Ecological Studies of Seabirds on Alcatraz Island, 2008

Acosta, Sara; Thayer, Julie; Merkle, William; Bishop, Stephanie

Alcatraz Island has become a breeding site for a number of seabird species in recent years. In 2008, we monitored the population size, breeding phenology, and productivity of Brandt’s and Pelagic Cormorants, Western and California Gulls, Pigeon Guillemots, and Black Oystercatchers on Alcatraz. Disturbance to wildlife is a concern on Alcatraz, given its status as a heavily-visited national park and its location in the center of the San Francisco Bay. During twice-weekly monitoring, we recorded and cataloged disturbances to seabird populations from March - August, 2008. Populations of Brandt’s and Pelagic Cormorants and California Gulls decreased on Alcatraz in 2008, while the Western Gull and Pigeon Guillemot populations increased. Overall productivity of seabirds was lower than long-term averages. The Brandt’s Cormorant breeding population decreased by 15% in 2008, the first decline in 8 years, and reproductive success was the lowest on record for this colony. Most sub-colonies started breeding activities late and some areas did not breed at all, including the North Foghorn, parts of the Model Industries, Barker Beach, Northern, and Southern sub-colonies. The Laundry Building sub-colony returned after a near absence in 2007 and expanded across parts of the Seawall area. Brandt’s Cormorants are beginning to expand to the east side of the island and a small number of birds were successful in breeding there in 2008. Productivity of Brandt’s Cormorants was 1.5 chicks fledged/pair in 2008, lower than the average 2.1 chicks fledged/pair in the past 10 years. The mean lay date in 2008 was 6 May. The Pelagic Cormorant population has decreased by nearly 100% since 1996 to only 1 breeding pair in 2008. The single nesting pair fledged one chick, the lowest productivity of Pelagic Cormorants on Alcatraz since 1999. Western Gull breeding numbers increased only slightly to 1061 breeding pairs in 2008 (not including nests removed under the NPS depredation permit) from 1037 pairs in 2007. Western Gulls fledged an average of 1.3 chicks/pair in 2008, lower than the long-term mean. We confirmed 31 Pigeon Guillemot sites in 2008, 10 more than in 2007 and just below the peak of 33 confirmed sites in 2004. One pair of Black Oystercatchers bred on the Seawall in 2008, but after laying in mid-June they lost their eggs within 2 weeks and did no relay attempts were seen. We first discovered California Gulls breeding on Alcatraz in 2004. Since then, this small colony behind the rubble piles on the Parade Ground peaked at 23 pairs in 2007, but decreased slightly to 21 pairs in 2008. They fledged an average of 1.2 chicks fledged/pair in 2008.

Ecological Studies of Seabirds on Alcatraz Island, 2008

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