The Black-crowned Night-Heron at Alcatraz Island, San Francisco Bay, California: 2003 Reproductive Success
Hothem RL. 2004. The Black-crowned Night-Heron at Alcatraz Island, San Francisco Bay, California: 2003 Reproductive Success. U.S Geological Survey. Dixon, CA
Nesting chronology, habitat use, subcolony use, and hatchability were documented for the black-crowned night-heron (Nycticorax nycticorax) nesting at Alcatraz Island, San Francisco Bay, California during 2003. Since 1990, total numbers of monitored 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 remained at a similar level (140) in 2003. A trend of increasing numbers of the Western Gull (Larus occidentalis), a predator on the black-crowned night-heron and a competitor for nest sites, continued in 2003, with the total number of gull nests reaching an all-time high of 933. Black-crowned night-heron nest success at Alcatraz Island in 2003 was 50%, fledging success was 90%, and overall reproductive success was 45%. Although fledging success was slightly higher than the average for the previous 13 years (88%), both the nest success and overall success were lower than the 13-year averages (64% and 56%, respectively). Predation, as well as interspecific competition with the Western Gull, habitat deterioration, and possible human disturbance are likely factors contributing to the relatively low numbers of nesting Night-Herons on Alcatraz in 2003.
- Type
- Unpublished Report
- Authors
- Hothem, Roger
- Date of Issue
- 2004
- Publisher
- U.S Geological Survey
- Units
- ALCA , GOGA , SFAN
- Keywords
- birds, black-crowned night-herons, breeding, nest monitoring, nesting, reproductive success
- Subjects
- Ecological Framework: Biological Integrity | Focal Species or Communities | Birds