Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993. Nests are monitored annually during repeated visits, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area (Kelly et al. 1993, 2006, 2007, 2008a).

Heron and egret monitoring results at West Marin Island: 2011 nesting season

Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993. Nests are monitored during repeated visits each year, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area.

Great Blue Herons (Ardea herodias) and Great Egrets (Casmerodius albus) are among the species known to have suffered eggshell thinning as a result of accumulating high levels of DDT and its derivatives in their body tissues (Vermeer and Reynolds 1970, Henny and Bethers 1971, Faber et al. 1972). Great Egret reproductive success in the San Francisco Bay region was recently diminished because eggs broke during incubation (Pratt 1972a) while the effect of eggshell thinning on reproductive success of Great Blue Herons was insignificant.

Breeding of Great Blue Herons and Great Egrets at Audubon Canyon Ranch, California, 1972-1973

A study of the breeding biology of Great Blue Herons (Ardea herodias) and Common Egrets (Casmerodius albus) was started in 1967 at Audubon Canyon Ranch, Marin County, California, where both species nest in the same heronry. I have reported on results of the first 2 years (1967, 1968) of the study (Pratt 1970). This paper presents findings on nesting success for 1969 and 1970 and adds data on clutch size for all 4 years. To facilitate comparison of nesting success with the 2 prior years, I include some previously published data.

Nesting success of Common Egrets and Great Blue Herons in the San Francisco Bay region

Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993. Nests are monitored during repeated visits each year, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area.

1993 heron and egret monitoring results at West Marin Island

Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993. Nests are monitored during repeated visits each year, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area.

Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993. Nests are monitored during repeated visits each year, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area.

1994 heron and egret monitoring results at West Marin Island

Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993. Nests are monitored during repeated visits each year, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area.

1995 heron and egret monitoring results at West Marin Island