Residues of organochlorine compounds occur commonly in environmental samples and have been associated with adverse effects in numerous avian species (Cooke 1973; L. F. Stickel 1973; W. H. Stickel 1975; Ohlendorf et al. 1977; H. M. Ohlendorf, R. W. Risebrough, and K. Vermeer, unpublished manuscript). The affected species are usually terminal consumers, generally those feeding on aquatic organisms (primarily fish) or birds.

Brown Pelicans (Pelecanus occidentalis), Double-crested Cormorants (Phalacrocorax auritus), and certain herons are among the species that have exhibited shell thinning in some regions (see Ohlendorf et al. 1977 for review). In 1972, we began a study to determine: (1) geographic differences in the occurrence of environmental pollutants in Anhingas (Anhinga anhinga) and waders in the eastern United States; (2) differences in environmental pollutant levels among those species nesting at the same localities; and (3) whether eggshell thickness had changed since the widespread use of organochlorine pesticides began in the mid-1940's.

The wader species included in our study were: Great Blue Heron (Ardea herodias), Green Heron (Butorides virescens), Little Blue Heron (Florida caerulea), Cattle Egret (Bubulcus ibis), Reddish Egret (Dichromanassa rufescens), Great Egret (Casmerodius albus), Snowy Egret (Egretta thula), Louisiana Heron (Hydranassa tricolor), Black-crowned Night Heron (Nycticorax nycticorax), Yellow-crowned Night Heron (Nyctanassa violacea), Least Bittern (Ixobrychus exilis), American Bittern (Botaurus lentiginosus), Wood Stork (Mycteria americana), Glossy Ibis (Plegadis falcinellus), White Ibis (Eudocimus albus), and Roseate Spoonbill (Ajaia ajaja).