Ecological Studies of Seabirds on Alcatraz Island, 2007

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 2007, 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, 2007. Populations of Brandt’s Cormorants, gulls, and Pigeon Guillemots on Alcatraz in 2007 increased, while the Pelagic Cormorant population decreased. Overall productivity of seabirds was lower than long-term averages. The Brandt’s Cormorant breeding population increased by 76% since 2006 to 1782 breeding pairs, the largest yet recorded for this colony. In 2007, several sub-colonies expanded, including the North Foghorn, Barker Beach, and Northern sub-colonies, while the Laundry Building sub-colony decreased significantly. Productivity of Brandt’s Cormorants was 2.0 chicks fledged/pair in 2007, slightly lower than the average 2.1 chicks fledged/pair in the past 9 years. The mean lay date in 2007 was 30 April. The Pelagic Cormorant population has decreased by 70% since 1996 to only 6 breeding pairs in 2007. Pelagic Cormorant productivity was only 0.4 chicks fledged/pair, an all time low and well below the 7-year average of 1.7 since 1999. Western Gull breeding numbers increased only slightly to 1037 breeding pairs in 2007 (not including nests removed under the NPS depredation permit) from 1033 pairs in 2006. Western Gulls fledged an average of 1.3 chicks/pair in 2007, lower than the long-term mean. We confirmed 21 Pigeon Guillemot sites in 2007, 5 more than in 2006, but still less than the peak of 33 confirmed sites in 2004. One pair of Black Oystercatchers bred on the Seawall in 2007, but after losing the eggs they moved for the first time to the east side of the island near the Powerhouse where they relaid but failed a second time. 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 23 pairs and fledged an average of 1.6 chicks fledged/pair in 2007. We documented 64 disturbances to Brandt’s Cormorants in 2007 during about 308 hours of observation. The majority of disturbance was due to kayaks and canoes, as well as other marine-based causes. In particular gunshots from sailboat races held in the bay became a new cause of disturbances throughout the season. The rate of disturbance was at its highest since 2003 and we are beginning to see that cumulative effects of disturbance may cause increased behavioral sensitivity in Brandt’s Cormorants. It also potentially takes only one ill-timed disturbance to 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, 2007

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