Significant decreases in eggshell thickness were found in 15 of 22 species of aquatic birds in Texas in 1970. Shell thickness reductions of 9 to 15 percent were found in white pelicans (Pelecanus erythrorhynchos), brown pelicans (P .occidentalis), and great blue herons (Ardea herodias). DDT family compounds were found in all eggs, and mean residues ranged from 0.4 ppm in white ibis (Eudocimus albus) to 23.2 ppm in great egrets (Casmerodius albus). GDDT residues were negatively correlated with shell thickness in five species; PCBs were negatively correlated in two. Residues in marine birds were generally lower and more uniform than levels in birds feeding in fresh and brackish water. DDT and dieldrin residues were higher in eggs from colonies near agricultural areas where these insecticides were heavily used; higher PCB residues were consistently associated with urban and industrial areas. Populations of five species have declined and deserve continued study: brown pelican, reddish egret (Dichromanassa rufescens), white-faced ibis (Plegadis chihi), laughing gull (Larus atricilla), and Forster's tern (Sterna forsteri). Population trends of four other species were undetermined and should be followed closely in future years.
