Ecological Studies of Seabirds on Alcatraz Island, 2005

Acosta, Sara; Thayer, Julie; Merkle, William; Hellwig, Christian

Alcatraz Island has become a breeding site for a number of seabird species in recent years. In 2005, we monitored the population size, breeding phenology, and productivity of Brandt’s Cormorants, Pelagic Cormorants, Western 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. While monitoring the cormorant sites and carrying out the twice-weekly census, we recorded and cataloged disturbances to seabird populations from March through mid-September, 2005. Productivity of seabirds on Alcatraz in 2005 was lower than in 2004, most likely affected by the marine climate anomaly in 2005. Productivity of Brandt’s Cormorants decreased to 1.8 chicks fledged/pair in 2005 compared to 2.4 chicks fledged/pair in 2004 and no double-brooding was attempted in 2005. Although productivity for the Brandt’s Cormorant was low, population size in 2005 was 821 breeding pairs, the largest yet recorded for this colony, an increase of 11% from 739 breeding pairs seen in 2004. In 2005 several sub-colonies expanded, including the Laundry Building sub-colony, the Northern sub-colony, the South Bricks sub-colony, and a new sub-colony formed below the Rubble Piles. Historically the largest sub-colony, the Southern sub-colony decreased significantly from 96 breeding pairs in 2004 to 37 breeding pairs in 2005. The first eggs of the season were laid between 23-27 April, over 5 weeks later than in 2004. The Pelagic Cormorant population has decreased by over 50% to only 6 breeding pairs. Pelagic Cormorant productivity was the lowest ever documented at 0.5 chicks fledged/pair. Western Gull breeding numbers increased slightly from 940 breeding pairs in 2004 to 1014 breeding pairs in 2005 (not including nests removed under the NPS depredation permit), an 8% increase. Western Gulls fledged an average of 1.4 chicks/pair in 2005, down from 1.9 chicks fledged/pair in 2004. We confirmed only 7 Pigeon Guillemot sites in 2005, a large decline from 33 confirmed sites in 2004. One pair of Black Oystercatchers bred on the Seawall in 2005; no chicks were hatched from 2 separate breeding attempts. We first discovered California Gulls breeding on Alcatraz in 2004. In 2005, a total of 14 nesting pairs were observed in a small colony on the walkway south of the Rubble Piles on the Parade Ground. At least 13 pairs laid eggs and at least 7 hatched chicks. Fledging could not be documented due to limited access for disturbance reasons. We documented 34 disturbances to Brandt’s Cormorants in 2005 during 339 hours of observation. The majority of disturbance was due to marine and air traffic around the island. This rate of disturbance was lower than that in the past six years of our observations; however, it takes only one ill-timed disturbance to potentially cause colony failure. Therefore, to continue to reduce disturbances, we recommend continued park vigilance in restricting human access and construction activities within view of the western cliffs after early February, especially if activities may influence the sensitive Pelagic Cormorant population. We also continue to encourage plans to post signs visible to both visitors on the island and to passing boaters that explain proximity restrictions with the threat of law enforcement. Alcatraz Island offers a unique opportunity for public to view seabird breeding activities up close, and this should be highlighted as a focal point of visitor education and outreach. Improved signage in sensitive areas, interpretive tours around the island, and increased staff training regarding awareness and sensitivity of bird colonies will help educate visitors as well as protect breeding seabirds.

Ecological Studies of Seabirds on Alcatraz Island, 2005

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