Sometimes treated as the sole member of the genus Halocyptena, a compound word derived from the Greek words for sea, speedy and winged, the Least Storm-Petrel is the smallest member of the Hydrobatidae. It is a uniformly sooty black bird with a wedge-shaped tail and a paler gray-brown upperwing bar. The flight is often fluttering. Although the Least Storm-Petrel is virtually confined as a breeding bird to the Gulf of California, where it nests under rocks and in crevices on most predator-free islands, as well as on three islands in the San Benito archipelago off western Baja California, during the non-breeding season (September–June) the species wanders at sea as far south as the equator. The species’ diet consists mostly of small plankton.