Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets at Marin Islands since 1979, and the annual reproductive success of Great Egrets and Great Blue Herons there since 1993. Nests are monitored annually, during repeated visits, from viewing positions on East Marin Island and by boat. This work is part of an ongoing, regional study of heron and egret colonies in the northern San Francisco Bay area (see previous reports in References Cited).
In 2015, all of the nesting Snowy Egrets and Black-crowned Night-Herons abandoned their nest sites on West Marin Island before completing the nesting cycle and, by 2 June 2015, only five Great Blue Heron nests and 26 Great Egret nests remained active (Kelly et al. 2015). The cause of the 2015 abandonments is unknown. In 2016, we found no evidence of successful nesting by herons or egrets on West Marin Island, where widespread nest failure resulted in overall reproductive success of zero for Great Egrets, Snowy Egrets, and Black-crowned Night-Herons at the Marin Islands (Kelly et al. 2017). However, seven Great Blue Heron nests were established on East Marin Island and a few of those nests were apparently successful in fledging young. In this report, we present our observations of heron and egret nesting activity on the Marin Islands in 2017.