The swallows and martins are a group of passerine birds in the family Hirundinidae which are characterised by their adaptation to aerial feeding. Swallow is used colloquially in Europe as a synonym for the Barn Swallow.
This family comprises two subfamilies: Pseudochelidoninae (the river martins of the genus Pseudochelidon) and Hirundininae (all other swallows and martins). Within the Old World, the name "martin" tends to be used for the squarer-tailed species, and the name "swallow" for the more fork-tailed species; however, there is no scientific distinction between these two groups. Within the New World, "martin" is reserved for members of the genus Progne. (These two systems are responsible for the Sand Martin being called "Bank Swallow" in the New World.) The entire family contains around 83 species in 19 genera.
The swallows have a cosmopolitan distribution across the world and breed on all the continents except Antarctica. It is believed that this family originated in Africa as hole-nesters; Africa still has the greatest diversity of species. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are non-migratory. A few species of swallow and martin are threatened with extinction by human activities, although other species have benefited from human changes to the environment and live around humans.
The Tree Swallow (Tachycineta bicolor), is a migratory passerine bird that breeds in North America and winters in Mexico, Central America and the Caribbean. It is a very rare vagrant to western Europe.
This swallow averages 13.5 cm (5 inches) long and weighs about 20 g. The bill is tiny. The adult Tree Swallow has iridescent blue-green upperparts, white underparts, and a very slightly forked tail. The female usually has duller colours than the male, often more greenish than the more bluish male. The juvenile plumage is dull grey-brown above and may have hint of a gray breast band.
Being highly social outside of the breeding season, tree swallows may form flocks of several thousand birds near roost sites. Flocks near Vacherie, Louisiana were estimated to contain well over 1 million birds during December 2009.
Tree Swallows nest in natural or artificial cavities near water and are often found in large flocks. They readily use nest boxes, including those built for bluebirds. Declines in cavity-builder populations are resulting in fewer natural nesting sites for Tree Swallows, although the swallow population remains healthy.
The Tree Swallow nest consists of multiple layers of grasses and thin twigs, and is lined with large feathers from other species. The female lays 4 to 7 white eggs and incubates them by herself. The eggs hatch in about 14 days and the hatchlings are altricial. The hatchlings typically fledge in 16–24 days. While there are young or eggs in the nest, adults frequently dive bomb intruders (including curious humans) and attempt to drive them from the area. Tree Swallows are known to "fight" over feathers in mid-air for reasons which are still under investigation. There is some speculation that this is a form of play.