Long-term decline in the thickness of eggshells of thrushes, Turdus spp., in Britain

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The thickness of eggshells of four species of thrush, Turdus spp., was estimated by an index based on the mass and linear dimensions of blown eggs in museum collections from Britain. Shell thickness was also measured directly for two species and was highly correlated with the index. Widespread declines in eggshell thickness since the nineteenth century were found in all species. There have been no previous reports of trends in eggshell thickness of this long duration and large spatial scale. The cause of the declines is unknown, but, for three of the four species, eggshell thinning began before the introduction of the organochlorine pesticide DDT, which caused eggshell thinning in predatory and fish–eating birds from 1947 onwards. The effect of acid deposition on the availability of calcium–rich prey is a plausible explanation.