Swallow, common name for various small birds of the family Hirundinidae. There are at least 78 species; all have long, sickle-shaped wings and long, forked tails. The plumage is generally dark, often with a metallic sheen. Many species have lighter underparts. The legs and feet are small and weak. Swallows can perch on wires or tree branches but are adapted to spend most of their time on the wing, feeding on insects. Many species are migratory.