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Hydrobates homochroa

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA AVES PROCELLARIIFORMES HYDROBATIDAE

Scientific Name: Hydrobates homochroa
Species Authority: (Coues, 1864)
Common Name(s):
English Ashy Storm-petrel, Ashy Storm Petrel, Ashy Storm-Petrel
Synonym(s):
Oceanodroma homochroa (Coues, 1864)
Taxonomic Notes: Hydrobates homochroa (del Hoyo and Collar 2014) was previously placed in the genus Oceanodroma.

Assessment Information [top]

Red List Category & Criteria: Endangered A2ce+3ce ver 3.1
Year Published: 2013
Date Assessed: 2012-05-03
Assessor(s): BirdLife International
Reviewer(s): Butchart, S.
Contributor(s): Keitt, B., McIver, B. & Wolf, S.
Facilitator/Compiler(s): Benstead, P., Calvert, R., Gilroy, J. & Symes, A.
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.

History:
2012 Endangered

Geographic Range [top]

Range Description: Hydrobates 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(~39°N) and the southernmost at Los Coronados Islands off northern Baja California, Mexico (~32°N) (Carter et al. 1992 unpublished data, McChesney et al. 2000, Brown et al. 2003, S. Wolf in litt. 2007).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primarily at Prince Island off San Miguel Island, Santa Barbara Island, and Santa Cruz Island (Carter et al. 1992 unpublished data).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.

Countries:
Native:
Mexico; United States; United States
Vagrant:
Ecuador
Range Map: Click here to open the map viewer and explore range.

Population [top]

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

Habitat and Ecology [top]

Habitat and 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 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).

Systems: Terrestrial; Marine

Threats [top]

Major Threat(s): 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 [top]

Conservation Actions: 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.


Citation: BirdLife International 2013. Hydrobates homochroa. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 23 November 2014.
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