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 1624 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 .
tree swallows are typically single-brooded , although they may attempt a second nest if the first fails early in the season .
there are records of parents raising two successful broods in a season .
they subsist primarily on a diet of insects , sometimes supplemented with small quantities of fruit .
they are excellent fliers and take off from their perch and acrobatically catch insects in their bills in mid-air .
