All Hands Ecology (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 References Cited).
As in other years, the colony site was occupied in 2015 by nesting Great Blue Herons, Great Egrets, Snowy Egrets, and Black-crowned Night-Herons; however, all of the nesting Snowy Egrets and Black-crowned Night-Herons abandoned the site before completing the nesting cycle. Small numbers of Great Blue Herons and Great Egrets persisted to successfully fledge their young. The cause of the abandonment is unknown, but we provide some details and possible explanations below.