Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 2008
Hothem RL and Brussee BE. 2008. Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 2008. U.S. Geological Survey. Sacramento, CA
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 2008, we monitored 101 night-heron nests, a 50% decrease from the 202 nests found in 2007. We monitored 82 snowy egret nests, a number similar to the 79 nests found in 2007. As in the previous 4 years, all snowy egret nests were located in the Greenhouse subcolony. Black-crowned night-heron incubation-period success at Alcatraz Island in 2008 was 39%, nestling-period success was 91%, and overall reproductive success was 35%, all increases from 2007. Incubation-period success and overall success, however, were lower than the 19-year averages. A primary cause of this poor reproductive success was the elevated rate of nest predation (29%), which was higher than the 19-year average (19%). Incubation period success (86%), nestling-period success (100%), and overall success (86%) for snowy egrets were all similar to rates observed in the previous two years. Predation on snowy egret nests was low (7%). The most common habitats for night-heron nests in 2008 were rubble (concrete, reinforcing rods, and other remnants of demolished buildings) (36%) and English ivy (Hedera helix) (26%). Snowy egrets nested primarily in Albizia (36%) and fig (Ficus carica) (46%). The numbers of breeding gulls on Alcatraz increased for the tenth year in a row, with 1061 Western Gull (Larus occidentalis) and 21 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 2008. 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 2008 represented about 20% of the total North San Francisco Bay population, while the total number of snowy egret nests at Alcatraz comprised about 24% of the North Bay nests.
- Type
- Unpublished Report
- Authors
- Hothem, Roger; Brussee, Brianne
- Date of Issue
- 2008
- Publisher
- U.S. Geological Survey
- Units
- ALCA , GOGA , SFAN
- Keywords
- Abundance (Biodiversity), Annual Report, Ardeidae (Herons, Aves (Birds), Bitteens), Ciconiiformes, Distribution (Biodiversity), egrets, Egretta thula (Snowy Egret), Monitoring, Nycticorax nycticorax (Black-crowned Night-heron), Observations, Presence (Biodiversity), Reproduction, Success