Like all birds dependent on freshwater and marine wetlands, members of the heron family are suffering progressive loss of essential habitat. In addition, agricultural and industrial pollution threatens their capacity to reproduce. Great Blue Herons (Ardea herodias) and Black-crowned Night-Herons (Nycticorax nycticorax) have declined in parts of their range and have been placed on the National Audubon Society's Blue List of species "which have recently given or are currently giving indications of non-cyclical population declines or range contractions either locally or widespread" (Tate 1981). Most herons nest in conspicuous, often widely separated colonies. Some colony sites are used repeatedly for many consecutive nesting seasons; others are deserted after a few years and new colonies are established elsewhere. Sites are sometimes reoccupied after several years of inactivity and the number of birds may increase or decrease dramatically. Evaluation of population trends requires knowledge of both the history of active and inactive colonies within a region and the population changes in individual colonies. This paper presents data on the location, population level, and history of 11 heron colonies known to have been occupied between 1967 and 1981 in Marin County, California (Figure1). Its purpose is to provide a baseline for future comparisons by indicating current and past status.