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).