|At Stroud Preserve in West Chester, Pennsylvania, US|
|Range of T. bicolor Breeding summer visitor Migration visitor Winter visitor|
The tree swallow (Tachycineta bicolor) is a migratory passerine bird that breeds in North America and winters in Mexico, Central America and the Caribbean. It is a very rare vagrant to western Europe.
Taxonomy and etymology
The tree swallow is usually placed in the genus Tachycineta, although it is sometimes placed in the genus Iridoprocne, along with the mangrove swallow, white-rumped swallow, white-winged swallow, and Chilean swallow.
The generally accepted genus name is from Ancient Greek takhykinetos, "moving quickly", and the specific bicolor is Latin and means "two-coloured". The other genus name, Iridoprocne, comes from the Greek iris, meaning rainbow, and Procne, a figure who supposedly turned into a swallow.
This swallow has a length between about 12 and 14 cm (4.7 and 5.5 in) and a weight of approximately 17 to 25.5 g (0.60 to 0.90 oz). The male has mostly glossy blue-green , with the wings and tail being blackish. The  and the cheek patch are white, although the coverts are grey-brown. The female can be differentiated from the male as the former is duller, and sometimes has a brown forehead. In addition, first-year females have brown upperparts, with a variable number of blue feathers. Second-year females sometimes retain some of this brown feathering. The juveniles can be distinguished be their brown upperparts and grey-brown washed breast.
Distribution and habitat
The tree swallow breeds in North America. Its range extends to north-central Alaska and up to the tree limit in Canada. It is found as far south as Tennessee in the eastern part of its range, California and New Mexico in the west, and Kansas in the centre. It occasionally breeds further south in the United States (US). The wintering range is primarily southern US coasts and south, along the Gulf Coast, to Panama and the Northwestern coast of South America, in addition to being found in the West Indies. When a swallow returns to nest, it usually does not change breeding sites.
The habitat of this swallow is primarily in open and wooded areas, especially those near water. The fact that it is able to habitate open areas is due to the construction of nest boxes in such regions.
Being highly social outside of the breeding season, tree swallows may form flocks of several thousand birds near roost sites. Flocks near Vacherie, Louisiana, were estimated to contain well over 1 million birds during December 2009.
The tree swallow usually renests in the same area to breed again; only about 14% of females and 4% of males disperse to breed at a new site per year. This dispersal, although, is influenced by breeding success; about 28% of breeders disperse after they fail to fledge a chick, compared to the 5% that disperse when they are successful. Most do not disperse far, usually breeding at sites less than 10 kilometres (6.2 mi) away from their original grounds. The nest is found in pre-existing holes, usually in trees and artificial structures such as pipes and fence posts, and in nestboxes. The nest hole is, on average, 3.4 metres (11 ft) above ground level, although about 45% of them are less than 2 metres (6.6 ft) above the ground. Nests are usually spaced 10 to 15 metres (33 to 49 ft) apart, and those that are closer in distance are usually further apart in terms of laying date. The nest cup itself is made from grass, moss, pine needles, and aquatic plants, and lined with feathers, all of which are collected mostly by the female.
Eggs are laid from early May to mid-June and chicks fledge between mid-June and July. When the eggs are laid is influenced by latitude, age of the female, and wing length of the female. On average, birds in the southern part of the breeding range breed earlier than those in the northern portion. The female's wing length and age are both inversely correlated with the timing of breeding. It generally lays when the temperature and the abundance of food (that the female can catch) are good enough for females to start laying eggs. These observations support the theory that the tree swallow is an income breeder (breeding based on food conditions during the laying season). This species is generally socially monogamous, but up to 8% of breeding males are polygynous. Polygyny is influenced by territory: males having territories with nestboxes at least 5 metres (16 ft) apart are more likely to be polygynous. It is suggested that this polygyny depends on the conditions during the laying season: better conditions, such as an abundance of food, allow females in polygyny who do not receive help in foraging to lay more eggs.
The tree swallow has high rates of extra-pair paternity, with 38% to 69% of nestlings being a product of extra-pair paternity, and 50% to 87% of broods containing at least one nestling that was the result of an extra-pair copulation. One factor that might contribute to this is that females have control over copulation, making paternity guards ineffective. Extra-pair paternity does not change the level of parental care the male contributes in the tree swallow, contrary to other birds. The tree swallow also differs in terms of the composition of extra-pair fathers; in a study that found the paternity of 35 extra-pair nestlings, it was determined that 25 extra-pair young were from fathers from sites near the nest where the female is, about 3 from sites within 2 kilometres (1.2 mi), and 7 that had fathers that were floaters (those present at breeding grounds that presumably do not breed). In the tree swallow, floating thus helps males in good condition produce more chicks, while allowing males in bad condition to be successful through parental care.
How extra-pair fathers are chosen and why females even breed with other males (because they can control copulation) is controversial. One theory, called the genetic compatibility hypothesis, states that increased offspring fitness results from increased heterozygosity, and thus that female tree swallows would prefer to mate with those that have more different alleles from them. This theory is justified on the basis that young produced from extra-pair mates usually are more heterozygous than within-pair offspring. In additional support of this theory, females are likely able to judge, after copulation, whether or not a male's sperm is of high quality and whether she should allow it to impregnate her. Another theory, called the good genes theory, says that females choose extra-pair males based on if they have good genes that would influence survival and mating success. This is supported on the basis that it is able to explain why some tree swallows do not have any extra-pair young, whereas others do. Although this is true, there is criticism for a lack of phenotypic difference between extra-pair males and pair-bonded males. But, there are results that may be in support of this theory. For example, in a 2007 study, it was found that in increased plumage brightness and increased age in extra-pair males, together, increased the number of extra-pair young.
Courtship for the tree swallow starts with a male attacking an unknown female; this can be stimulated by the female doing a wing-fluttering flight, which is possibly an invitation to court. The male may then take a vertical posture, with a raised tail, slightly spread, and with wings slightly drooped. This stimulates the female to try and land on the male's back, but he flies to prevent this; this is repeated. After the male courts the female, he flies to his chosen nest site, and the female inspects it. The pair bond takes time to develop. During copulation, the male hovers over the female, and then mounts her, giving aggression calls. He then makes with the female while holding her neck feathers in his bill and standing on her slightly outstretched wings. Copulation occurs multiple times.
The tree swallow lays a clutch of two to eight, although usually four to seven, pure white, and translucent at laying, eggs that measure about 19 by 14 centimetres (7.5 by 5.5 in). These eggs are incubated by the female, usually after the second-to-last egg is laid, for 11 to 20 days, although most hatch after 14 to 15 days. They hatch slightly asynchronously, with an average of 28 hours between the time of the first and final egg is laid. The laying order predicts the hatching order, with eggs generally hatching in the order they were laid. When a brood hatches asynchronously, a weight hierarchy is established, with nestlings hatched earlier weighing more than those hatched later. This allows for the female to prioritize which chick to give food to in times of food shortage, although this weight difference is less pronounced about 12 days into nesting, suggesting that this brood-reduction only has a significant effect early in the nestling period. Infanticide of the chicks and eggs sometimes occurs when a male is replaced by another male. Infanticide usually does not occur when the clutch is not complete, as replacement males then have a chance to fertilize at least one egg. When the male arrives during incubation, it sometimes commits infanticide, but other times adopts the eggs, as there is a chance that some eggs were sired from the replacement male. If the replacement male arrives after the chicks hatch, however, infanticide is usually committed, though the female will sometimes prevent this.
The sex ratio of the hatchlings is male biased in females of better condition, and these males produced by the females in better condition are themselves in better condition. This is hypothesized to be because males have more variability in reproductive success, thus meaning a female in better condition will produce a male in better condition that may have better reproductive success (more than that of a female of similar condition).
Nestling tree swallows are able to thermoregulate at a capacity of 75% compared to the adult at an average age of 9.5 days when out of the nest, and from nine to four days when in the nest (depending on the size of the brood).
The tree swallow forages 0 to 50 metres (160 ft) above the ground singly or in groups. Its flight is a mix of flapping and gliding. During the breeding season, this is mostly within 4 kilometres (2.5 mi) of the nest site. When it is foraging for nestlings, although, it usually goes up to 200 metres (660 ft) from the nest, mostly staying in sight of it, and forages at a height up to 12 metres (39 ft). In addition to being caught in flight, insects are sometimes taken from the ground, water, vegetation, and vertical surfaces.
The tree swallow eats mostly insects, with some molluscs, spiders, and fruit. In North America, flies make up about 40% of the diet, with beetles and ants supplementing it. Otherwise, the diet is about 90% flies. The seed and berry food is mainly from the genus Myrica, which is mainly taken in the all four of the Northern Hemisphere seasons except summer. Crustaceans were also found to be important in the wintering diet in a study on Long Island, New York.
Both sexes feed the nestlings, although the male feeds the chicks less than the females. The hatching order affects how much a chick is fed; last-hatched nestlings (in cases where hatching is asynchronous) are likely fed less than those hatched earlier. Nestlings closer to the entrance of the nest are also more likely to be fed, as are those who started begging first. The diet itself is composed mostly of insects, with insects in the orders Diptera, Hemiptera, and Odonata making up most of the diet. These insects are mostly up to 10 millimetres (0.39 in) in size, but sometimes are up to 60 millimetres (2.4 in) in length. In nests near lakes acidified by humans, calcium supplements, primarily fish bones, crayfish exoskeletons, clam shells, and the shells of bird eggs, are more important in the diet of nestlings. To get these calcium supplements, the adult tree swallow travels further than usual; sometimes up to 650 metres (2,130 ft) away from the nest.
The tree swallow is vulnerable to various parasites, such as the blood parasite Trypanosoma. It is also susceptible to the flea Ceratophyllus idius and the feather mites Pteronyssoides tyrrelli, Trouessartia, and (likely) Hemialges. It is also probably afflicted by lice of the genera Brueelia and Myrsidea. There is a correlation between the number of fleas on a bird and the number of young it is caring for. This relationship is speculated to be a cause of an improved microclimate for fleas due to a larger clutch.
The tree swallow is considered to be least concern by the International Union for Conservation of Nature. This is due to the bird's large range of about 834,000 square kilometres (322,000 sq mi), and its stable population, estimated to be about 20,000,000 individuals. In some parts of the United States, the range of this swallow has extended South, likely due to changes in land use, the reintroduction of beavers, and nestboxes installed for bluebirds. Tree swallows are negatively impacted by the clearing of forests, and there is a documented impact of the reduction of marshes, which reduce the habitat available for wintering. The tree swallow also has to compete for nest sites with the common starling and house sparrow, both introduced to North America. Acid rain that acidifies lakes can force it to go relatively long distances to find calcium-rich items, and it can result in it eating some plastic. Other chemicals, like pesticides and other pollutants, can become highly concentrated in eggs, and PCBs are associated with the abandonment of a pair's clutch.
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|Wikimedia Commons has media related to Tree swallow.|
|Wikispecies has information related to Tachycineta bicolor|
- Tree swallow - Tachycineta bicolor - USGS Patuxent Bird Identification InfoCenter
- Tree swallow species account – Cornell Lab of Ornithology
- How to create and manage nest box projects for tree swallows
- "Tree swallow media". Internet Bird Collection.
- Tree swallow photo gallery at VIREO (Drexel University)
- Tree swallow, borealforest.org