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 upperparts, 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 upperparts, with some blue feathers. Juveniles have brown upperparts and a grey-brown-washed breast.
Tree swallows breed in the United States and Canada. They winter along southern US coasts south, along the Gulf Coast, to Panama and the northwestern coast of South America, and in the West Indies. The breeding habitat of these birds is primarily open and wooded areas, especially those near water; these may include marshes, ponds, bogs, wooded swamps, and lakes. Tree swallows prefer to rest in cane or reed beds over water, but may also be found over land and on trees and wires.
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.
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.
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 have also to 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 sands mine sites can also negatively affect Tree swallows by increasing the presence of toxins, in nestlings.
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.