Reproductive Success of Black-crowned Night-Herons (Nycticorax nycticorax) at Alcatraz Island, San Francisco Bay, California, 1990-2002

Hothem, Roger

Nesting chronology, habitat use, subcolony use, and hatchability were documented for Black-crowned Night-Herons (Nycticorax nycticorax) nesting at Alcatraz Island during 1990-2002. Reproductive success was estimated using the Mayfield method and compared among years. Totals of monitored nests per year ranged from 68 in 2001 to 341 in 1996, with a trend toward declining numbers since 1996. A trend of increasing numbers of nests of Western Gulls, the primary competitor with the Black-crowned Night-Herons, was identified during the same period. Calculated nest success has generally declined since 1994, with success less than the 13-year average of 63.9% each of the last six years. Although fledging success declined to its nadir in 1997, the 13-year average was 87.6%, and the rate has generally increased during the past 5 years. The net result, however, has been that the overall reproductive success of Black-crowned Night-Herons at Alcatraz Island has been below the 13-year average of 56.3% since 1996. During the study, the average number of chicks fledged per nest ranged from 0.46 to 1.27 chicks per monitored nest, far below the required two chicks per nest required for a sustained population. Embryos in 5 of the 187 failed Black-crowned Night-Heron eggs that were examined contained deformed embryos. In 1990 and 1991 eggs were analyzed for a wide range of contaminants, but none appeared to be sufficiently elevated to have caused the observed deformities. Based on these relatively low levels of contaminants, a high hatchability rate (94.5%), and relatively low levels of embryotoxicity, contaminants did not appear to significantly affect Black-crowned Night-Heron reproduction at Alcatraz Island. However, predation by Common Ravens (Corvus corax) and Western Gulls (Larus occidentalis), interspecific competition with the Western Gulls, habitat deterioration, and possible human disturbance are likely factors contributing to the decline in Black-crowned Night-Heron reproduction on Alcatraz Island in recent years.