Eggshell Thinning and DDE

Abstract

Blus, Gish, Belisle and Prouty allege that DDE causes eggshell thinning in brown pelicans and that the relationship is logarithmic¹. In support of their conclusion that a cause and effect relationship between DDE and shell thinning exists, they offer as evidence Wiemeyer and Porter's² work with kestrels. A critical review, however, shows that Wiemeyer and Porter have not shown DDE causes shell thinning; on the contrary, the thinnest eggshells in both years of that study were laid by the control group. The control birds laid eggs with shells ranging in thickness from 130 µm–210 µm in both years, while the DDE-treated birds laid eggs ranging in thickness from 165 µm–203 µm and 153 µm–185 µm for the respective years of the study. Blus et al. also state that DDE levels reported in Wiemeyer and Porter's experimental kestrels approximate environmental levels found in wild falcons. Such a claim is not borne out by Cade et al.³ who have performed extensive tissue analysis, and a comparison shows that the levels in the experimental kestrels exceed levels accumulated in wild falcons by a factor of about two. Despite this, Wiemeyer and Porter only report mean shell thinning of 9.7%, considerably less than the 17% and 35% shell thinning reported by Blus et al. in the South Carolina and California brown pelican colonies. To support their conclusions, the authors state that concentrations of residues in the female determine shell thickness, a claim which is unreferenced, largely hypothetical, and without consideration of contradictory experimental evidence.