Tree swallow  (Tachycineta bicolor)

The tree swallow is classified as Least Concern (LC) on the IUCN Red List (1).

The tree swallow (Tachycineta bicolour) is a medium-sized swallow species with beautiful, glossy, almost iridescent blue-green upperparts and contrasting white underparts (2) (3) (4). The wings of the tree swallow are long and the tail only shallowly forked (2) (3) (4), while the beak is small and black and the legs are dark reddish-brown or brownish-grey (2) (4).

The female tree swallow is slightly duller than the male, sometimes with a browner forehead (3). In its first year, the female is brown on the back, with only a faint greenish-blue sheen, and only begins to develop the iridescent blue-green upperparts from about two years old (2) (3) (4). Juvenile tree swallows have grey-brown upperparts, dull white underparts and a faint brown band across the chest (2) (3).

The song of this species is a series of repeated whistles and twitters, often given by the male at dawn (2) (3) (4). The tree swallow also has a diverse range of calls, including a ‘buli-duli-dulit’ contact call, a ‘she-she-she-shet’, given near the nest, a ‘peeh’ or ‘pee-deeh’ alarm call, and an aggressive ‘zjiht’ (3) (4).

Although quite similar in appearance to the violet-green swallow (Tachycineta thalassina), the tree swallow differs in having no white above and behind the eye, and no white on the sides of the rump. Juvenile tree swallows resemble the sand martin (Riparia riparia), but are larger and do not have a distinct dark breast band (2) (3) (4).

The breeding range of the tree swallow extends across northern and central North America, from Alaska and Canada, south to California, Nevada, Arizona, New Mexico, Texas, Alabama, Georgia and North Carolina in the United States (2) (3) (4) (5).

After the breeding season, the tree swallow migrates south to spend the winter along the coasts of the southern USA, as well as in the Caribbean and south to the northern coasts of South America (2) (3) (4) (5). Some tree swallows occasionally winter as far north as eastern Massachusetts and Long Island, New York, and this species is also sometimes recorded outside of its usual range, in Bermuda, Greenland, and even as far east as the United Kingdom (3) (4) (5).

The tree swallow typically inhabits open areas near to water and fields, including marshes, wooded swamps, lake shores, beaver ponds and other wetland margins (2) (3) (4) (5). As its common name suggests, it requires trees for nesting and roosting (3) (4).

The diet of the tree swallow consists mainly of flying insects, such as flies, beetles, ants, grasshoppers and dragonflies, although it will also take spiders and some crustaceans and molluscs (3) (4). Feeding usually takes place over open water or fields, often in areas sheltered from the wind, and prey is caught in flight or picked off the ground, from water, or from vertical surfaces or vegetation (2) (3) (4). Large numbers of tree swallows may gather to feed on insect swarms. The tree swallow is an adept flier that tends to glide more than many other swallow species (3) (4).

In addition to taking insects and other prey, the tree swallow is unusual among swallows in being able to survive on seeds and berries during periods of bad weather, particularly the waxy fruits of Myrica species (bayberries or wax myrtle). This allows it to winter further north and return to its breeding grounds earlier than other species, as it reduces its dependence on the availability of insects (2) (4).

The tree swallow breeds between May and July (3) (4). The male tree swallow generally arrives at the breeding grounds before the female, and immediately establishes a territory around a nest site. Since suitable nest sites are often limited, competition can be intense, and territories may be aggressively defended (4). Despite this, tree swallows commonly nest in loose groups, with the nests spaced at least 10 to 15 metres apart (3) (4).

The nest itself is usually built in a hole in a tree, often in a natural cavity or in an abandoned woodpecker hole. The tree swallow also readily uses artificial nest boxes, and has been known to sometimes nest in the eaves of buildings, in holes in fence posts, or even in a hole in the ground (3) (4). The female tree swallow performs most of the nest building, constructing an open cup of grass, pine needles, moss, aquatic plants and rootlets, and lining it with the feathers of other birds, such as waterfowl. Between 2 and 8 white eggs are laid, and are incubated by the female for 11 to 19 days (2) (3) (4). Both adults feed the chicks, which leave the nest at around 18 to 22 days and are fed for at least a further 3 days (3) (4).

The tree swallow typically raises a single brood each year (3). This species can breed from a year old if the opportunity arises (4), and it has been recorded living for up to 12 years (3). At the end of the breeding season, tree swallows congregate in vast flocks, which sometimes number into the hundreds of thousands and form dense clouds as the birds come together to roost at night (2) (3) (4).

The tree swallow is a widespread and abundant species (5), and its populations are thought to be increasing in parts of its range (2) (4). This species may have benefitted from the creation of reservoirs and the reintroduction of beavers, which may create more suitable habitat, as well as from the provision of artificial nest boxes (3).

There are not known to be any major threats to the tree swallow, but it may potentially be affected by forest clearance and the removal of the dead trees in which it nests, as well as by competition for nest sites with introduced birds such as the European starling (Sturnus vulgaris) and house sparrow (Passer domesticus) (3) (4).

The tree swallow is also vulnerable to pesticides and other pollutants, which are passed on to it from its prey and may affect its breeding success (3) (4). In addition, water pollution and the draining of marshes in its wintering grounds reduce suitable habitat, while acid rain may increase the acidity of wetlands and so influence the availability of the tree swallow’s prey (3) (4).

There are not known to be any specific conservation measures in place for the tree swallow. However, this species would benefit from forest management methods which leave dead trees standing and so do not remove vital nesting sites (4).

ARKive is supported by OTEP, a joint programme of funding from the UK FCO and DFID which provides support to address priority environmental issues in the Overseas Territories, and Defra

Find out more about the tree swallow and its conservation:

• BirdLife International - Tree swallow:
  http://www.birdlife.org/datazone/speciesfactsheet.php?id=7078
• The Cornell Lab of Ornithology: All About Birds - Tree swallow:
  http://www.allaboutbirds.org/guide/Tree_Swallow/id

More information on bird conservation in the Americas:

• American Bird Conservancy (ABC):
  http://www.abcbirds.org/
• National Audubon Society:
  http://www.audubon.org/