In 1972 and 1973 we collected 1,339 clutches of eggs of anhingas (Anhinga anhinga) and 17 species of waders (herons, egrets, bitterns, ibises, and storks). We analyzed the eggs for organochlorine residues and compared shell thickness of these eggs and others collected since 1946 (and now in museum collections) with shell thickness of eggs collected before the widespread use of organochlorine pesticides. Eggshells were significantly thinner in one or more regions for the post-1946 samples of anhinga, great blue heron, black-crowned night heron, and wood stork. We detected no significant change in the other species.

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