This species account is dedicated in honor of Bill Ellison, a member of the Cornell Lab of Ornithology's Administrative Board.
Leach's Storm-Petrel, also known as Leach's Petrel and Mother Cary's Chicken, is the most widespread procellariiform breeding in the Northern Hemisphere. More than eight million pairs nest in burrows or crevices on Atlantic islands from Norway to Massachusetts and on Pacific islands from Baja California to Hokkaido, Japan. Outside the long nesting season, these seabirds disperse widely in the Atlantic and Pacific oceans, well away from land and mainly in the tropics. Millions more nonbreeders, mostly immatures, remain at sea year-round, although many of them visit colonies during the nesting season. Small and dark and not usually gregarious or attracted to ships, this species is inconspicuous at sea. Even at nesting islands, individuals fly to and from their subterranean nests only at night. Many aspects of their lives remain mysteries.
Breeding populations in the North Atlantic Ocean are quite similar to those in the Aleutian Islands of the North Pacific and are considered the same subspecies. Taxonomic treatment of populations with increasingly dark-rumped individuals southward in the eastern Pacific is still tentative. Of interest and worth continued investigation are the distinct populations with different egg-laying seasons on Guadalupe Island off Baja California (Power and Ainley 1986). Much work remains to determine the nonbreeding distributions of these populations. The smaller, dark-rumped Swinhoe's Storm-Petrel (Oceanodroma monorhis), nesting off Japan, Korea, China, and Russia, is so similar it has been considered a race of Leach's Storm-Petrel; the two are appropriately considered a superspecies.
Like all storm-petrels (as far as is known), this species feeds on zooplankton and nekton at the surface of the sea, pecking at individual organisms while hovering, resting briefly on the surface, or, rarely, pattering with wings partly spread. Nesting in burrows dug on wooded or treeless islands or in crevices among rocks, individuals return faithfully each year to their colony. As individuals fly about over the colonies at night, they utter a loud Chatter Call; in the nest they produce a Purring Call, often as a duet, for many minutes at a time. Like all Procellariiformes, this species is long-lived and lays only one egg per year. Both sexes incubate and feed the young, which develops slowly and becomes very fat before slimming down to flying weight in the fall.