The tree swallow has glossy blue-green upperparts, with the exception of the blackish wings and tail, and white underparts.

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.

An aerial insectivore, the tree swallow forages both alone and in groups, eating mostly insects, in addition to mollusks, spiders, and fruit.

The effect of disease can become stronger as a tree swallow gets older, as some parts of the immune system decline with age.

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.

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.

[5] Some authors continued to use this classification, with the addition of Tumbes swallow; however, genetic evidence supports the existence of a single genus, Tachycineta.

[7] The generally accepted genus name is from Ancient Greek takhykinetos, "moving quickly", and the specific bicolor is Latin and means "two-coloured".

[8] The other genus name, Iridoprocne, comes from the Greek iris, meaning rainbow, and Procne, a figure who supposedly turned into a swallow.

[11] A study based on such nuclear DNA placed the tree swallow in the most basal position within Tachycineta as a whole (as a sister group to the rest of the genus).

A 2014 study, for example, found that broods for whom white noise was played were less likely to crouch or stop begging in response to alarm calls.

[7] It roosts every night during the non-breeding season, preferring to rest in cane or reed beds over water, but it is also found over land and on trees and wires.

[26] Because of the large amount of research on the tree swallow and how it willingly breeds in nest boxes, biologist Jason Jones recommended that it be considered a model organism.

[7] The nest cup itself is made from grass, moss, pine needles, and aquatic plants collected mostly by the female,[14] and is lined with feathers gathered primarily by the male in fights.

[33] 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.

However, temperatures in Nova Scotia (where Annapolis Valley is) are generally lower than those in Saskatchewan, possibly explaining the unexpected result.

He then makes cloacal contact with the female while holding her neck feathers in his bill and standing on her slightly outstretched wings.

[45] However, a 2005 paper discovered a slight negative correlation between a pair's genetic similarity and the proportion of extra-pair young in their nest.

[48] A 2018 study weakly supported this context dependent hypothesis, finding that extra-pair offspring were more likely to fledge than within-pair offspring in experimentally enlarged broods; however, neither telomere length (a correlate of survival and reproductive success) nor size 12 days after hatching were significantly different among these young, and no significant differences between the two types were found in non-enlarged broods.

These theories are based on genetic constraint, where an allele resulting in a maladaptive behavior is maintained because it also contributes to a beneficial phenotype.

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,[52] like within-pair copulation rate.

[20] It seems to alter the intensity of its attacks based on which predator approaches;[73] a 1992 study found that ferrets elicited a more vigorous defense than black rat snakes,[20] and a 2019 thesis similarly discovered that black rat snake models were dived at the least and eastern chipmunk models the most.

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, due to the colder temperatures near the start of the breeding season.

[81] Higher quality female tree swallows (as measured by laying date) are able to maintain their reproductive effort while diverting resources to fight an immune challenge.

[84] Because of this immunosenescence (a decrease in immune function with age), older females infected with a disease generally visit their nest less, resulting in their nestlings growing slower.

[87] 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 nest boxes installed for bluebirds.

The tree swallow is negatively impacted by the clearing of forests and the reduction of marshes, the latter reducing the habitat available for wintering.

[89] Acidification of lakes can force this swallow to go relatively long distances to find calcium-rich items, and can result in chicks eating plastic.

[14] Contamination from oil sands mine sites can negatively affect tree swallows by increasing the presence of toxins, as measured by the activity of ethoxyresorufin-o-deethylase (a detoxification enzyme) in nestlings.

This normally has little influence on nestling and fledging,[90] though extreme weather can reveal the effects: a 2006 study found that nestlings from wetlands most polluted by oil sands processing material were more than 10 times more likely to die than those from a control site during periods of synchronized cold temperatures and heavy rainfall, compared to the lack of difference in mortality between the groups when the weather was less extreme.