An urban success story: Breeding seabirds on Alcatraz Island, California, 1990-2002

Abstract

Located in the San Francisco Bay estuary, Alcatraz Island has in recent years become an important breeding site for seabirds, including Brandt's Cormorant Phalacrocorax penicillatus, Pelagic Cormorant P. pelagicus, Pigeon Guillemot Cepphus columba and Western Gull Larus occidentalis. We synthesized available data on breeding populations from 1990-2002 for those four species and calculated annual productivity in 1997-2002 for Brandt's Cormorants and 1999-2002 for Pelagic Cormorants and Western Gulls. Population trends and productivity were compared to those on larger, offshore Southeast Farallon Island (SEFI). Brandt's Cormorants established a colony that grew to more than 460 breeding pairs during the study period. Small populations of Pelagic Cormorants and Pigeon Guillemots on Alcatraz remained relatively stable during 1995-2002. Breeding Western Gulls increased at a 6.0% annual rate from 1990-2002. Productivity of Brandt's Cormorants and Western Gulls on Alcatraz was higher than on SEFI in the years monitored. We propose that high availability of forage fishes in the San Francisco Bay estuary, combined with seasonal protection of nesting habitat, contributed to the recent growth and success of Alcatraz seabird populations. Because seabirds on Alcatraz breed in close proximity to myriad human activities, they present a unique opportunity to investigate the ecology of urban seabird populations.