Among the four dark storm-petrels nesting on North America's west coast islands, the Ashy Storm-Petrel is the most northern. Confined to California islands and nearby waters, this smoke-gray seabird blends in well with its foggy environment. Like other storm-petrels, Ashy Storm-Petrels arrive and depart their breeding colonies only at night; unlike most others, they do not travel far from their colonies after breeding, and the breeding season is spread out over many months.
This small seabird weighs 1.3 ounces, and is eight inches in length, with a wingspan of 18 inches. The Ashy Storm-Petrel, as the name implies, is an entirely gray seabird roughly the size of a Purple Martin, with a forked tail. It is smaller and grayer than the similar Black and Leach's Storm-Petrels, but larger than the Least Storm-Petrel—the other dark species found in the area.
The Ashy Storm-Petrel can only be found on the islands off California and in the adjacent waters. This species' limited year-round range extends from Cape Mendecino, California to northern Baja, just south of the U.S. - Mexico border. Breeding colonies occur on offshore islands in the area, including the Southeast Farallons, San Miguel, Santa Barbara, Santa Cruz, Anacapa, San Clemente and Los Coronados (in Mexico). On most of the larger islands listed above, breeding occurs on outlying rock formations, free of mammalian predators.
Ashy Storm-Petrels feed near their nesting islands in the offshore waters of the California current, one of the richest regions of the world ocean. The birds breed on rocky islands among talus slopes, from just above sea level to the highest, interior portions of nesting islands. Smaller islands and offshore rocks where resources are not sufficient normally to sustain predatory mammals such as rats and foxes are preferred for nesting.
The diet of Ashy Storm-Petrels includes small fish, young squid, and crustaceans, which it finds on the ocean surface at night. Spiny lobster young have been found in the stomachs of Ashy Storm-Petrels in their southern range. These petrels feed mostly at night; sightings of feedings during the day are rare. Known to scavenge, they are attracted to fishing vessels hauling their nets and to fish-oil slicks. Food is obtained by sitting on the water and, if the prey item is large, by tearing pieces off with the bill. Birds also pick small prey items, such as crustaceans, from the sea surface using the beak as they hover into the wind, especially when preying on one euphausiid (
Thysanoessa spinifera) that swarms at the sea surface. Unfortunately, items ingested also include plastic particles that are now common on the sea surface.
Ashy Storm-Petrels nest in cavities on offshore islands and move to and from their colonies at night, which may help them avoid predation by gulls, falcons, owls and other enemies. Nest cavities are hidden among the rock crevices, generally on the driest portion of islands, where vegetation is not so dense. Little if any nest material is used inside the cavity. Like other storm petrels, female Ashy Storm-Petrels lay only one white egg, sometimes marked with faint red-brown dots. Both sexes incubate the egg for about 45 days. Chicks are fed via regurgitation by both parents and head off to sea about 84 days after hatching. Periods of incubation and chick growth are very long by avian standards. The chicks of some pairs can be half grown at the time when other pairs lay their egg.
Ashy Storm-Petrels do not perform long-distance migration. Individuals may disperse, but not to great distances and only during the molting period in the autumn.
Ainley, D. 1995. Ashy Storm-Petrel (
Oceanodroma homochroa). In
The Birds of North America, No. 185 (A. Poole and F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D. C.
BirdLife International (2006)
Species Factsheet: Oceanodroma homochroa, Ashy Storm-petrel
Cooper, D. S.
California Important Bird Areas. Audubon California, Sacramento, CA. 2002.
