Ashy Storm-petrel Hydrobates homochroa


Justification of Red List Category
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.

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 temperatures leading to reduced colony attendance.

Distribution and population

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 (U.S.A.) (~39°N) and the southernmost at Los Coronados Islands off northern Baja California, Mexico (~32°N) (Carter et al. 1992 unpublished data, McChesney et al. 2000, Brown et al. 2003, S. Wolf in litt. 2007).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primarily at Prince Island off San Miguel Island, Santa Barbara Island, and Santa Cruz Island (Carter et al. 1992 unpublished data).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 kilometres 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 1995, 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 of the population breeding on the South Farallon Islands (Sydeman et al. 1998) and the other 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.


The species 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).


The species is threatened by increasing predation from native species. The seasonal residency of Burrowing Owls Athene cunicularia in the Farallons has increased, with autumn-visitant owls likely remaining through the winter due to the abundance of mice available since the eradication of rabbits has allowed more vegetation to go to seed. This has resulted in hyperpredation of Ashy Storm-petrels, especially following seasonal decreases in the mouse population (Mills 2016 and Nur et al. in press in Carter et al. 2016). Further declines in the Farallon colonies are thought to have occurred due to heavy predation by a large, and growing, population of Western Gulls Larus occidentalis (Ainley et al. 1974, Penniman et al. 1990, Sydeman et al. 1998 in Carter et al. 2016).

Invasive species also pose a threat. On Anacapa Island, Ashy Storm-petrels were not confirmed breeding until after the eradication of House Rats Rattus rattus and appeared to be restricted to the most inaccessible locations on the island, while subsequently nesting records have been found in areas accessible to researchers (Harvey et al. 2016). House Rats remain present on several islands on which Ashy Storm-petrel has been recorded to breed (e.g. San Clemente and San Miguel), but the current impact is unclear (Carter et al. 2016). Mice Mus musculus are also suspected to depredate eggs and chicks (Ainley et al. 1990 in Angel et al. 2009). Introduced Barn Owls Tyto alba are known to predate upon Ashy Storm-petrels, representing their primary avian predator on Santa Cruz Island in 1995-1996 (McIver 2002); however, depredation by the Barn Owl was less significant in 2005-2007 (McIver et al. 2009).

There is strong evidence for the impact of chemical pollution on the Ashy Storm-petrel. In 1992-1997, high levels of organochlorine pollutants, such as DDT and PCBs, were found in Ashy Storm-petrel eggs at Santa Cruz Island (Fry 1994, Carter et al. 2008a,b). This caused extensive eggshell thinning and potentially embryo mortality, leading to low hatching success, averaging 62% on the island between 1995 and 1998. Regulations in 1970 caused a decline in ocean pollutant levels around Santa Cruz Island, implying that the levels of contamination measured in the 1990s were not representative of the historic maximum. By 2008, pollutant levels in eggs on Santa Cruz had decreased (Carter et al. 2008a) and mean hatching success in 2005-2007 was significantly higher than that in 1995-1998 (McIver et al. 2009), similar to the mean seen in the less polluted Fallarones in 1972-1983 (Ainley et al. 1990b).

Rising sea levels and increasing storm surge due to climate change is an increasing concern (National Research Council 2012) due to the potential increase in flooding of low-lying nesting habitat in sea caves and offshore rocks during the breeding season. High-water events in 2008, 2010 and 2011 in the Cave of Birds’ Eggs caused low levels of adult and chick mortality, as well as the loss of some areas of cave-floor nesting habitat (McIver et al. 2016).

Storm-petrels have been demonstrated to be negatively impacted by the action of chronic and acute oil incidents, with effects extending to reduced reproductive success in the following season. However, most of the impacts do not appear to persist beyond a few years. Breeding populations are not generally suppressed, which may be explained by the recruitment of non-breeding individuals in a species that are limited by breeding sites. They are unlikely to be retrieved dead after an oil spill due to their small size and propensity for being scavenged (ATTC 2001) and, as such, will be underrepresented in counts of this type. The American Trader oil spill is thought to have had some effect on the Ashy Storm-petrel (ATTC 2001). Chronic oiling events do occur within the Ashy Storm-petrel’s range in central California, including the S.S. Jacob Luckenbach, sunk in the Gulf of the Farallones in 1954 and responsible for a number of oiling events since, as well as incidents of illegal offshore dumping of tank washing and bilge dumping (Hampton et al. 2003). The congregation of Ashy Storm-petrel in Monterey Bay in the autumn means major spill in this area would have the potential to affect relatively large numbers of birds (Sydeman et al. 1998).

Lights on oil rigs and fishing boats represent a significant source of light pollution, with the entire Ashy Storm-petrel population subject to the impacts of squid boat lights while foraging (McIver et al. 2016), and 37% of the total population exposed to the effects of bright lighting on offshore energy platforms (U.S. Fish and Wildlife Service 2015). This is likely to cause some level of mortality, but the effect on the population is unknown.

Disturbance by kayakers also poses a threat to the Ashy Storm-petrel, with the lowest breeding success recorded at sites easily accessible by kayak. Natural and artificial nesting habitats are fragile and prone to movement or collapse if stepped upon. During the breeding season, which coincides with the peak numbers of kayakers, Storm-petrel adults, chicks, and eggs within nest sites are vulnerable to being crushed or disturbed by unaware human visitors (McIver et al. 2011). However, signs preventing tourists from entering caves have been deployed and no evidence of disturbance by the general public has been recorded since (McIver et al. 2011).

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 depredation 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.


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.


Text account compilers
Symes, A., Benstead, P., Fjagesund, T., Calvert, R., Gilroy, J., Hermes, C., Martin, R., Miller, E., Stuart, A.

McIver, W., Keitt, B., Wolf, S.

Recommended citation
BirdLife International (2021) Species factsheet: Hydrobates homochroa. Downloaded from http://www.birdlife.org on 18/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 18/10/2021.