Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 2009

Hothem, Roger; Brussee, Brianne

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 2009, we monitored 143 night-heron nests, a 42% increase from the 101 nests found in 2008. We monitored 85 snowy egret nests, a number similar to the 82 nests found in 2008. As in the previous 5 years, all snowy egret nests were located in the Greenhouse subcolony. Black-crowned night-heron incubation-period success at Alcatraz Island in 2009 was 24%, nestling period success was 80%, and overall reproductive success was 19%, all decreases from 2008 and all lower than the 20-year averages. Incubation-period success and overall success were the lowest observed during the 20-year study. A primary cause of this poor reproductive success was the elevated rate of nest predation (46%), more than two times higher than the 20-year average (20.4%), and the highest rate of predation in the 20-year study. Incubation-period success (75%) and overall success (75%) for snowy egrets were lower than rates observed in the previous four years; nestling success (100%) was similar to previous years. Predation on snowy egret nests was low (7%). The most common habitats for night-heron nests in 2009 were century plant (Agave americana) (28%) and English ivy (Hedera helix) (28%), followed by rubble (concrete, reinforcing rods, and other remnants of demolished buildings) (24%). Snowy egrets nested primarily in plume albizia (Paraserianthes lophantha) (33%) and fig (Ficus carica) (36%). The numbers of breeding gulls on Alcatraz decreased for the first time since 1998, with 888 Western Gull (Larus occidentalis) and 14 California Gull (Larus californicus) nests counted. As found in previous years, interspecific competition with gulls was the most likely factor limiting reproductive success of the night-herons on Alcatraz Island in 2009. 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 2009 exceeded the total found in South San Francisco Bay and comprised about 10% of the total North San Francisco Bay population. The total number of snowy egret nests at Alcatraz was similar to the total found in the South San Francisco Bay and comprised about 21% of the North Bay snowy egret nests.