Real Dictionary

cliff swallow in American English
a North American swallow (Petrochelidon pyrrhonota) that builds its gourdlike nest of mud, grass, etc. against a cliff or under the eaves of a building
noun: any of several North American birds of the genus Hirundo, esp. H. pyrrhonota, that live colonially and build bottle-shaped mud nests on cliffs and walls

cliff swallow in British English
noun: an American swallow, Petrochelidon pyrrhonota, that has a square-tipped tail and builds nests of mud on cliffs, walls, etc