Ecological Studies of Seabirds on Alcatraz Island, 2006

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

Alcatraz Island has become a breeding site for a number of seabird species in recent years. In 2006, we monitored the population size, breeding phenology, and productivity of Brandt’s Cormorants, Pelagic Cormorants, Western Gulls, 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. While monitoring the cormorant sites and carrying out the twice-weekly census, we recorded and cataloged disturbances to seabird populations from March - August, 2006. In an attempt to reduce human caused disturbances to seabirds and raise awareness of seabird natural history, we built upon past public outreach efforts and began a coordinated outreach program in partnership with GGNRA and GGNPC. In 2006 we delivered public outreach programs to target audiences including on-island staff, island visitors, personal watercraft operators, and school children, and made recommendations for further outreach centered on Alcatraz seabirds. Populations of Brandt’s Cormorants and gulls on Alcatraz in 2006 increased, while guillemot and Pelagic Cormorant populations decreased. Overall productivity of seabirds was lower than long-term averages, likely affected by the second consecutive year of negative marine climate anomalies. Productivity of Brandt’s Cormorants was 1.9 chicks fledged/pair in 2006, lower than the average 2.1 chicks fledged/pair in the past 9 years. No doublebrooding was attempted in 2006. The Brandt’s Cormorant breeding population, however, increased 23% to 1010 breeding pairs, the largest yet recorded for this colony. In 2006 several sub-colonies expanded, including the Barker Beach sub-colony, the South Bricks sub-colony, and the new sub-colony which formed below the Rubble Piles in 2005. Historically the largest sub-colony, the Southern sub-colony decreased significantly from 96 breeding pairs in 2004 to only 16 breeding pairs in 2006. The mean lay date in 2006 was 7 May, over 1 week later than the long-term mean of 27 April since 1997. The Pelagic Cormorant population decreased by over 50% to only 6 breeding pairs in 2005 and had 7 pairs in 2006. Pelagic Cormorant productivity was only 1.0 chicks fledged/pair, below the 6-year average of 2.0 since 1999. Western Gull breeding numbers increased slightly to 1033 breeding pairs in 2006 (not including nests removed under the NPS depredation permit), only a 2% increase. Western Gulls fledged an average of 1.4 chicks/pair in 2006, lower than the long-term mean. We confirmed only 16 Pigeon Guillemot sites in 2006, 9 more than in 2005, but still less than the 33 confirmed sites in 2004. One pair of Black Oystercatchers bred on the Seawall in 2006; and for the first time in recorded history were successful in fledging 2 chicks in one season. We first discovered California Gulls breeding on Alcatraz in 2004. Since then, this small colony behind the rubble piles on the Parade Ground has grown to 21 pairs and fledged an average of 1.7 chicks fledged/pair in 2006. We documented 27 disturbances to Brandt’s Cormorants in 2006 during 363 hours of observation. The majority of disturbance was due to humans on the island, most often due to the removal of the visual barrier just outside of the Model Industries Building. The rate of disturbance was lower than that in the past seven 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 which is at risk of disappearance from Alcatraz. We also recommend the re-installation of a visual barrier at the Model Industries Building.

Ecological Studies of Seabirds on Alcatraz Island, 2006

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