California Condors and DDE: a re‐evaluation

Abstract

Eggshells of wild California Condors Gymnogyps californianus were much thinner in the 1960s, when DDT was used heavily, than during earlier pre‐DDT and later reduced‐DDT periods. However, eggshell thickness was more strongly linked to egg size (mass) than to measured levels of p,p′DDE (the primary metabolite of DDT). Egg size was consistent within individual females and yielded correlation coefficients with shell thickness ranging from 0.49 to 0.97, depending on the period and the analysis assumptions used. Measured DDE levels, although often substantial, provided only a weak correlation (r = −0.33) with shell thickness. In part, the absence of a strong DDE/thickness correlation may have been an artefact of losses of DDE from fragment membranes over time. Nevertheless, the extreme (28–29%) shell thinning of the 1960s was not linked with clearly increased egg‐breakage or nest‐failure rates, and one female of the 1980s with 25.6% shell thinning was the most productive female of her era. Some eggs with over 30% shell thinning hatched successfully, and broken eggs closely resembled hatched eggs in shell thickness, strongly suggesting that shell thinning was not an important cause of breakage. The apparent absence of harmful effects from the extreme shell thinning of the 1960s may have resulted from (1) the fact that historic pre‐DDT condor eggs were on average 16.7% thicker shelled for their mass than predicted by the overall egg mass/shell thickness curve for birds, and (2) a possible egg‐size decline or sampling bias toward small‐egged females in the 1960s. That DDE was an important cause of the Condor's decline appears unlikely from overall available data.