Ashy Storm-Petrel Hydrobates homochroa Scientific name definitions

Among the four, dark storm-petrel species that nest on xeric islands along the Pacific coast from northern Baja California (Mexico) to northern California (United States), the Ashy Storm-Petrel has the most northerly distribution. Foraging in the near-coast waters, this species' dark smoke-gray color befits its often foggy environment. Like other storm-petrel species, the Ashy Storm-Petrel arrives and departs its nesting areas only at night—unlike most others, it is nonmigratory and has no expansive post-breeding dispersal. Rather, the Ashy Storm-Petrel frequents its nesting islands for most of the year (except to some degree during the autumn molt), and feeds nearby in offshore waters of the California Current, one of the richest ocean regions on the globe.

Like other species in the Order Procellariiformes, the female Ashy Storm-Petrel produces only a single egg in any reproductive season. Unlike other storm-petrels that migrate and lay eggs synchronously with other colony members, egg-laying by the Ashy Storm-Petrel is asynchronous and spread over several months. The chicks of some pairs can be half grown at the time when other pairs lay their egg. Both incubation and nestling periods are very long by avian standards. Consistent with the extended occupation of the colony and nesting duties, molt in the Ashy Storm-Petrel also is prolonged and overlaps broadly with breeding; this differs from the segregated molt schedules of migratory procellarids.

The global breeding population of the Ashy Storm-Petrel is not known, but is estimated to be on the order of 8,200 birds (range 3,000–13,250). Fluctuation in population size also is poorly known. Numbers may have been negatively affected by increased abundance of the predatory Western Gull (Larus occidentalis), whose population has grown immensely in recent decades, and introduced rodents that have invaded the larger islands where this storm-petrel nests or formerly nested. Owls attracted to the rodents, including native species on some islands, also take storm-petrels. Native mesopredators, such as foxes and skunks on some of the nesting islands, limit the availability of appropriate nesting habitat for the Ashy Storm-Petrel.

Although these factors may contribute to population instability, breeding birds are distributed among 33 known breeding sites, and may nest at around 15 others where only their presence thus far has been detected. Despite the seemingly high number of breeding sites, suitable nesting habitat is limited, contributing to a high prevalence of nonbreeding individuals in the population. Owing to significant exchange among breeding sites, the definition of what constitutes a “colony” in this species is problematic. Almost all breeding sites are afforded a measure of protection from the direct encroachment of human civilization, as almost all are within federal and state public lands. Regardless, a number of them are vulnerable to various negative environmental factors.