Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California: 2007

Hothem, Roger; Bergen, Darrin; Hellwig, Christian

As part of a program initiated in 1990, we documented nesting chronology, habitat use, subcolony use, hatchability, and reproductive success for black-crowned night-herons (Nycticorax nycticorax) and snowy egrets (Egretta thula) nesting at Alcatraz Island, San Francisco Bay, California. In 2007, we found 202 night-heron nests, a 44% increase from the 140 nests found in 2006, and nearly twice the number found in 2005. We also found 79 snowy egret nests, more than twice the previous high of 36 recorded in 2005. As in the previous three years, all snowy egret nests were located in the Greenhouse subcolony. Black crowned nightheron incubation-period success at Alcatraz Island in 2007 was 29%, nestling-period success was 83%, and overall reproductive success was 24%, all decreases from 2006. All success rates were lower than the 18-year averages, and incubation-period success and overall success were the lowest recorded during the study. A primary cause of this poor reproductive success was the elevated rate of nest predation (40%), the highest observed during the 18-year study and more than twice the overall mean (18.5%). Incubation-period success (89%), nestling-period success (98%), and overall success (87%) for snowy egrets were higher than the night-herons and were similar to those observed for snowy egrets the previous two years. Predation on snowy egret nests was low (6%). The most common habitats for night-heron nests in 2007 were English ivy (Hedera helix) (27%), mirrorbush (Coprosma baueri) (16%), Albizia sp. (15%), and rubble (concrete, reinforcing rods, and other remnants of demolished buildings) (10%). Snowy egrets nested primarily in Albizia (48%) and fig (Ficus carica) (34%). The trend of increasing numbers of the Western Gull (Larus occidentalis), a predator on night-heron and snowy egret eggs and chicks and a competitor for nest sites, continued in 2007, with the estimated total number of gull nests reaching an all-time high of 1,060. We assumed that much of the predation, especially on night-heron eggs and chicks, was by gulls, based on the close proximity of the two species’ nests and the highly aggressive nature of the gulls to both adult and young night-herons. Predation on night-heron eggs by common ravens (Corvus corax), which was frequently observed, likely also reduced nesting success. In 2007, the Alcatraz colony had the most night-heron and snowy egret nests of all the North San Francisco Bay colonies. Within the Central Bay area, the percentage of night-herons nesting at Alcatraz increased from 41% in 2006 to 64% in 2007, while the percentage of snowy egrets increased from 23% in 2006 to 65% in 2007.

Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California: 2007

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