Cranes are a family, the Gruidae, of large, long-legged, and long-necked birds in the group Gruiformes. The 15 species of cranes are placed in four genera. Unlike the similar-looking but unrelated herons, cranes fly with necks outstretched, not pulled back. Cranes live on all continents except Antarctica and South America.
They are opportunistic feeders that change their diets according to the season and their own nutrient requirements. They eat a range of items from suitably sized small rodents, fish, amphibians, and insects to grain and berries.
Cranes construct platform nests in shallow water, and typically lay two eggs at a time. Both parents help to rear the young, which remain with them until the next breeding season.
Some species and populations of cranes migrate over long distances; others do not migrate at all. Cranes are solitary during the breeding season, occurring in pairs, but during the nonbreeding season, they are gregarious, forming large flocks where their numbers are sufficient.
Most species of cranes have been affected by human activities and are at the least classified as threatened, if not critically endangered. The plight of the whooping cranes of North America inspired some of the first US legislation to protect endangered species.
Cranes are very large birds, often considered the world's tallest flying birds. They range in size from the demoiselle crane, which measures 90 cm (35 in) in length, to the sarus crane, which can be up to 176 cm (69 in), although the heaviest is the red-crowned crane, which can weigh 12 kg (26 lb) prior to migrating. They are long-legged and long-necked birds with streamlined bodies and large, rounded wings. The males and females do not vary in external appearance, but males tend to be slightly larger than females.
The plumage of cranes varies by habitat. Species inhabiting vast, open wetlands tend to have more white in their plumage than do species that inhabit smaller wetlands or forested habitats, which tend to be more grey. These white species are also generally larger. The smaller size and colour of the forest species is thought to help them maintain a less conspicuous profile while nesting; two of these species (the common and sandhill cranes) also daub their feathers with mud to further hide while nesting.
Most species of cranes have some areas of bare skin on their faces; the only two exceptions are the blue and demoiselle cranes. This skin is used in communication with other cranes, and can be expanded by contracting and relaxing muscles, and change the intensity of colour. Feathers on the head can be moved and erected in the blue, wattled, and demoiselle cranes for signaling, as well.
Also important to communication is the position and length of the trachea. In the two crowned cranes, the trachea is shorter and only slightly impressed upon the bone of the sternum, whereas the trachea of the other species is longer and penetrates the sternum. In some species, the entire sternum is fused to the bony plates of the trachea, and this helps amplify the crane's calls, allowing them to carry for several kilometres.
Distribution and habitat
The cranes have a cosmopolitan distribution, occurring across most of the world continents. They are absent from Antarctica and, mysteriously, South America. East Asia is the centre of crane diversity, with eight species, followed by Africa, which holds five resident species and wintering populations of a sixth. Australia, Europe. and North America have two regularly occurring species each. Of the four crane genera, Balearica (two species) is restricted to Africa, and Leucogeranus (one species) is restricted to Asia; the other two genera, Grus (including Anthropoides and Bugeranus) and Antigone, are both widespread.
Most species of cranes are dependent on wetlands and require large areas of open space. Most species nest in shallow wetlands. Some species nest in wetlands, but move their chicks up onto grasslands to feed (while returning to wetlands at night), whereas others remain in wetlands for the entirety of the breeding season. Even the demoiselle crane and blue crane, which may nest and feed in grasslands (or even arid grasslands or deserts), require wetlands for roosting at night. The only two species that do not always roost in wetlands are the two African crowned cranes (Balearica), which are the only cranes to roost in trees.
Some crane species are sedentary, remaining in the same area throughout the year, while others are highly migratory, travelling thousands of kilometres each year from their breeding sites. A few species have both migratory and sedentary populations.
Behaviour and ecology
The cranes are diurnal birds that vary in their sociality by season. During the breeding season, they are territorial and usually remain on their territory all the time. In contrast in the non-breeding season, they tend to be gregarious, forming large flocks to roost, socialize, and in some species feed. Species that feed predominately on vegetable matter in the non-breeding season feed in flocks to do so, whereas those that feed on animals usually feed in family groups, joining flocks only during resting periods, or in preparation for travel during migration. Large aggregations of cranes are important for safety when resting and also as places for young unmated birds to meet others.
Calls and communication
Cranes are highly vocal and have a large vocabulary of specialized calls. The vocabulary begins soon after hatching with low, purring calls for maintaining contact with their parents, as well as food-begging calls. Other calls used as chicks include alarm calls and "flight intention" calls, both of which are maintained into adulthood. The cranes' duet calls are most impressive. They can be used for individual recognition.
The cranes as a family consume a wide range of food, both animal and plant matter. When feeding on land, they consume seeds, leaves, nuts and acorns, berries, fruit, insects, worms, snails, small reptiles, mammals, and birds. In wetlands, roots, rhizomes, tubers, and other parts of emergent plants, other molluscs, small fish and amphibians are also consumed, as well. The exact composition of the diet varies by location, season, and availability. Within the wide range of items consumed, some patterns emerge; the shorter-billed species usually feed in drier uplands, while the longer-billed species feed in wetlands.
Cranes employ different foraging techniques for different food types. Tubers and rhizomes are dug for and a crane digging for them remains in place for some time digging and then expanding a hole to find them. In contrast both to this and the stationary wait and watch hunting methods employed by many herons, they forage for insects and animal prey by slowly moving forwards with their heads lowered and probing with their bills.
Where more than one species of cranes exists in a locality, each species adopts separate niches to minimise competition and niche overlap. At one important lake in Jiangxi Province in China, the Siberian cranes feed on the mudflats and in shallow water, the white-naped cranes on the wetland borders, the hooded cranes on sedge meadows, and the last two species also feed on the agricultural fields along with the common cranes.
Cranes are perennially monogamous breeders, establishing long-term pair bonds that may last the lifetime of the birds. Pair bonds begin to form in the second or third years of life, but several years pass before the first successful breeding season. Initial breeding attempts often fail, and in many cases, newer pair bonds dissolve (divorce) after unsuccessful breeding attempts. Pairs that are repeatedly successful at breeding remain together for as long as they continue to do so. In a study of sandhill cranes in Florida, seven of the 22 pairs studied remained together for an 11-year period. Of the pairs that separated, 53% was due to the death of one of the pair, 18% was due to divorce, and the fate of 29% of pairs was unknown. Similar results had been found by acoustic monitoring (sonography/frequency analysis of duet and guard calls) in three breeding areas of common cranes in Germany over 10 years.
Cranes are territorial and generally seasonal breeders. Seasonality varies both between and within species, depending on local conditions. Migratory species begin breeding upon reaching their summer breeding grounds, between April and June. The breeding season of tropical species, however, is usually timed to coincide with the wet or monsoon seasons. Territory sizes also vary depending on location. Tropical species can maintain very small territories, for example sarus cranes in India can breed on territories as small as one hectare where the area is of sufficient quality and disturbance by humans is minimised. In contrast, red-crowned crane territories may require 500 hectares, and pairs may defend even larger territories than that, up to several thousand hectares. Territory defence is usually performed by the male. Because of this, females are much less likely to retain the territory than males in the event of the death of a partner.
Taxonomy and systematics
The 15 living species of cranes are placed in four genera. A molecular phylogenetic study published in 2010 found that the genus Grus, as then defined, was polyphyletic. In the resulting rearrangement to create monophyletic genera, the Siberian crane was moved to the resurrected monotypic genus Leucogeranus, while the sandhill crane, the white-naped crane, the sarus crane, and the brolga were moved to the resurrected genus Antigone. Some authorities recognize the additional genera Anthropoides (for the demoiselle crane and blue crane) and Bugeranus (for the wattled crane) on morphological grounds.
The fossil record of cranes leaves much to be desired. Apparently, the subfamilies were well distinct by the Late Eocene (around 35 mya). The present genera are apparently some 20 mya old. Biogeography of known fossil and the living taxa of cranes suggests that the group is probably of (Laurasian?) Old World origin. The extant diversity at the genus level is centered on (eastern) Africa, making it all the more regrettable that no decent fossil record exists from there. On the other hand, it is peculiar that numerous fossils of Ciconiiformes are documented from there; these birds presumably shared much of their habitat with cranes back then already. Cranes are sister taxa to Eogruidae, a lineage of flightless birds; as predicted by the fossil record of true cranes, eogruids were native to the Old World. A species of true crane, Grus cubensis, has similarly become flightless and ratite-like.
Fossil genera are tentatively assigned to the present-day subfamilies:
- Palaeogrus (Middle Eocene of Germany and Italy – Middle Miocene of France)
- Pliogrus (Early Pliocene of Eppelsheim, Germany)
- Camusia (Late Miocene of Menorca, Mediterranean)
- "Grus" conferta (Late Miocene/Early Pliocene of Contra Costa County, US)
Sometimes considered Balearicinae
Sometimes considered Gruidae incertae sedis
- Eobalearica (Ferghana Late? Eocene of Ferghana, Uzbekistan)
- Probalearica (Late Oligocene? – Middle Pliocene of Florida, US, France?, Moldavia and Mongolia) – A nomen dubium?
- Aramornis (Sheep Creek Middle Miocene of Snake Creek Quarries, US)
In mythology and symbolism
The cranes' beauty and spectacular mating dances have made them highly symbolic birds in many cultures with records dating back to ancient times. Crane mythology is widely spread and can be found in areas such as India, the Aegean, South Arabia, China, Korea, Japan, and Native American cultures of North America. In northern Hokkaidō, the women of the Ainu people performed a crane dance that was captured in 1908 in a photograph by Arnold Genthe. In Korea, a crane dance has been performed in the courtyard of the Tongdosa Temple since the Silla Dynasty (646 CE).
In Mecca, in pre-Islamic South Arabia, Allāt, Uzza, and Manāt were believed to be the three chief goddesses of Mecca, they were called the "three exalted cranes" (gharaniq, an obscure word on which 'crane' is the usual gloss). See The Satanic Verses for the best-known story regarding these three goddesses.
In China, several styles of kung fu take inspiration from the movements of cranes in the wild, the most famous of these styles being Wing Chun, Hung Gar (tiger crane), and the Shaolin Five Animals style of fighting. Crane movements are well known for their fluidity and grace.
The Greek for crane is Γερανος (geranos), which gives us the cranesbill, or hardy geranium. The crane was a bird of omen. In the tale of Ibycus and the cranes, a thief attacked Ibycus (a poet of the sixth century BCE) and left him for dead. Ibycus called to a flock of passing cranes, which followed the attacker to a theater and hovered over him until, stricken with guilt, he confessed to the crime.
Pliny the Elder wrote that cranes would appoint one of their number to stand guard while they slept. The sentry would hold a stone in its claw, so that if it fell asleep, it would drop the stone and waken. A crane holding a stone in its claw is a well-known symbol in heraldry, and is known as a crane in its vigilance.
Aristotle describes the migration of cranes in the History of Animals, adding an account of their fights with Pygmies as they wintered near the source of the Nile. He describes as untruthful an account that the crane carries a touchstone inside it that can be used to test for gold when vomited up. (This second story is not altogether implausible, as cranes might ingest appropriate gizzard stones in one locality and regurgitate them in a region where such stone is otherwise scarce.)
Throughout Asia, the crane is a symbol of happiness and eternal youth. In Japan, the crane is one of the mystical or holy creatures (others include the dragon and the tortoise) and symbolizes good fortune and longevity because of its fabled life span of a thousand years. The crane is a favourite subject of the tradition of origami or paper folding. An ancient Japanese legend promises that anyone who folds a thousand origami cranes will be granted a wish by a crane. After World War II, the crane came to symbolize peace and the innocent victims of war through the story of schoolgirl Sadako Sasaki and her thousand origami cranes. Suffering from leukemia as a result of the atomic bombing of Hiroshima and knowing she was dying, she undertook to make a thousand origami cranes before her death at the age of 12. After her death, she became internationally recognised as a symbol of the innocent victims of war and remains a heroine to many Japanese girls.
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- Hayes, M.A. (2005): Divorce and extra-pair paternity as alternative mating strategies in monogamous sandhill cranes. MS thesis, University of South Dakota, Vermilion, S.D.. 86 p. PDF fulltext at the International Crane Foundation's Library
|Wikimedia Commons has media related to:|
- Saving Cranes website (ICF)
- Craneworld website, mainly in German
- individual recognition of cranes using frequency analysis of their calls
- Gruidae videos on the Internet Bird Collection
- Crane sounds on xeno-canto.org
- Cranes of the World, by Paul Johnsgard