Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 2006

Hothem, Roger; Bergen, Darrin

As part of a monitoring program initiated in 1990, we documented nesting chronology, habitat use, subcolony use, and hatchability for black-crowned night-herons (Nycticorax nycticorax) and snowy egrets (Egretta thula) nesting at Alcatraz Island, San Francisco Bay, California. In 2006, we monitored 140 night-heron nests, a 37% increase from the 102 nests found in 2005. We also monitored 35 snowy egret nests, a number similar to the 36 nests found in 2005. As in 2004 and 2005, all snowy egret nests were located in the Greenhouse subcolony. Black-crowned night-heron hatching success at Alcatraz Island in 2006 was 54%, fledging success was 86%, and overall reproductive success was 47%, all increases from 2005, but all somewhat lower than the 17-year averages. Hatching success (92%), fledging success (100%), and overall success (92%) for snowy egrets were all similar to those observed in 2005, and all were higher than for night-herons both years. Although the nest predation rate (21%) on night-heron nests was slightly higher than the 17-year mean (17%), it was 5-11% lower than the previous 7 years. The percentage of nests with unknown hatch (31%) and fledging success (34%) were both higher than the average for the 17 years of the study. This was likely related to an earlier cessation of nest monitoring in 2006 than in previous years to prevent adverse interspecific conflicts between gulls and night-herons and egrets. About one-half of all night-heron nests monitored in 2006 were in either English ivy (Hedera helix) (26%) or rubble (concrete, reinforcing rods, and other remnants of demolished buildings) (23%). The trend of increasing numbers of the Western Gull (Larus occidentalis), a predator on night-heron and snowy egret eggs and chicks and a competitor for nest sites, continued in 2006, with the estimated total number of gull nests reaching an all-time high of 1,054. As found in previous years, interspecific competition with gulls was the most likely factor limiting reproductive success of the nightherons on Alcatraz Island in 2006. Common ravens (Corvus corax) again nested on Alcatraz and likely contributed to predation on an unknown number of night-heron nests. The total number of night-heron nests found on Alcatraz Island in 2006 represented about 20% of the total North San Francisco Bay population. In addition, the total number of snowy egret nests at Alcatraz comprised about 9.5% of the North Bay nests in 2006.