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.

In 1972 the Environmental Protection Agency imposed an almost total ban on use of DDT, but even before this there were local restrictions on its use. The study reported on here has continued without interruption since 1967, allowing a comparison of reproductive success in these two affected species before and after restrictions on use of DDT.

This paper presents observations on nesting success, population levels, laying dates and clutch size of Great Blue Herons and Great Egrets during the 1972 and 1973 nesting seasons at Audubon Canyon Ranch near Stinson Beach, Marin County, California. I reported results for the earlier years elsewhere (Pratt 1970, 1972a, 1972b). Relevant data from the first five years of the study are included here for comparison. Nesting success of Great Blue Herons has been reported for Canada (Vermeer 1969, McAloney 1973) and for Oregon (Henny and Bethers 1971). Teal (1965) reported on nesting success of Great Egrets in Georgia.