Tree Swallow

## Description

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, within which its phylogenetic placement is debated. 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 dark brown, and the legs and feet 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 occurring earlier 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 for around 14 to 15 days. The chicks hatch slightly asynchronously, allowing the female to prioritize which chicks to feed in times of food shortage. They 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, in addition to molluscs, spiders, and fruit. The nestlings, like the adult, primarily eat insects, fed to it by both sexes. 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.

## Appearance

The Tree swallow is a migratory American swallow that spends most of its time flying and chasing after insects in acrobatic twists and turns. It has glossy blue-green upper parts, with the exception of the blackish wings and tail, and white underparts. Its bill is black, the eyes dark brown, and the legs and feet pale brown. The female is generally duller than the male, and the first-year female has mostly brown upper parts, with some blue feathers. Juveniles have brown upper parts and a grey-brown-washed breast.

## Distribution

The Tree swallow is a migratory American swallow that spends most of its time flying and chasing after insects in acrobatic twists and turns. It has glossy blue-green upper parts, with the exception of the blackish wings and tail, and white underparts. Its bill is black, the eyes dark brown, and the legs and feet pale brown. The female is generally duller than the male, and the first-year female has mostly brown upper parts, with some blue feathers. Juveniles have brown upper parts and a grey-brown-washed breast.

## Behavior

Tree swallows are sociable outside of the breeding season, forming flocks sometimes numbering thousands of birds. They roost every night in big flocks and these roosting sites are generally located 100 to 150 kilometers (62 to 93 mi) apart. Tree swallows forage up to 50 meters (160 ft) above the ground singly or in groups. Their flight is a mix of flapping and gliding. As well as being caught in flight, insects are sometimes taken from the ground, water, vegetation, and vertical surfaces. Tree swallows' song consists of three parts: the chirp, the whine, and the gurgle. The chirp call is made to stimulate the nestlings to beg or (in some populations) when their mate leaves or enters the next cavity. The whine call may be given alone as the anxiety call and occasionally made in response to certain predators. The gurgle, as when it appears at the end of the song, is likely involved in pair bonding. The chatter call is used to advertise nest sites (also known as the "nest-site advertising call") and is also given to intruding conspecifics. A short high-pitched submission call is sometimes uttered after an aggressive encounter with another Tree swallow. The alarm call is given in reaction to predators and other intruders and can serve to induce older nestlings to crouch and stop begging when a predator is near.

## Diet

Tree swallows are carnivores (insectivores). They eat mostly insects, with some mollusks, spiders, fruit, berries, and seeds.

## Reproduction

Tree swallows are monogamous and form strong pair bonds although some males are polygynous and may mate with more than one female. Breeding can start as soon as early May and it can end as late as July. During courtship, a male attacks an unknown female. This can be stimulated through the wing-fluttering flight by the female, which may be an invitation to court. The male may then take a vertical posture, with a raised and slightly spread tail and wings flicked and slightly drooped. This prompts the female to try to land on the male's back, but he flies to prevent this; this is repeated. Tree swallows nest either in isolated pairs or loose groups, in both natural and artificial cavities. The nest cup itself is made from grass, moss, pine needles, and aquatic plants collected mostly by the female and is lined with feathers gathered primarily by the male in fights. The feathers may function to insulate the nest, decreasing incubation time. The female incubates the clutch of 2 to 8 (but usually 4 to 7) pure white eggs for around 14 to 15 days. The chicks hatch blind, helpless, and sparsely covered with down. They generally fledge about 18 to 22 days after hatching and start to breed at one year of age.

## Population

### Population threats

Tree swallows are not endangered at present, however, they are negatively impacted by the clearing of forests and the reduction of marshes; this reduces the habitat available for wintering. Tree swallows also compete for nest sites with Common starlings, House sparrows, bluebirds, and House wrens. Acidification of lakes can force Tree swallows to go relatively long distances to find calcium-rich items and can result in chicks eating 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. Contamination from oil sand mine sites can also negatively affect Tree swallows by increasing the presence of toxins, in nestlings.

### Population number

According to Partners in Flight resource, the total breeding population size of the Tree swallow is 19,000,000 birds. Overall, currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.

## Fun Facts for Kids

While migrating, Tree swallows often use stop-over sites, spending an average of 57 days in these areas during autumn. To get to their wintering range, the birds often use one of three flyways: the Western flyway, west of the Rocky Mountains; the Central flyway, between the Rocky Mountains and the Great Lakes, stretching south into Eastern Mexico; and the Eastern flyway, from the Great Lakes east.

Tree swallows not only drink but also bathe on the wing. They skim their bodies against the surface and then rise quickly shaking off water drops.

After finding a suitable place to nest, the male Tree swallow perches near it and calls frequently. A lack of sites can cause fights between birds, sometimes resulting in deaths. Tree swallows usually defend an area around their nest with a radius of about 4.6 meters (15 ft), as well as extra nests inside of that territory, by blocking the entrance to the nest and chasing intruders.

Tree swallows line their nests with feathers to help to keep the eggs and nestlings warm, so they can grow faster. In order to collect feathers, they can sometimes even pull off some feathers from the duck’s back.

Tree swallows feed their nestlings about 10 to 20 times per hour. The parents often use the chirping call to stimulate their chicks to beg. This is used more frequently with younger chicks, as they beg less than older chicks when the parent arrives with food but does not call.