We summarized historical knowledge from 1912 to 1998 of 13 known breeding colonies of Ashy Storm-Petrels (Oceanodroma homochroa) at Santa Cruz Island, California. Breeding was first reported by naturalists at Painted Cave (1912), Scorpion Rocks (1928), and Orizaba Rock (1937). During early seabird colony surveys of the Channel Islands, breeding was also noted at Cavern Point Cove Caves (1970), Diablo Rocks (1976), and Gull Island (1977). During 1991–1998, more extensive surveys of nearly all sea caves and offshore rocks documented additional breeding at Bat Cave, Cave of the Birds' Eggs, Del Mar Cove Cave, Del Mar Rock, Dry Sandy Beach Cave, Shipwreck Cave, and Willows Anchorage Rocks. Historical impacts apparently occurred from specimen collecting and interference by other nesting seabirds; however, guano harvesting may have had major impacts on nesting habitats at Scorpion Rocks (main rock) and Orizaba Rock. Impacts from organochlorine pollutants were likely extensive, and population sizes at all colonies may have been lower in the 1960s and 1970s when organochlorine pollution was at its highest level. Standardized monthly nest monitoring during 1995–1998 was used to measure reproductive success and population size at 5 colonies, creating a baseline for long-term trend monitoring that has continued annually to 2016. Mist net captures were used to estimate population sizes at 2 colonies in 1991. Estimated population size for Santa Cruz Island in 1991–1998 was 338 pairs, greater than previously reported for 1976–1977 (45–55 pairs), but the more recent estimate includes more colonies (especially 3 large colonies in sea caves), different methods of estimation, and likely some recovery from impacts associated with organochlorine pollution.
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