Leach’s Storm-Petrel Oceanodroma leucorhoa
White-rumped O. l. leucorhoa
Dark-rumped O. l. chapmani
Variation is complex, the named subspecies are not always white- or dark-rumped as implied here. Taxonomy was revised by Ainley (1980. Auk 97:837-853). Nominate O. l. leucorhoa is widespread in the north, usually white-rumped, and individuals from the North Atlantic are indistinguishable from those in the North Pacific. The subspecies O. l. chapmani differs from leucorhoa in averaging smaller in size, in some very subtle differences in proportions, and in being mainly dark-rumped (few leucorhoa are dark-rumped), but do not differ significantly in voice. They apparently differ in foraging habitat preference, and may differ in other ways as well, but all differences are clinal with many intermediate birds off central and southern California. Two other populations nest on Guadalupe Island off Mexico and could wander to waters off California. The winter breeding population there is named O. l. cheimomnestes and the summer breeding population O. l. socorroensis.