Reproductive Success of the Black-crowned Night-Heron and Snowy Egret at Alcatraz Island, San Francisco Bay, California: 2004
Hothem RL and Meckstroth AM. 2005. Reproductive Success of the Black-crowned Night-Heron and Snowy Egret at Alcatraz Island, San Francisco Bay, California: 2004. USGS, Western Ecological Research Center. Dixon, CA
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 during 2004. Since 1990, total numbers of monitored night-heron nests per year have ranged from 68 in 2001 to 341 in 1996. There was a significant trend toward declining numbers between 1996 and 2001, but numbers increased in 2002 to 141 and have been relatively stable since then, with 133 nests monitored in 2004. We monitored 32 snowy egret nests in 2004, the highest number in this 15-year study. A trend of increasing numbers of the Western Gull (Larus occidentalis), a predator on the black-crowned night-heron and snowy egret and a competitor for nest sites, continued in 2004, with the total number of gull nests reaching an all-time high of 940. Black-crowned night-heron hatching success at Alcatraz Island in 2004 was 71%, fledging success was 85%, and overall reproductive success was 60%. Although fledging success in 2004 was slightly lower than the average for the previous 14 years (88%), both the hatching success and overall success were higher than the 14-year averages (63% and 56%, respectively). For snowy egrets, hatching success (72%), fledging success (93%), and overall success (67%) were all slightly higher than for night-herons. Predation, interspecific competition with the Western Gull, habitat deterioration, and possible human disturbance are likely factors that adversely affected reproductive success of wading birds on Alcatraz Island in 2004.
- Type
- Unpublished Report
- Authors
- Hothem, Roger; Meckstroth, Anne
- Date of Issue
- 2005-08-01
- Publisher
- USGS, Western Ecological Research Center
- Units
- ALCA , GOGA , SFAN
- Keywords
- Abundance (Biodiversity), Alcatraz Island, Annual Report, Birds, Black-crowned Night-Heron, Distribution (Biodiversity), Disturbance, Egretta thula, Monitoring, Nycticorax nycticorax, Observations, Presence (Biodiversity), Reproductive Success, San Francisco Bay, snowy egret