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Ashy Storm-petrel Hydrobates homochroa
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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.

Taxonomic source(s)
Brooke, M. de L. 2004. Albatrosses and Petrels Across the World. Oxford University Press, Oxford.
del Hoyo, J.; Collar, N. J.; Christie, D. A.; Elliott, A.; Fishpool, L. D. C. 2014. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Barcelona, Spain and Cambridge UK: Lynx Edicions and BirdLife International.

Taxonomic note
Hydrobates homochroa (del Hoyo and Collar 2014) was previously placed in the genus Oceanodroma.

Oceanodroma homochroa (Coues, 1864)

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.

Distribution and 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 (USA) (~39°N) and the southernmost at Los Coronados Islands off northern Baja California, Mexico (~32°N) (Carter et al. 1992 unpublished data, McChesney et al. 2000, Brown et al. 2003, S. Wolf in litt. 2007). 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) (S. Wolf in litt. 2007). 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primarily at Prince Island off San Miguel Island, Santa Barbara Island, and Santa Cruz Island (Carter et al. 1992 unpublished data). Breeding is also suspected at one mainland site in California (Brown et al. 2003). 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) (Ainley 1995, Howell and Webb 1995a, S. Wolf in litt. 2007). 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 Islands (Carter et al. 2007). Autumn congregations of 4000-6000 birds have been recorded in Monterey Bay (Ainley 1995). The breeding population has been estimated at 5,200-10,000 individuals (Carter et al. 1992 unpublished data, Ainley 1995), with about half breeding on the South Farallon Islands (Sydeman et al. 1998) and half in the Channel Islands (Carter et al. 1992 unpublished data). A study on the South Farallon Islands found declines in breeding birds of 42% in 1972-1992 (Sydeman et al. 1998), 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 sites (S. Wolf in litt. 2007). Variation in per capita breeding productivity is thought to be related to fluctuating oceanographic conditions (Sydeman et al. 1998), but consistent declines in productivity were noted on Southeast Farallon between 1990 and 2006 (Ainley et al. 1990, S. Wolf in litt. 2007), suggesting a genuine temporal decline. Recent population trends have not been determined.

Population justification
The population is estimated to number 5,200-10,000 individuals, equivalent to 3,500-6,700 mature individuals.

Trend justification
A study on the South Farallon Islands found declines in breeding birds of 42% in 1972-1992 (Sydeman et al. 1998), equivalent to c.23% in 10 years or 78% over three generations, and declines in reproductive success have also been noted on Southeast Farallon Island. On Santa Cruz Island, nest-site monitoring during 1995-2006 showed declines in the number of breeding birds at two of five monitored sites (S. Wolf in litt. 2007). Declines are thought to have been driven by increased levels of predation and pollutants, though it may have been exaggerated by consecutive years of abnormal sea-surface temeratures leading to reduced colony attendance.

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 October (Ainley et al. 1990). At Santa Cruz Island, Ashy Storm-petrel nesting activity spans March through December (del Hoyo et al. 1992, S. Wolf in litt. 2007). Birds feed at sea on planktonic crustaceans and small fish and visit the colony at night. Foraging during the breeding season occurs mainly over continental shelves (92 - 98%), with aggregations coinciding spatially and seasonally with the spawning aggregations of sardines and anchovies (Adams and Takekawa 2008).

Foraging areas are threatened by organochlorine and oil pollution (Coulter and Risebrough 1973, Carter et al. 1992 unpublished data, Sydeman et al. 1998). At Anacapa Islands, introduced rats have probably reduced colony size (Carter et al. 1992 unpublished data, Ainley 1995, Sydeman et al. 1998), though these rats have now been eradicated (S. Wolf in litt. 2007). Predation by expanding Western Gull Larus occidentalis populations, as well as Burrowing Owls Athene cunicularia and Barn Owls Tyto alba, may be partly responsible for keeping numbers low at South Farallon, Santa Barbara and Anacapa islands (Ainley 1995). Bright lights used by near-shore squid fishing and other commercial and recreational vessels during the breeding season could increase predation levels (Carter et al. 1992 unpublished data), as well as cause mortality by attraction to lighted structures (S. Wolf in litt. 2007). Ashy Storm-petrels are sensitive to human disturbance at their nest sites and may abandon their nests with frequent disturbance (McIver 2002). Consequently, disturbance from sea kayaker visits is a potential threat to nesting birds (McIver 2002). 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 species (S. Wolf in litt. 2007).

Conservation Actions Underway
Most of the Californian population nest on protected and specially managed islands. Conservation Actions 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.

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(1): 9-17.

Ainley, D. G. 1995. Ashy Storm-petrel Oceanodroma homochroma. In: Poole, A.; Gill, F. (ed.), The birds of North America, No. 185, pp. 1-12. The Academy of Natural Sciences, and The American Ornithologists' Union, Philadelphia, and Washington, DC.

Ainley, D. G., Henderson, R. P.; Strong, C. S. 1990. Leach's Storm-petrel and Ashy Storm-petrel. Seabirds of the Farallon Islands: Ecology, dynamics and structure of an upwelling-system community, pp. 128-162. Stanford University Press, Stanford, CA, USA.

Brown, A.; Collier, N.; Robinette, D.; Sydeman, W. J. 2003. A potential new colony of Ashy Storm-petrels on the mainland coast of California, USA. Waterbirds 26: 385-388.

Carter, H. R., McIver, W. R.; Adams, J.; J. Y. Takekawa, J. Y. 2007. Population monitoring of Ashy Storm-Petrels and Cassin’s Auklets at Santa Cruz Island, California, in 2006.

Carter, H. R.; McChesney, G. J.; Jaques, D. L.; Strong, C. S.; Parker, M. W.; Takekawa, J. E.; Jory, D. L.; Whitworth, D. L. 1992. Breeding populations of seabirds in California, 1989-1991. Population estimates. U.S. Fish and Wildlife Service, Dixo

Coulter, M. C.; Risebrough, R. W. 1973. Shell-thinning in eggs of the Ashy Petrel (Oceanodroma homochroa) from the Farallon Islands. Condor 75: 254-255.

del Hoyo, J.; Elliot, A.; Sargatal, J. 1992. Handbook of the Birds of the World, vol. 1: Ostrich to Ducks. Lynx Edicions, Barcelona, Spain.

Howell, S. N. G.; Webb, S. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, Oxford.

McChesney, G. J.; H. R. Carter, H. R.; Parker, M. W. 2000. Nesting of Ashy Storm-petrels and Cassin's Auklets in Monterey County, California. Western Birds 31: 178-183.

McIver, W. R. 2002. Breeding phenology and reproductive success of Ashy Storm-petrels (Oceanodroma homochroa) at Santa Cruz Island, California, 1995-1998. MSc, Humboldt State University.

Sydeman, W. J.; Nur, N.; McLaren, E. B.; McChesney, G. J. 1998. Status and trends of the Ashy Storm-petrel on Southeast Farallon Island, California, based upon capture-recapture analyses. Condor 100: 438-447.

Further web sources of information
Audubon WatchList

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Text account compilers
Benstead, P., Calvert, R., Gilroy, J. & Symes, A.

Keitt, B., McIver, B. & Wolf, S.

IUCN Red List evaluators
Butchart, S., Taylor, J.

Recommended citation
BirdLife International (2016) Species factsheet: Hydrobates homochroa. Downloaded from on 25/10/2016. Recommended citation for factsheets for more than one species: BirdLife International (2016) IUCN Red List for birds. Downloaded from on 25/10/2016.

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.

Key facts
Current IUCN Red List category Endangered
Family Hydrobatidae (Northern Storm-petrels)
Species name author (Coues, 1864)
Population size 3500-6700 mature individuals
Population trend Decreasing
Distribution size (breeding/resident) 205,000 km2
Country endemic? No
Links to further information
- Additional Information on this species