Ibis (2003), 145, 673–680 © 2003 British Ornithologists’ Union Blackwell Publishing Ltd. Breeding biology and conservation of the Black-vented Shearwater Puffinus opisthomelas BRADFORD S. KEITT, 1,4 * BERNIE R. TERSHY 3,4 & DONALD A. CROLL 2,4 1 Ocean Sciences, 2 Ecology and Evolutionary Biology, and 3 Institute of Marine Sciences, University of California, Santa Cruz, CA 95064, USA 4 Island Conservation and Ecology Group, Long Marine Laboratory, 100 Shaffer Road, University of California, Santa Cruz, CA 95060, USA The Black-vented Shearwater Puffinus opisthomelas is endemic to the Pacific coast of Baja California, Mexico. We studied the breeding biology of this species at Natividad Island in 1997 and 1998. The colony at Natividad Island covers approximately 2.5 km 2 and we estimated there to be 114 455 (± 27 520 95% CI) burrows in the colony. In 1997 burrow occupancy was 66.9%, providing a population estimate of 76 570 (± 18 411 95% CI) breeding pairs, representing about 95% of the world’s population of this species. In 1997 the peak in egg laying occurred in early March and hatching began on 7 May. The incubation period averaged 51 days (± 6 sd) and chick rearing averaged 69 days (± 3 sd). In 1998 burrow occu- pancy was lower (19.6%) and nest initiation was later (peak egg laying in mid-April), perhaps a result of El Niño conditions that prevailed in the Eastern Pacific at that time. We calculated that the development of the town and roads on Natividad Island have destroyed over 15% (26 532 burrows) of the breeding habitat on the island. The Black-vented Shearwater Puffinus opisthomelas is endemic to islands off the Pacific coast of Mexico and is the only shearwater that breeds on islands in the California Current. Anecdotal observations of this species at its breeding sites began in the late 1800s (Anthony 1896). However, there have been no in-depth investigations of the breeding biology of this species. Everett (1988) summarized the existing literature on the Black-vented Shearwater and provided insight from his own visits to the nesting islands. Current knowledge is limited to some egg and chick dates and cursory population estimates. Here we provide new data on the breeding biology of the Black-vented Shearwater, including population size, breeding phenology and breeding success. We also discuss impacts of the human settlement on the Shearwater population on Natividad Island, the world’s largest breeding colony. STUDY AREA AND SPECIES Black-vented Shearwaters are reported to breed on seven islands and islets off the Pacific coast of Baja California, Mexico: Guadalupe and two offshore islets (Afuera and Negro), the three San Benito Islands, and Natividad Island (27°52′N 115°10′W, Everett 1988, Fig. 1). In general, population esti- mates for this species have been based on cursory burrow counts or best guesses based on numbers of birds observed at sea around known colonies. On Guadalupe Island shearwaters were reported to be ‘rather common’ by Anthony (1900). Surveys on the Guadalupe islet of Negro in the late 1960s and early 1970s indicated that 100–150 pairs bred there (Jehl & Everett 1985). Burrow counts on the Guadalupe islets of Negro and Afuera in the mid-1960s indicated a total of 150 breeding pairs for both islets (Delong & Crossin 1968). Jehl and Everett (1985) reported counts of flocks on the water around Guadalupe Island that indicated approximately 500 – 2500 pairs in the region, and suggested that other undocu- mented colonies may exist or that the Negro and Afuera colonies are larger than previous estimates. Actual breeding locations on Guadalupe Island have never been documented and it has been suggested that feral cats may have eliminated most, if not all, breeders from the island (Everett & Pitman 1993). Population estimates for San Benito Islands include *Corresponding author. Email: bkeitt@islandconservation.org