Reproductive Success of the Black-Crowned Night Heron and Snowy Egret at Alcatraz Island, San Francisco Bay, California: 2005

Hothem, Roger; Feltman, Skylar

As part of a monitoring program begun in 1990, we documented nesting chronology, habitat use, subcolony use, and hatchability for the black-crowned night heron (Nycticorax nycticorax) and snowy egret (Egretta thula) nesting at Alcatraz Island, San Francisco Bay, California. We monitored 102 night-heron nests in 2005, the second fewest recorded during the 16-year study. We also monitored 36 snowy egret nests, the most recorded in any year of the study. Black-crowned night heron hatching success at Alcatraz Island in 2005 was 42%, fledgling success was 81%, and overall reproductive success (34%) was the lowest recorded in the study. As in 2004, all snowy egret nests were located in the Greenhouse Subcolony. Hatching success (91%), fledgling success (100%), and overall success (91%) were all higher for snowy egrets than for night-herons. For the third consecutive year, the Greenhouse had more night-heron and snowy egret nests than any other subcolony. The predation rate (31%) on night-heron nests was similar to that found each year since 1998, while the presumed predation rate on hatched nests (13%) was similar to the highest rates observed in the study. Fewer unknown nest fates (14%) were recorded than the average (19%), but the percentage unknown fledgling success (28%) was similar to that aveage for the 16 years of the study (30%). In 1992, 78.5% of the night-heron nests were in Mirrorbush (Coprosma baueri), but, primarily due to plant senescence, only 2% of the nests were found this habitat in 2005. Nesting by night-herons in 2005 was primarily in rubble (concrete, reinforcing rods, and other remnants of demolished buildings) (32.4%) and rose (Rosa spp.) (20.6%). A trend of increasing numbers of the Western Gull (Larus occidentalis) continued in 2005, with the total number of gull nests reaching an all-time high of 1007. Adverse interactions with gulls, including predation on black-crowned night-heron eggs and chicks and competition for nest sites, especially in the rubble subcolonies, appeared to be the most likely factor to limiting reproductive success of the night-herons on Alcatraz Island in 2005. Common ravens (Corvus corax) again nested on Alcatraz and likely contributed to predation on an unknown number of night-heron nests. Despite fewer night-heron nests on Alcatraz Island than in most previous years, the total found on Alcatraz Island in 2005 still represented about 15% of the North San Francisco Bay population and about 12% of the total San Francisco Bay breeding population. In addition, the total number of nests of the snowy egret breeding population at Alcatraz comprised nearly 12% of the North Bay nests in 2005.

Reproductive Success of the Black-Crowned Night Heron and Snowy Egret at Alcatraz Island, San Francisco Bay, California: 2005

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