The incidence of DDT (parent compound) declined significantly in the eggs of Black-crowned Night-Herons (Nycticorax nycticorax), White-faced Ibis (Plegadis chihi), and Black-necked Stilts (Himantopus mexicanus), and showed a downward trend in Snowy Egrets (Egretta thula). Mean DDE residues (breakdown product of DDT) in eggs declined significantly in Black-crowned Night-Herons and showed a downward trend in Black-necked Stilts and Snowy Egrets; however, no trend was apparent for White-faced Ibis. Some night-herons, egrets, and ibis laid thin-shelled eggs that cracked; eggshell thickness was negatively correlated with DDE residues in the egg contents. Ibis were the most sensitive species to DDE- induced shell thinning. Generally, reduced reproductive success for ibis started at lower DDE egg residues (3 ppm) than for either Snowy Egrets (5 ppm) or night-herons (8 ppm). Egg residue profiles, band recoveries, biotelemetry studies, and residues in fish and other prey items implicate the southwestern United States (a wintering area) as an important source of DDT-DDE that caused the most serious reproductive problem encountered during the study (in night-herons nesting at Ruby Lake, Nevada). Available evidence indicates that the three other species, together with night- herons from Oregon and Idaho, winter in Latin America.
