Justification This storm-petrel qualifies as Endangered because studies suggest that its small population may be declining very rapidly over three generations (48 years) owing to a variety of threats.

Family/Sub-family Hydrobatidae

Species name author (Coues, 1864)

Taxonomic source(s) AOU checklist (1998 + supplements), Brooke (2004), Sibley and Monroe (1990, 1993), Stotz et al. (1996)

Identification 20 cm. An all dark storm-petrel that is difficult to identify. The pale wash on the underwing forms a distinct bar and is an important feature, as are the pale grey edges of the uppertail coverts. Similar spp. Very similar to the Black Storm-petrel O. melania but paler, smaller and with a relatively longer tail that is held upswept in flight. Voice At the nest a rising and falling purring can be heard.

Population estimate

Population trend

Range estimate (breeding/resident)

Country endemic?

5,200 - 10,000

decreasing

227,000 km²

No

Range & population Oceanodroma homochroa breeds on a small number of island groups and offshore rocks within the California Current System, the northernmost being off Mendocino County, California (~39°N) and the southernmost at Los Coronados Islands off northern Baja California, Mexico (~32°N)^2,8,9,10. Breeding has been confirmed at only six major island groups (South Farallon, San Miguel, Santa Cruz, Santa Barbara, San Clemente, and Los Coronado Islands) and three groups of offshore rocks (Castle Rock/Hurricane Point, Double Point, and Bird Rocks)⁸. Major colonies, containing the vast majority of the world population, occur on the South Farallon Islands in central California and the Channel Islands in southern California, primarily at Prince Island off San Miguel Island, Santa Barbara Island, and Santa Cruz Island². Breeding is also suspected at one mainland site in California¹⁰. At sea, Ashy Storm-petrels remain within the central and southern California Current System year-round, preferring continental slope waters (200-2000 m deep) that are within a few kilometers of the coast in some areas (e.g. Monterey Bay) and more than 50 km offshore in other areas (e.g. Gulf of the Farallones)^4,8,11. High densities are known to congregate in some areas, e.g. the continental shelf-break in the western Santa Barbara Channel, and in the Santa Cruz Basin that separates Santa Cruz, San Nicolas, and Santa Barbara Islands¹². Autumn congregations of 4000-6000 birds have been recorded in Monterey Bay¹¹. The breeding population has been estimated at 5,200-10,000 individuals^1,2, with about half breeding on the South Farallon Islands⁵ and half in the Channel Islands². A study on the South Farallon Islands found declines in breeding birds of 42% in 1972-1992⁵, equivalent to c.23% in 10 years or 78% over three generations. On Santa Cruz Island, nest-site monitoring during 1995-2006 showed declines in the number of breeding birds at two of five monitored sites⁸. Variation in per capita breeding productivity is thought to be related to fluctuating oceanographic conditions⁵, but consistent declines in productivity were noted on Southeast Farallon between 1990 and 2006^7,8, suggesting a genuine temporal decline. Recent population trends have not been determined.

Ecology: Breeds in rock crevices and burrows in colonies on offshore islands. The breeding season is protracted, and eggs are laid asynchronously, with some pairs laying eggs while other pairs are in the midst of chick-rearing. At Southeast Farallon Island, Ashy Storm-petrels visit the colony year-round, and most breeding activity is concentrated in February through October⁷. At Santa Cruz Island, Ashy Storm-petrel nesting activity spans March through December^6,8. Birds feed at sea on planktonic crustaceans and small fish and visit the colony at night.

Threats Foraging areas are threatened by organochlorine and oil pollution^2,3,5. At Anacapa Islands, introduced rats have probably reduced colony size^1,2,5, though these rats have now been eradicated⁸. Predation by expanding Western Gull Larus occidentalis populations, as well as Burrowing Owls Speotyto cunicularia and Barn Owls Tyto alba, may be partly responsible for keeping numbers low at South Farallon, Santa Barbara and Anacapa islands¹. Bright lights used by near-shore squid fishing and other commercial and recreational vessels during the breeding season could increase predation levels², as well as cause mortality by attraction to lighted structures⁸. Ashy storm-petrels are sensitive to human disturbance at their nest sites and may abandon their nests with frequent disturbance¹³ Consequently, disturbance from sea kayaker visits is a potential threat to nesting birds¹³.Future changes in climate could also affect this species, for example through declines in primary productivity associated with warming and reduced upwelling, sea level rises affecting nest site availability, or the effects of ocean acidification (caused by increasing carbon dioxide absorption) on crustacean prey species⁸.

Conservation measures underway Most of the Californian population nest on protected and specially managed islands.

Conservation measures proposed Eradicate introduced predators from nesting islands, and ensure they remain free of introduced species. Conduct studies to determine the magnitude of threats posed by native predators, as well as pollution in foraging areas. Investigate effects of artificial lights from commercial and recreational vessels on predation and breeding success at colony sites. Continue long-term population monitoring at the South Farallon Islands, including analysis of recent mist-netting data at Southeast Farallon Island to determine recent population trends. Monitor populations in the Channel Islands. Monitor the effects of global warming on populations at sea and on breeding colonies.

References 1. Ainley (1995). 2. Carter et al. (1992, unpublished data). 3. Coulter and Risebrough (1973). 4. Howell and Webb (1995a). 5. Sydeman et al. (1998). 6. del Hoyo et al. (1992). 7. Ainley et al. (1990). 8. S. Wolf in litt. (2007). 9. McChesney et al. (2000). 10. Brown et al. (2003). 11. Ainley (1995). 12. Carter et al. (2007). 13. McIver (2002).

Further web sources of information

Audubon WatchList

Text account compilers Phil Benstead (BirdLife International), James Gilroy (BirdLife International)

Contributors Brad Keitt (Island Conservation), Bill McIver, Shaye Wolf (Center for Biological Diversity)

IUCN Red List evaluators Jeremy Bird (BirdLife International), Stuart Butchart (BirdLife International), James Gilroy (BirdLife International)

Recommended citation BirdLife International (2009) Species factsheet: Oceanodroma homochroa. Downloaded from http://www.birdlife.org on 9/2/2010

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

To provide new information to update this factsheet or to correct any errors, please email BirdLife

To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums