Ashy storm-petrel (Oceanodroma homochroa)

GenusOceanodroma (1)
SizeLength: 20 cm (2)
Wingspan: 46 cm (3)
Weight37 g (3)

Classified as Endangered (EN) on the IUCN Red List (1).

The ashy storm-petrel is so named for its smoky-grey plumage, barely discernible amidst the dense fog that frequently shrouds California’s coastal waters (4). Only the subtle, pale wash of the underwings and pale edges of the uppertail coverts provide a reliable means of distinguishing it from the similarly grey storm-petrels that overlap its range (2). Less defining features include a deeply forked tail and a dark coloured hooked bill (3) (5). Its vocalisations are poorly described but in the nest environment a rising and falling purr is often heard (2).

The ashy storm-petrel is a year-round resident of coastal waters of California and northern Mexico with breeding colonies concentrated on a small number of islands and islets from off the coast of Mendocino County, California in the north, to Los Coranados off northern Baja, Mexico in the south (2) (3). The vast majority of the total population nest in colonies on the South Farallon Islands and the Channel Islands in central and southern California respectively (2).

Nesting sites are commonly located on sparsely vegetated rocky slopes on rugged islands. These often tend to be small islands that lack sufficient resources to support predators such as rats. Feeding grounds are normally close to the nesting sites in the nutrient rich upwelled waters that characterise the offshore waters of California’s coastline (2) (3).

In contrast with the long-distance migratory habits of most other storm petrel species, ashy storm-petrels typically remain within the vicinity of the breeding colonies year round (3) (6). The breeding season by comparison with other species is also unusually protracted and asynchronous, occurring from February through to October, with some chicks fledging while others are barely hatching (2) (3). The basic nests, which comprise little, if any, supplementary bedding, are made in cavities within rock crevices (3) (6). A single egg is incubated by both parents for approximately 45 days before hatching. Under the cover and relative safety of night, the adults journey to and from the nest on foraging bouts (3). Small fish, squid and crustaceans form the bulk of the ashy storm-petrel’s diet. It feeds either by resting on the surface and tearing flesh from larger prey or by hovering near the surface and using its bill to catch smaller prey such as crustaceans. The growing chicks are fed regurgitated food by the parents and will eventually fledge after around 84 days (2) (3).

Between the 1880s and 1970s, the breeding population of ashy storm-petrels at the South Farallon Islands was stable. However, in the early 1990s, a twenty-year study reported an alarming decline of 42 percent in the population (2) (3). More recently, these findings have been corroborated by other studies showing continued declines on South Farallon and also at several other important breeding sites (2). No one single cause is considered responsible for the decline but rather a combination of several negative pressures. This includes predation of petrels by gulls and owls, and on the larger islands, introduced mammals such as rats, mice and cats. In particular, rapid growth in the population of predatory gulls is of considerable concern at many of the nesting sites. Other reported threats include oil and chemical pollution in foraging areas, light pollution from fishing vessels increasing the vulnerability of petrels to predation, ingestion of plastics and human disturbance. Furthermore, there is long-term concern that climate change may disturb the ocean currents and processes of coastal upwelling, which form an integral part of a food web supporting ashy storm-petrels and many other organisms (2) (3).

In light of its restricted range, and a current population size that is almost certainly less than the 5,000 to 10,000 breeding birds estimated in the early to mid-1990s, the ashy-storm petrel is listed as Endangered on the IUCN Red List (1) (2). Fortunately, despite the threats described, most of the breeding colonies are on islands either protected from development or subject to sensitive management (2) (3). Current conservation proposals include eradication of all introduced predators from nesting islands, an investigation of the impacts of light, chemical and plastics pollution, and continued monitoring of population trends (2). Presently, the ashy storm-petrel is designated by the California Department of Fish and Game as a Bird Species of Conservation Concern, but as of May 2008 it is also under review for protection under the Endangered Species Act (ESA) (6). If listed under the ESA, the ashy storm-petrel will benefit from the compulsory development of a recovery plan, protection and restoration of critical habitat, scientific research and public education (7) (8).

For further information on the conservation of ashy storm petrel see:

For more information on this and other bird species please see:

Authenticated (23/04/10) by Mark Tasker, Head of Marine Advice, Joint Nature Conservation Committee.

  1. IUCN Red List (October, 2007)
  2. BirdLife International (September, 2008)
  3. National Audubon Society (September, 2008)
  4. Ainley, D. (1995) Ashy Storm-Petrel (Oceanodroma homochroa). In: Poole, A. (Ed) The Birds of North America. Cornell Lab of Ornithology, Ithaca. Available at:
  5. Center for Biological Diversity (September, 2008)
  6. Adams, J. and Takekawa, J.Y. (2008) At sea distribution of radio-marked ashy storm-petrels Oceanodroma homochroa captured on the California Channel Islands. Marine Ornithology, 36: 9 - 17.
  7. Taylor, M.F.J., Suckling, K.F. and Rachlinski, J.J. (2005) The effectiveness of the endangered species act: a quantitative analysis. Bioscience, 55(4): 360 - 367.
  8. Clark, J.A., Hoekstra, J.M., Boersma, P.D. and Kareiva, P. (2002) Improving U.S. Endangered Species Act recovery plans: key findings and recommendations of the SCB Recovery Plan Project. Conservation Biology, 16: 1510 - 1519.