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 (Kelly and Nur 2015a, b, Kelly et al. 2006).
The Marin Islands supported large numbers of nesting herons and egrets from at least as early as 1979 through 2015 (Table 1). In 2015, all the nesting Snowy Egrets and Black-crowned Night-Herons abandoned the site before completing the nesting cycle and there were substantially reduced numbers of Great Egrets and Great Blue Herons (Kelly and Fischer 2018). Since the 2016 season, West Marin Island has supported zero nesting herons or egrets, and East Marin Island has supported only small numbers of nesting Great Blue Herons. Here I present my detailed observations of heron and egret nesting activity on the Marin Islands in 2019.