Six swallow species inhabit the American Midwest during the warmer months of the year. All are graceful fliers and feed primarily on flying insects; this trait makes them especially beneficial to humans but makes them vulnerable to periods of cold weather or heavy rain (since they also feed on berries, tree swallows are least susceptible to these foul weather events).
Tree swallows, identified by their blue-green back, white underparts and slightly forked tail, are the only species to winter along southern U.S. coasts and generally arrive in the Midwest by late March. Migrating in huge flocks, they pair off to look for nest sites in tree cavities or man-made nest boxes. The other five species arrive by late April and, like tree swallows, favor open country with nearby lakes or wetlands.
Barn swallows are easily identified by their deeply forked tail and, as their name implies, usually nest within barns or under their eaves. Bank swallows, smallest of the group, nest in large colonies, digging burrows into the side of riverbanks or quarry walls; their best field mark is a brown band crossing their white chest. Cliff swallows, identified by their squared tail and buff-colored rump feathers, also nest in colonies, often gathering beneath bridges, cliff ledges or the roofs of rural buildings.
Purple martins, our largest and heaviest swallow, have dark purple plumage; the females and immature birds have light gray underparts. Nesting in tree cavities or man-made "martin houses," these birds are often seen in the evening sky, hawking for insects with chimney swifts and common nighthawks. Finally, northern rough-winged swallows, less conspicuous than their cousins, have dull brown backs, a faint brown wash on their chests and slightly forked tails. They nest in pairs or small groups, usually along streams; their nests are placed in the hollows of bridges, in drainage pipes and in crevices on rocky bluffs.
By late summer, swallows begin to form large, mixed flocks, often gathering on power lines in rural areas. Come October, the tree swallows will depart for coastal areas of the southern U.S. and Mexico while the rest will head for Central and South America.
