Among the earliest of our summer birds to return to their breeding grounds, tree swallows usually appear in the American Midwest by mid March. First sightings are often of large flocks in the barren, drowned trees of reservoir backwaters, where they feast on insects rising from the lake or adjacent wetlands. While insects may be unavailable during periods of cold March weather, tree swallows supplement their diet with berries when necessary.
Though they arrive in large flocks, these swallows soon pair off to locate a tree cavity or nest box in which to raise their young; their aggressive behavior and attraction to blue bird boxes has interfered with efforts to protect those beloved songbirds. Once established at a nest site, tree swallows swoop across open fields, lakes and wetlands to snare flying insects; as they tilt in the sun, their iridescent blue-green back, contrasting with white underparts, makes identification easy. In addition, since their cousins (barn, cliff, bank and rough-winged swallows) generally arrive a month later, those seen in March are almost always tree swallows.
By late summer, tree swallows gather in large flocks and begin to depart for wintering grounds across the southernmost U.S. and Mexico. There they often roost in massive colonies, producing avian clouds as they wheel about before nightfall.
