Reproductive success of a colony of common egrets (Casmerodius albus) in California declined between 1967 and 1970. Successful nesting attempts decreased from 52 to 28%, and nests losing eggs increased from 30 to 54%. Reproductive success of great blue herons (Ardea herodias) in this colony showed no comparable trends over this period. Mean thicknesses of all egret and heron eggshells recovered from the floor of the heronry were 15·2 and 10·4% lower, respectively, than pre-1947 values. Eggs of both species, broken during incubation, were 17% below normal thickness.