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.

Faunal Remains in California Condor Nest Caves

“…Our findings appear to be inconsistent with the conclusion of Snyder and Meretsky (2003) that a reduced thickness of California Condor eggshells is an association with smaller eggs rather than an effect of an environmental factor. We have, however, demonstrated very significant differences in both eggshell thickness and structure between recent eggs laid in central and southern California; we now show that there is no relationship between eggshell thinning of the central California eggs and egg size.…”

“…The mean shell thickness of the eggs laid by TAMA (0.77 ± .06 mm, n = 8), was 12% higher than the pre-1944 mean and 54% higher than the mean shell thickness of eggs laid by AC8 in the wild in the 1980s (0.50 ± 0.04 mm, n = 8). The mean shell thickness of eggs of the Santa Barbara female was intermediate (0.60 ± 0.04 mm, n = 10, Snyder and Meretsky 2003).…”

“…We are undertaking follow-up studies to determine whether this pattern is typical of condor eggs. Snyder and Meretsky (2003) stated, without supporting data, that eggshell thickness is usually "greater at the equator than at the poles." Although shell fragments were taken from the equatorial region of the intact eggs from central California, it was not possible to determine the area of the egg from which the relatively small southern California fragments had come.…”

“…Twenty six of these eggs were produced by only three females, the Santa Barbara female, TAMA, and AC8 (Adult Condor 8) (studbook numbers 10, 11, and 12, respectively, designated by Snyder and Meretsky (2003) as SBF, CCF, and SSF. An additional egg reported by Snyder and Meretsky (2003) to have been laid in 1980 by AC8 is attributed by the studbook (Mace 2010) only to a "wild" female and is not included. In this sample, a high variability of shell thickness was associated with variation in egg size among individual females.…”

The history of Patuxent: America’s wildlife research story

“…Neither was there any evidence of chronic failure of eggs to hatch after fullterm incubation. As documented in Snyder and Meretsky (2003), the correlation between eggshell thickness and DDE levels in eggshell membranes was weak; instead, eggshell thickness was highly correlated with egg size, indicating that the thin eggshell fragments collected in the 1960s could have come from relatively small eggs rather than from structurally weak eggs. One female in the 1980s was laying eggs whose shell thickness was nearly 25 percent less than the historical mean, but her eggs were also very small and she was the most successful female of her period in producing fledglings.…”