USGS1983
Post-DDT (post-1947) eggshell thickness was examined in samples of Turkey Vulture, Black Vulture, and Crested Caracara eggs from several parts of the United States. Highly significant post-DDT decreases in eggshell thickness indices of at least 10 percent were found in Turkey Vulture eggs from California, Florida, and Texas and in Black Vulture eggs from Texas and Florida. Over one-third of the Black VUlture eggs and about 30 percent of the Turkey Vulture eggs from Texas showed thinning exceeding 20 percent, a level associated with reproductive failure and population decline in other species. A strong negative correlation was found between eggshell thickness indices and DDE residues extracted from eggshell membranes in California and Texas samples of Turkey Vulture eggs and in Texas Black Vulture eggs. Crested Caracara eggs from Texas and Florida showed mean changes in eggshell thickness indices of only -5.6 and -8.2 percent, respectively, although thinning in a few eggs from both states exceeded 20 percent. Most of the post-DDT Old World vulture eggs examined appeared to be of normal thickness, with low DDE residue levels in eggshell membranes; but single eggs of Egyptian Vulture from India, Cinereous Vulture from Spain, and White-headed Vulture from Zambia showed apparent thinning. Further monitoring of vulture populations in tropical regions, where DDT use is still increasing, is recommended.