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 (5.3 in) long and weighs about 20 g (0.71 oz). 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 grey 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. source