This family is formed by a single genus containing the Frigate-birds—rather large dark-coloured oceanic birds, which far surpass all other Steganopodes in expanse of wing, and greatly resemble Accipitrine birds in their mode of flight. Although, as in other families of the present order, all four toes are united by a membrane, the web is much less developed than in the allied forms. The tarsus is broad and very short. Furcula anchylosed to keel of sternum ; nostrils not pervious. Cervical vertebra) 15 ; syringeal muscles present; ambiens and femoro-caudal developed, the other characteristic thigh-muscles wanting.
Frigate-birds breed on oceanic islands. The nest of stick* is placed on rocks or on bushes, and a single white egg is laid.