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.
Fun Fact 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.