Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 2010 Annual Data Summary

Hothem, Roger; Brussee, Brianne; Coates, Peter

As part of a monitoring program initiated in 1990, we documented nesting chronology, habitat use, subcolony use, hatchability, incubation-period success, and nestling-period success for black-crowned night-herons (Nycticorax nycticorax) and snowy egrets (Egretta thula) at Alcatraz Island, San Francisco Bay, California. In 2010, we monitored 196 night-heron nests, a 37% increase from the 143 nests found in 2009 and 94 snowy egret nests, an increase from the 85 nests found in 2009. As in the previous 6 years, all snowy egrets nested exclusively in the Greenhouse subcolony; night-herons nested in 11 other subcolonies. Black-crowned night-heron incubation-period success in 2010 was 38%, nestling-period success was 76%, and overall reproductive success was 29%, all increases from 2009, but all lower than the 21-year averages. Incubation-period success, nestling success, and overall success were all 100% for snowy egrets, but rainfall and earlier than expected nesting restricted visits to the snowy egret subcolony, and, as a result, the fates of more nests were unknown. Predation on snowy egret nests was not observed in 2010 but could have occurred after monitoring ceased. The percentage of nightheron nests destroyed by predators in 2010 (32%) was similar to the average observed over the past 13 years (30%). To better document nest fate, 21 night-heron nests were monitored using micro-cameras and digital video recorders (DVR). The cameras revealed that ravens preyed on more nests (both eggs and chicks) than previously assumed. The night-heron colony on Alcatraz was likely the largest in the North San Francisco Bay, and the total number was about 2.5 times more than were observed in all of South San Francisco Bay in 2010. The 94 snowy egret nests at Alcatraz in 2010 ranked second to West Marin Island’s 175 nests in North San Francisco Bay, but the total was 30 more than were counted at all the sites surveyed in South San Francisco Bay.