Shell thinning in avian eggs by environmental pollutants

Abstract

Literature has been reviewed concerning shell thinning in avian eggs by environmental pollutants. Field evidence indicates that the declines in shell thickness observed in certain species in North America and Great Britain since the Second World War have been largely caused by residues of pp′-DDE or other compounds or metabolites of the DDT group. In North America, polychlorinated biphenyls and cyclodiene insecticides have played no more than minor roles, although in Britain cyclodienes have probably made a significant contribution. Mercury compounds are not apparently associated with the shell thickness declines. Results from controlled experiments, in which laying birds have been exposed to pollutants, generally support these suggestions. Laboratory investigations indicate an interspecific difference in shell thinning response, gallinaceous species tending to be the most resistant and falcons the most susceptible. In a laying bird, organochlorine residues affect many biochemical mechanisms known to be essential for proper shell formation and the extent of the contribution of each affected mechanism towards decreasing shell thickness probably depends on variables such as species and environmental conditions. There is no evidence to suggest that one mechanism is always dominant irrespective of the conditions. In North America shell thinning has often been associated with population decreases, but in Britain declines in shell thickness are not thought to be responsible for the population decreases observed in certain raptor species.