Reproductive Success of the Black-crowned Night Heron at Alcatraz Island, San Francisco Bay, California, 1990-2002

Hothem, Roger; Hatch, Daphne

Nesting chronology, habitat use, subcolony use, and hatchability were documented for the Black-crowned Night Heron (Nycticorax nycticorax) nesting on Alcatraz Island, San Francisco Bay, California 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 of declining numbers since 1996. An increase in the numbers of Western Gull (Larus occidentalis), the Black-crowned Night Heron's primary competitor, occurred during the same period. Overall reproductive success of the Black-crowned Night Heron on Alcatraz Island was below the 13 year average of 56.4% since 1996. During the study, the average number of chicks fledged per nest each year ranged from 0.46 to 1.27, which is less than the two chicks per nest suggested as a requirement for sustained population. Embryos in five of the 187 failed Black-crowned Night Heron eggs were deformed. In 1990 and 1991, eggs were analyzed for a wide range of contaminents, but none appeared to be sufficiently elevated to have caused the observed deformities. Based on these relatively low level of contaminents, a high hatchability rate (94.5%), and relatively low levels of embryotoxicity, contaminents did not appear to significantly affect Black-crowned Night Heron reproduction at Alcatraz Island. However, predation by the Common Raven (Corvus corax) and Western Gull, interspecific competition with the Western Gull, habitat deterioration, and possible human disturbance are likely factors contributing to the decline in the Black-crowned Night Heron reproductive success on Alcatraz Island in recent years.

Reproductive Success of the Black-crowned Night Heron at Alcatraz Island, San Francisco Bay, California, 1990-2002

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