Eggshell pigmentation indicates pesticide contamination.
Persistent pesticide residues remain a significant environmental pollutant, reducing breeding success of predatory birds even where such pesticides have been banned for many years. The ability to detect such residues inexpensively without destructive sampling would be a valuable tool for ecotoxicological monitoring. We suggest that assaying eggshell colour might provide such a tool. Recent studies have revealed two mechanisms affecting eggshell pigmentation in, and believed to be specific to, passerine birds. In the great tit Parus major, reddish protoporphyrin speckles on their otherwise white eggs coincide precisely with thin shell and are related to calcium shortage. In the pied flycatcher Ficedula hypoleuca, the intensity of the eggshell's blue-green ground-colour (pigment biliverdin) is related to the female's health (immunocompetence) at laying. We studied the effect of the insecticide DDT (notorious for reducing eggshell thickness in raptors by blocking calcium availability to the shell gland) on the eggshell pigmentation of Eurasian sparrowhawks Accipiter nisus (Falconiformes), whose eggshells show both protoporphyrin spotting and a blue-green biliverdin-based ground colour. Eggshells showing protoporphyrin spots as an internalized pigment layer also showed a strong correlation between DDE levels (DDT metabolite) and shell thickness. The shells were also significantly thinner at these spots. Eggshell thickness was uncorrelated with DDE levels in eggs that lacked such spotting. The green-ness of the ground colour, related to biliverdin pigment, also increased with DDE levels but, unlike the protoporphyrins, this increase was independent of eggshell thickness. We also noted that the colour of eggs collected before 1930 was consistent with the environmental absence of DDT. Synthesis and applications. DDT occurrence in predatory birds indicates ecosystem-wide contamination. Correlation between eggshell colour and DDT concentration indicates that pigmentation might be used as a non-destructive assay for DDT (which is still in widespread use, particularly for combating malaria) in eggs. Further studies should allow our findings to be applied to other raptor species. Our results corroborate the associations of protoporphyrin pigmentation with shell thinning and biliverdin with female immunocompetence described for passerines, but their occurrence in a falconiform implies that these effects evolved long before the appearance of the order Passeriformes.