Tree Swallow

Handsome aerialists with deep-blue iridescent backs and clean white fronts, Tree Swallows are a familiar sight in summer fields and wetlands across northern North America. They chase after flying insects with acrobatic twists and turns, their steely blue-green feathers flashing in the sunlight. Tree Swallows nest in tree cavities; they also readily take up residence in nest boxes. This habit has allowed scientists to study their breeding biology in detail, and makes them a great addition to many a homeowner’s yard or field.

Migrating and wintering Tree Swallows can form enormous flocks numbering in the hundreds of thousands. They gather about an hour before sunset and form a dense cloud above a roost site (such as a cattail marsh or grove of small trees), swirling around like a living tornado. With each pass, more birds drop down until they are all settled on the roost. Tree Swallows winter farther north than any other American swallows and return to their nesting grounds long before other swallows come back. They can eat plant foods as well as their normal insect prey, which helps them survive the cold snaps and wintry weather of early spring. Tree Swallows have helped researchers make major advances in several branches of ecology, and they are among the best-studied bird species in North America. Still, we know little about their lives during migration and winter.