Tree swallow

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Tree swallow
Tree swallow at Stroud Preserve.jpg
At Stroud Preserve, near West Chester, Pennsylvania, US
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Passeriformes
Family: Hirundinidae
Genus: Tachycineta
T. bicolor
Binomial name
Tachycineta bicolor
(Vieillot, 1808)
Tree Swallow-rangemap.gif
Range of T. bicolor      Breeding summer visitor     Migration visitor     Winter visitor

Hirundo bicolor Vieillot, 1808
Iridoprocne bicolor Coues, 1878

The tree swallow (Tachycineta bicolor) is a migratory bird of the family Hirundinidae. Found in the Americas, the tree swallow was first described in 1807 by French ornithologist Louis Vieillot as Hirundo bicolor. It has since been moved to its current genus, Tachycineta, where its phylogenetic placement is in debate. The tree swallow has glossy blue-green upperparts, with the exception of the blackish wings and tail, and white underparts. The bill is black, the eyes are dark brown, and the legs and feet are pale brown. The female is generally duller than the male, and the first-year female has mostly brown upperparts, with some blue feathers. Juveniles have brown upperparts, and a grey-brown-washed breast. The tree swallow breeds in the US and Canada. It winters along southern US coasts south, along the Gulf Coast, to Panama and the northwestern coast of South America, and in the West Indies.

The tree swallow nests either in isolated pairs or loose groups, in both natural and artificial cavities. Breeding can start as soon as early May, although this date is advancing because of climate change, and it can end as late as July. This bird is generally socially monogamous (although about 8% of males are polygynous), with high levels of extra-pair paternity. This can benefit the male, but since the female controls copulation, the lack of resolution on how this behaviour benefits females makes the high level of extra-pair paternity puzzling. The female incubates the clutch of two to eight (but usually four to seven) pure white eggs, usually for 14 to 15 days. The chicks hatch slightly asynchronously, allowing for the female to prioritize which chicks to feed in times of food shortage. The chicks generally fledge about 18 to 22 days after hatching. The tree swallow is sometimes considered a model organism, due to the large amount of research done on it.

An aerial insectivore, the tree swallow forages both alone and in groups, eating mostly insects. Molluscs, spiders, and fruit are also found in the diet. The nestlings, like the adult, primarily eat insects, fed to it by both the male and the female. This swallow is vulnerable to parasites, but, when on nestlings, these do little damage. The effect of disease can become stronger as a tree swallow gets older, as some parts of the immune system decline with age. Acquired T cell-mediated immunity, for example, decreases with age, whereas both innate and acquired humoral immunity do not. Because of its large range and stable population, the tree swallow is considered to be least concern by the International Union for Conservation of Nature. In the US, it is protected by the Migratory Bird Treaty Act of 1918, and in Canada by the Migratory Birds Convention Act. This swallow is negatively affected by human activities, such as the clearing of forests; acidified lakes can force a breeding tree swallow to go long distances to find calcium-rich food items to feed to its chicks.

Taxonomy and etymology[edit]

The tree swallow was described as Hirundo bicolor by Louis Pierre Vieillot in his Histoire naturelle des oiseaux de l'Amérique Septentrionale, published in 1807.[2] The genus it is currently placed in, Tachycineta, was established by Jean Cabanis in 1850.[3] This swallow is sometimes placed in the genus Iridoprocne, along with the mangrove swallow, white-rumped swallow, white-winged swallow, and Chilean swallow.[4] The placement was first made by Elliott Coues,[5] on the basis of its plumage.[6] The tree swallow is also called the white-bellied swallow, for its white underparts.[7]

The generally accepted genus name is from Ancient Greektakhykinetos, "moving quickly", and the specific bicolor is Latin and means "two-coloured".[8] The other genus name, Iridoprocne, comes from the Greekiris, meaning rainbow, and Procne, a figure who supposedly turned into a swallow.[5]

The position of the tree swallow in relation to other members of Tachycineta is unresolved. In studies based on mitochondrial DNA, this bird was given basal placement (meaning it was the first offshoot in the species tree) within the North American-Caribbean clade, i.e. it was basal to the violet-green swallow, which itself was basal to the golden and Bahama swallow. Although mitochondrial DNA is advocated to be a better indicator of evolutionary changes because of its short coalescence time (the time ago that two random gene copies had a common ancestor),[9] analyses based on it can suffer because mitochondrial DNA is only inherited from the mother, thus likely making it worse than nuclear DNA from multiple loci at representing the phylogeny of a whole group. A study based on such nuclear DNA, contrasting with mitochondrial DNA studies, placed the tree swallow in the most basal position within Tachycineta as a whole (as a sister group to the rest of the genus).[10]


A juvenile tree swallow

The tree 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 upperparts, the wings and tail being blackish. The underparts[11] and the cheek patch are white,[7] although the underwing coverts are grey-brown.[11] The bill is black, eyes are dark brown, and the legs and feet are pale brown.[7] The female can be differentiated from the male as the former is duller, and sometimes has a brown forehead. The first-year female also has brown upperparts, with a variable number of blue feathers.[11] This brown feathering likely has the function of allowing exploration of nest sites, as the male is usually less aggressive to a first-year female.[12] The second-year female sometimes retains some of this brown feathering. The juveniles can be distinguished by their brown upperparts and grey-brown-washed breast.[11]


Dickcissel male perched on a metal pole singing, with neck stretched and beak open.

Songs & calls

Song of the tree swallow:
Listen to tree swallow on xeno-canto

The tree swallow's song is three long and descending notes ending with a liquid warble. This song is repeated.[7] Its alarm call is a "peeh" or "pee-deeh";[11] this call can serve to silence older nestlings that are begging while a predator is near.[13]

Distribution and habitat[edit]

The tree swallow breeds in North America. Its range extends to north-central Alaska and up to the tree line 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 US.[7] The wintering range is primarily southern US coasts and south, along the Gulf Coast, to Panama and the northwestern coast of South America. It also is found wintering in the West Indies. Vagrants are sometimes found in the Arctic Circle, the northern Pacific, Greenland, and Europe.[11] When a swallow returns to nest, it usually does not change breeding sites.[14]

The habitat of this swallow is primarily in open and wooded areas, especially those near water. The fact that it is able to live in open areas is due to the construction of nest boxes in such regions.[7]


Tree swallows fighting

Because of the numerous studies on various aspects of this bird's biology, it has been recommended that the tree swallow be considered a model organism.[12] Although it is aggressive during the breeding season, the tree swallow is sociable outside of it, forming flocks sometimes numbering thousands of birds.[7]


A natural nest hole

The tree swallow usually nests 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, though, is influenced by breeding success; of the breeders that fail to fledge a chick, about 28% disperse, 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.[14] It nests both in loose groups and isolated pairs. When nesting in loose groups, nests are usually spaced 10 to 15 metres (33 to 49 ft) apart,[11] and those that are closer in distance are usually further apart in terms of laying date.[15] In natural cavities, the tree swallow nests about 27 metres (89 ft) apart from its neighbor.[16] After a nest is found in pre-existing holes, usually in trees and artificial structures such as pipes and fence posts, and in nestboxes,[11] the male perches near it and calls frequently. A lack of nest sites can cause fights between birds, sometimes resulting in deaths. This bird usually defends an area around the nest with a radius of about 4.6 metres (15 ft),[7] as well as extra nests inside of that territory.[11] These territories are defended by blocking the entrance to the nest and by chasing intruders.[7] 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.[16] The nest cup itself is made from grass, moss, pine needles, and aquatic plants, which are collected mostly by the female,[11] and lined with feathers, which are mostly collected by the male in fights.[17] The feathers may serve the function of insulating the nest, helping to decrease incubation time[18] and likely preventing hypothermia in chicks. In addition to an increased mass at fledging for chicks,[17] eggs cool slower in nests with feathers than those without.[19] However, a study published in 2018 did not find a significant correlation between the number of feathers in nests that were artificially warmed versus those that were not. Additionally, it found that nests in St. Denis, Saskatchewan, used significantly less feathers than those in Annapolis Valley, despite the former being further north.[20]

A pair mating

Courtship starts with a male tree swallow 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 cloacal contact with the female while holding her neck feathers in his bill and standing on her slightly outstretched wings. Copulation occurs multiple times.[7]

Eggs are laid from early May to mid-June and chicks fledge between mid-June and July,[11] although the laying date is getting later because of climate change.[21] When the eggs are laid is influenced by latitude,[11] age of the female, and wing length of the female, which allows it to forage more efficiently.[22] On average, birds in the southern part of the breeding range breed earlier than those in the northern portion.[11] The female's wing length and age are both inversely correlated with the timing of breeding.[22] 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).[23] This species is generally socially monogamous, but up to 8% of breeding males are polygynous.[11] 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.[24]

The inside of a tree swallow nest

The tree swallow has high rates of extra-pair paternity, 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.[11] One factor that might contribute to this is that females have control over copulation, making paternity guards ineffective.[25] Although this is true, a 2009 study found correlational evidence between the copulation frequency and how far away laying was in terms of time, and that this likely influenced the result of brood paternity, there being a correlation between within-pair copulation frequency and the proportion of young the within-pair male sired. But, a 1993 and 1994 study did not find this latter correlation when it was studied.[26] 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 three from sites within 2 kilometres (1.2 mi), and seven 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.[27]

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 males 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. Further supporting 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.[28] 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.[25] 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 increased plumage brightness and increased age in extra-pair males, together, increased the number of extra-pair young.[29] Another theory suggests that extra-pair paternity is context dependent, meaning that extra-pair young outperform within-pair young in certain situations, whereas within-pair young do better than extra-pair young in other environments. For example, extra-pair young have less developmental plasticity compared to within-pair young, meaning that within-pair young outperform extra-pair young when both were born in relatively unchallenging environments.[30]

Studies attempting to prove the adaptability of extra-pair paternity for females have been criticized for the lack of positive effect that increased offspring fitness would have when compared with the potential cost of decreased fitness for the female.[31] Thus, theories based on the non-adaptivity of extra-pair paternity for females have been postulated. These theories are based on genetic constraint, where pleiotropic effects (when an allele affects multiple phenotypes) produce benefits to behaviours that are otherwise maladaptive. The theory of intersexual antagonistic pleiotropy says that strong selection for extra-pair paternity in males (as seen in this bird[31]) overrides the weak selection against extra-pair paternity in females. The hypothesis of intrasexual antagonistic pleiotropy, meanwhile, argues that extra-pair paternity is present because the genes regulating it have pleiotropic effects on aspects of female fitness,[32] like within-pair copulation rate.[31]

A tree swallow egg

The tree swallow lays a clutch of two to eight, although usually four to seven,[11] pure white, and translucent at laying, eggs that measure about 19 by 14 millimetres (0.75 by 0.55 in).[33] These eggs are incubated by the female,[11] usually after the second-to-last egg is laid,[34] for 11 to 20 days,[33] although most hatch after 14 to 15 days. About 88% of nests produce at least one nestling, although this is lowered by poor weather and a younger breeding female.[11] They hatch slightly asynchronously, with an average of 28 hours between the time the first and final egg is laid. The laying order predicts the hatching order, the eggs generally hatching in the order they were laid. When a brood hatches asynchronously, a weight hierarchy is established, the 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.[34] 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, infanticide is usually committed, though the female will sometimes prevent this.[35]

Nests produced by females of better condition often have sex ratios skewed towards high quality males. This is hypothesized to be because males have more variability in reproductive success, thus meaning a female in better condition can produce a male in good condition that may have better reproductive success (more than that of a female of similar condition).[36]

The growth of nestling tree swallows is influenced by their environment. The growth of younger nestlings—those two to four days old—is positively influenced by a higher maximum temperature and an older age, and negatively influenced by a later hatching date. In older nestlings—those from nine to eleven days of age—growth is positively influenced, albeit to a lesser extent, by a higher maximum temperature and more abundant insects, and negatively influenced, again to less effect, by a later hatching date and an older age.[37] In terms of thermoregulation, 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).[38] These nestlings generally fledge after 18 to 22 days, with about 80% fledging success. Like hatching success, this value is reduced by unfavourable weather and a younger female.[11] Chicks may be preyed on by snakes[39] and raccoons.[40] This predation can be exacerbated by begging calls.[41] For their first year, the tree swallow has an average annual mortality rate of 79%, and a rate of 40% to 60% afterwards. The maximum lifespan recorded for this bird is 12 years.[11]


Flying in Central New York, US

The tree swallow forages up 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, though, it usually goes up to 200 metres (660 ft) from the nest,[11] mostly staying in sight of it, and forages at a height up to 12 metres (39 ft).[42] As well as being caught in flight,[7] insects are sometimes taken from the ground, water, vegetation, and vertical surfaces.[11]

The tree swallow eats mostly insects, with some molluscs, spiders, and fruit. In North America, flies make up about 40% of the diet, supplemented with beetles and ants. Otherwise, the diet is about 90% flies.[11] The insects taken are a mix of aquatic and terrestrial organisms; the former is an important source of omega-3 highly unsaturated fatty acids. This is because, although the tree swallow can convert the precursor α-Linolenic acid into high unsaturated fatty acids like docosahexaenoic acid, it cannot do so in the quantities needed.[43] The seed and berry food is mainly from the genus Myrica, which is mainly taken in 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.[11]

A chick being fed

Both sexes feed the nestlings, although the male feeds the chicks less than the females. There are about 10 to 20 feedings per hour.[11] 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.[44] Nestlings closer to the entrance of the nest are also more likely to be fed, as are those who started begging first.[45] The diet itself is composed mostly of insects, those in the orders Diptera, Hemiptera, and Odonata making up most of the diet.[11] 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.[42] In nests near lakes acidified by humans, calcium supplements, primarily fish bones, crayfish exoskeletons, clam shells, and the shells of bird eggs, are harder to find. This forces the adult tree swallow to travel further than usual—sometimes up to 650 metres (2,130 ft) away from the nest—to get these calcium supplements.[46]


Despite its aerial skills, the tree swallows is susceptible to a wide range of predators, especially near their nests. Variously eggs, nestlings and (sometimes) adults are known to fall victim to black rat snakes,[47] raccoons,[48] American black bears,[49] as well as chipmunks, weasels, domestic cats, and deermice.[50] Raptorial birds are considered the main threat to this swallow. When it is flying, sharp-shinned hawks, kestrels, merlins, prairie falcons and peregrine falcons are predators, while perched and roosting birds are sometimes ambushed by great horned owls, barred owls, red-tailed hawks, red-shouldered hawks, bald eagles, golden eagles and black-billed magpies.[50][51][52] Whilst evasive flight is the usual antipredator response in free-flying swallows, mobbing behavior is common around the nest. A mobbing swallow rarely contacts the predator, instead swarming and dive bombing predators while making aggressive calls. It seems to alter the intensity of its attacks based on which predator approaches, with snakes and raptorial birds often drawing the most spirited attacks. It is likely that the tree swallow selects roosting sites in part by their accessibility by predators.[50][53]

Parasites and immunology[edit]


Blow-fly in the genus Protocalliphora

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.[54] Nestlings also suffer from parasites, like blow-flies of the genus Protocalliphora,[55] which results in a loss of blood by nestlings.[56] These parasites, though, do not seem to have a significant effect on nestlings, and are found in a majority of nests. A study published in 1992 found that only 5.5% of the variation in nestling mass could be explained by the effects of parasitism.[55]


In the breeding female tree swallow, humoral immunocompetence (HIC) is inversely correlated with laying date. This means that, on average, a bird that lays its egg earlier has a stronger antibiotic response to an antigen than a bird that lays its egg later. A tree swallow that is handicapped by wing-clipping generally has a lower HIC. These relationships could be interpreted as supporting the conclusion that a female that lays earlier acquires a higher HIC, but the authors of the study that found the correlations believed this unlikely. Instead, they thought it showed that a better quality female is able to lay earlier due to that quality. The fact that a handicapped bird has a higher HIC suggests that it is sensitive to increases in workload.[57]

In the tree swallow, some components of the immune system deteriorate with age. Acquired T cell-mediated immunity, for example, declines with age in the female tree swallow. But, the age of a female does not affect both the acquired and innate humoral immunity. The lack of deterioration in the former contrasts with some other studies of passerines.[58] Because of this immunosenescence (a decrease in immune function with age), older females infected with a disease can visit their nest less and this can have a more pronounced effect on the growth of the nestlings than an infection in a younger female. Older females are also likely to lose weight because of an infection.[59]


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),[1] and its stable population, estimated to be about 20,000,000 individuals.[11] It is protected in the US by the Migratory Bird Treaty Act of 1918,[60] and in Canada by the Migratory Birds Convention Act.[61] In some parts of the US, 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.[11] Acidification of lakes can force it to go relatively long distances to find calcium-rich items, and can result in chicks eating plastic.[46] 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.[11]


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