BREEDING OF GREAT BLUE HERONS AND GREAT EGRETS AT AUDUBON CANYON RANCH, CALIFORNIA, 1972-1973

Abstract

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.