Night-Herons on Alcatraz Island
Project Status: Ongoing
The U.S. Geological Survey has been monitoring black-crowned night-herons (Nycticorax nycticorax) and snowy egrets (Egretta thula) nesting on Alcatraz Island in the San Francisco Bay since 1990. The monitoring aims to describe how many birds nest on the island, where they nest, their reproductive success, and to detect any trends in the nesting populations. Among the goals of this monitoring is to provide management recommendations for the protection of herons and egrets on the heavily visited island.
For black-crowned night herons, monitoring of 196 nests in 2010 revealed that overall reproductive success was 29 percent; higher than in 2009, but lower than the 21-year average. Thirty-two percent of nests were destroyed by predators, and video monitoring of several nests revealed that ravens (there is one pair that nests on Alcatraz) were responsible for more of this predation than was previously assumed. The night heron nests were spread out across 11 sub-colonies around Alcatraz.
Monitoring of 94 snowy egret nests showed their reproductive success to be 100 percent, although monitoring was not as complete as usual because rainfall and earlier than average nesting allowed for fewer monitoring visits than normal. No predation of snowy egret nests was observed, and as in recent years, all of the egret nests were found in one section of the island.
Nest counts reveal that Alcatraz is a very significant breeding location in the San Francisco Bay area. Alcatraz’s night heron colony was likely the largest colony in the region in 2010, and the snowy egret colony was second in size only to the West Marin Island colony.