: Their Ecology and Breeding Systems London: Academic Press ISBN 0-12-735420-4.
- Jones DN, Dekker, René WRJ, Roselaar, Cees S (1995). The Megapodes. Bird Families of the World 3. Oxford University Press: Oxford. ISBN 0-19-854651-3
- Elliot A (1994). "Family Megapodiidae (Megapodes)" in Handbook of the Birds of the World. Volume 2; New World Vultures to Guineafowl (eds del Hoyo J, Elliott A, Sargatal J) Lynx Edicions:Barcelona. ISBN 84-87334-15-6
- Metz VG, Schreiber EA (2002). "Great Frigatebird (Fregata minor)" In The Birds of North America, No 681, (Poole, A. and Gill, F., eds) The Birds of North America Inc: Philadelphia
- Young, Euan. Skua and Penguin. Predator and Prey.. Camebridge University Press, 1994, p. 453.
- Ekman, J (2006). "Family living amongst birds". Journal of Avian Biology 37 (4): 289–98. doi:10.1111/j.2006.0908-8857.03666.x.
- Cockburn A (1996). "Why do so many Australian birds cooperate? Social evolution in the Corvida". In Floyd R, Sheppard A, de Barro P. Frontiers in Population Ecology. Melbourne: CSIRO. pp. 21–42.
- Cockburn, Andrew (2006). "Prevalence of different modes of parental care in birds" (Free full text). Proceedings: Biological Sciences 273 (1592): 1375–83. doi:10.1098/rspb.2005.3458. PMC 1560291. PMID 16777726.
- Gaston AJ (1994). Ancient Murrelet (Synthliboramphus antiquus). In The Birds of North America, No. 132 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington, D.C.: The American Ornithologists' Union.
- Schaefer, HC; Eshiamwata, GW; Munyekenye, FB; Bohning-Gaese, K (2004). "Life-history of two African Sylvia warblers: low annual fecundity and long post-fledging care". Ibis 146 (3): 427–37. doi:10.1111/j.1474-919X.2004.00276.x.
- Alonso, JC; Bautista, LM; Alonso, JA (2004). "Family-based territoriality vs flocking in wintering common cranes Grus grus". Journal of Avian Biology 35 (5): 434–44. doi:10.1111/j.0908-8857.2004.03290.x.
- Davies N (2000). Cuckoos, Cowbirds and other Cheats. T. & A. D. Poyser: London ISBN 0-85661-135-2
- Sorenson, M (1997). "Effects of intra- and interspecific brood parasitism on a precocial host, the canvasback, Aythya valisineria". Behavioral Ecology 8 (2): 153–61. doi:10.1093/beheco/8.2.153.
- Spottiswoode, C. N.; Colebrook-Robjent, J. F.R. (2007). "Egg puncturing by the brood parasitic Greater Honeyguide and potential host counteradaptations". Behavioral Ecology 18 (4): 792. doi:10.1093/beheco/arm025.
- Clout, M; Hay, J (1989). "The importance of birds as browsers, pollinators and seed dispersers in New Zealand forests". New Zealand Journal of Ecology 12: 27–33.
- Gary Stiles, F. (1981). "Geographical Aspects of Bird-Flower Coevolution, with Particular Reference to Central America". Annals of the Missouri Botanical Garden 68 (2): 323–51. doi:10.2307/2398801. JSTOR 2398801.
- Temeles, E; Linhart, Y; Masonjones, M; Masonjones, H (2002). "The Role of Flower Width in Hummingbird Bill Length–Flower Length Relationships". Biotropica 34 (1): 68–80.
- Bond, William J.; Lee, William G.; Craine, Joseph M. (2004). "Plant structural defences against browsing birds: a legacy of New Zealand's extinct moas". Oikos 104 (3): 500–08. doi:10.1111/j.0030-1299.2004.12720.x.
- Wainright, S; Haney, J; Kerr, C; Golovkin, A; Flint, M (1998). "Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska". Marine Ecology 131 (1): 63–71.
- Bosman, A; Hockey, A (1986). "Seabird guano as a determinant of rocky intertidal community structure". Marine Ecology Progress Series 32: 247–57. doi:10.3354/meps032247.
- Bonney, Rick; Rohrbaugh, Jr., Ronald (2004). Handbook of Bird Biology (Second ed.). Princeton, NJ: Princeton University Press. ISBN 0-938027-62-X.
- Dean W, Siegfried R, MacDonald I (1990). "The Fallacy, Fact, and Fate of Guiding Behavior in the Greater Honeyguide". Conservation Biology 4 (1) 99–101. Blackwell-PDF
- Singer, R.; Yom-Tov, Y. (1988). "The Breeding Biology of the House Sparrow Passer domesticus in Israel". Ornis Scandinavica 19 (2): 139–44. doi:10.2307/3676463. JSTOR 3676463.
- Dolbeer, R (1990). "Ornithology and integrated pest management: Red-winged blackbirds Agleaius phoeniceus and corn". Ibis 132 (2): 309–22. doi:10.1111/j.1474-919X.1990.tb01048.x.
- Dolbeer, R; Belant, J; Sillings, J (1993). "Shooting Gulls Reduces Strikes with Aircraft at John F. Kennedy International Airport". Wildlife Society Bulletin 21: 442–50.
- Reed, KD; Meece, JK; Henkel, JS; Shukla, SK (2003). "Birds, Migration and Emerging Zoonoses: West Nile Virus, Lyme Disease, Influenza A and Enteropathogens". Clinical medicine & research 1 (1): 5–12. doi:10.3121/cmr.1.1.5. PMC 1069015. PMID 15931279.
- Brown, Lester (2005). "3: Moving Up the Food Chain Efficiently.". Outgrowing the Earth: The Food Security Challenge in an Age of Falling Water Tables and Rising Temperatures. earthscan. ISBN 978-1844071852.
- Simeone, A; Navarro, X (2002). "Human exploitation of seabirds in coastal southern Chile during the mid-Holocene". Rev. Chil. Hist. Nat 75 (2): 423–31. doi:10.4067/S0716-078X2002000200012.
- Hamilton, S. (2000). "How precise and accurate are data obtained using. an infra-red scope on burrow-nesting sooty shearwaters Puffinus griseus?". Marine Ornithology 28 (1): 1–6.
- Keane, Aidan; Brooke, M.de L.; McGowan, P.J.K. (2005). "Correlates of extinction risk and hunting pressure in gamebirds (Galliformes)". Biological Conservation 126 (2): 216–33. doi:10.1016/j.biocon.2005.05.011.
- "The Guano War of 1865–1866". World History at KMLA. Retrieved 18 December 2007.
- Cooney, R; Jepson, P (2006). "The international wild bird trade: what's wrong with blanket bans?". Oryx 40 (1): 18–23. doi:10.1017/S0030605306000056.
- Manzi, M; Coomes, O. T. (2002). "Cormorant fishing in Southwestern China: a Traditional Fishery under Siege. (Geographical Field Note)". Geographic Review 92 (4): 597–603. doi:10.2307/4140937. JSTOR 4140937.
- Pullis La Rouche, G. (2006). Birding in the United States: a demographic and economic analysis. Waterbirds around the world. Eds. G.C. Boere, C.A. Galbraith and D.A. Stroud. The Stationery Office, Edinburgh, UK. pp. 841–46. JNCC.gov.uk, PDF
- Chamberlain, DE; Vickery, JA; Glue, DE; Robinson, RA; Conway, GJ; Woodburn, RJW; Cannon, AR (2005). "Annual and seasonal trends in the use of garden feeders by birds in winter". Ibis 147 (3): 563–75. doi:10.1111/j.1474-919x.2005.00430.x.
- Routledge, S; Routledge, K (1917). "The Bird Cult of Easter Island". Folklore 28 (4): 337–55. doi:10.1080/0015587X.1917.9719006.
- (Matthew 26:34)(Mark 14:30)(Luke 22:34)
- Lukas, SE; Benedikt, R; Mendelson, JH; Kouri, E; Sholar, M; Amass, L (1992). "Marihuana attenuates the rise in plasma ethanol levels in human subjects". Neuropsychopharmacology 7 (1): 77–81. doi:10.1371/journal.pbio.0040014. PMID 1326277.
- Ingersoll, Ernest (1923). Archive.org, "Birds in legend, fable and folklore". Longmans, Green and co. p. 214
- Hauser, A. J. (1985). "Jonah: In Pursuit of the Dove". Journal of Biblical Literature 104 (1): 21–37. doi:10.2307/3260591. JSTOR 3260591.
- Thankappan Nair, P. (1974). "The Peacock Cult in Asia". Asian Folklore Studies 33 (2): 93–170. doi:10.2307/1177550. JSTOR 1177550.
- Tennyson A, Martinson P (2006). Extinct Birds of New Zealand Te Papa Press, Wellington ISBN 978-0-909010-21-8
- Meighan, C. W. (1966). "Prehistoric Rock Paintings in Baja California". American Antiquity 31 (3): 372–92. doi:10.2307/2694739. JSTOR 2694739.
- Clarke, CP (1908). "A Pedestal of the Platform of the Peacock Throne". The Metropolitan Museum of Art Bulletin 3 (10): 182–83. doi:10.2307/3252550. JSTOR 3252550.
- Boime, Albert (1999). "John James Audubon: a birdwatcher's fanciful flights". Art History 22 (5): 728–55. doi:10.1111/1467-8365.00184.
- Chandler, A (1934). "The Nightingale in Greek and Latin Poetry". The Classical Journal 30 (2): 78–84. JSTOR 3289944.
- Lasky, E. D. (1992). "A Modern Day Albatross: The Valdez and Some of Life's Other Spills". The English Journal 81 (3): 44–46. doi:10.2307/820195. JSTOR 820195.
- Carson, A (1998). "Vulture Investors, Predators of the 90s: An Ethical Examination". Journal of Business Ethics 17 (5): 543–55. doi:10.1023/A:1017974505642.
- Enriquez PL, Mikkola H (1997). "Comparative study of general public owl knowledge in Costa Rica, Central America and Malawi, Africa". pp. 160–66 In: J.R. Duncan, D.H. Johnson, T.H. Nicholls, (Eds). Biology and conservation of owls of the Northern Hemisphere. General Technical Report NC-190, USDA Forest Service, St. Paul, Minnesota. 635 pp.
- Lewis DP (2005). Owlpages.com, Owls in Mythology and Culture. Retrieved on 15 September 2007
- Dupree, N (1974). "An Interpretation of the Role of the Hoopoe in Afghan Folklore and Magic". Folklore 85 (3): 173–93. doi:10.1080/0015587X.1974.9716553. JSTOR 1260073.
- Fuller E (2000). Extinct Birds (2nd ed.). Oxford University Press, Oxford, New York. ISBN 0-19-850837-9
- Steadman D (2006). Extinction and Biogeography in Tropical Pacific Birds, University of Chicago Press. ISBN 978-0-226-77142-7
- "BirdLife International announces more Critically Endangered birds than ever before". Birdlife International. 14 May 2009. Retrieved 15 May 2009.
- Kinver, Mark (13 May 2009). "Birds at risk reach record high". BBC News Online. Retrieved 15 May 2009.
- Norris K, Pain D (eds, 2002). Conserving Bird Biodiversity: General Principles and their Application Cambridge University Press. ISBN 978-0-521-78949-3
- Brothers, NP (1991). "Albatross mortality and associated bait loss in the Japanese longline fishery in the southern ocean". Biological Conservation 55 (3): 255–68. doi:10.1016/0006-3207(91)90031-4.
- Wurster, D; Wurster, C; Strickland, W (1965). "Bird Mortality Following DDT Spray for Dutch Elm Disease". Ecology 46 (4): 488–99. doi:10.2307/1934880.; Wurster, C. F.; Wurster, D. H.; Strickland, W. N. (1965). "Bird Mortality after Spraying for Dutch Elm Disease with DDT". Science 148 (3666): 90–91. doi:10.1126/science.148.3666.90. PMID 14258730.
- Blackburn, T; Cassey, P; Duncan, R; Evans, K; Gaston, K (2004). "Avian Extinction and Mammalian Introductions on Oceanic Islands". Science 305 (5692): 1955–58. doi:10.1126/science.1101617. PMID 15448269.
- Butchart, S; Stattersfield, A; Collar, N (2006). "How many bird extinctions have we prevented?". Oryx 40 (3): 266–79. doi:10.1017/S0030605306000950.
|Find more about Bird at Wikipedia's sister projects|
|Definitions and translations from Wiktionary|
|Media from Commons|
|Learning resources from Wikiversity|
|News stories from Wikinews|
|Quotations from Wikiquote|
|Source texts from Wikisource|
|Textbooks from Wikibooks|
|Species directories Aves from Wikispecies|
|The Wikibook Dichotomous Key has a page on the topic of: Aves|
- Avibase—The World Bird Database
- Birdlife International—Dedicated to bird conservation worldwide; has a database with about 250,000 records on endangered bird species.
- Bird biogeography
- Birds and Science from the National Audubon Society
- Cornell Lab of Ornithology
- Bird at the Encyclopedia of Life
- Essays on bird biology
- International Ornithological Committee
- North American Birds for Kids
- Sora—Searchable online research archive; Archives of the following ornithological journals The Auk, Condor, Journal of Field Ornithology, North American Bird Bander, Studies in Avian Biology, Pacific Coast Avifauna, and the Wilson Bulletin.
- The Internet Bird Collection—A free library of videos of the world's birds
- The Institute for Bird Populations, California
- list of field guides to birds, from the International Field Guides database
- RSPB bird identifier—Interactive identification of all UK birds