Egg shell characteristics and incidence of shell breakage for grey herons Ardea cinerea exposed to environmental pollutants

Abstract

Egg shell characteristics and incidences of shell breakage have been studied for the grey heron Ardea cinerea at several colonies in eastern England. Eggs collected in the late 1960s and in the 1970s had shells which were significantly thinner than shells collected before 1946. For a sample taken in 1973, shell thickness, shell strength, mammillary layer thickness and palisade layer thickness were all inversely related to the concentration of pp'-DDE(+ pp'-DDT + pp'-TDE) in the egg contents. Shell thinning was considered to be due to a reduced availability of shell components leading to a depressed rate of shell formation. A separate mechanism resulted in increased mammillae height relative to mammillary layer thickness. For comparison, the structure of other types of thin shells was also examined. Incidence of egg shell breakage was positively related to the residue levels of pp′-DDE or dieldrin in intact eggs. Eggs with shells thinner than 240 μm were unlikely to survive incubation. Since such eggs tend to contain higher-than-average residues, sampling intact eggs leads to a mean egg residue figure that is lower than the real mean for the colony. Measuring shell thickness is suggested as a rapid and economical bioassay of DDE levels. Shell thinning leads to a proportionally greater reduction in shell strength, and breakage of shells less than 240 μm in thickness was probably due to a combination of shell fragility and aberrant behaviour by the brooding adults. Breakage of thicker shells was believed to be mainly due to deliberate destruction. Despite more than half of the pairs breaking their shells during some years, heron populations have not declined overall perhaps because of the ability of this species to lay repeat clutches. There are indications that egg residues of DDE and other organochlorines are decreasing, and shell thickness and the incidence of shell breakage are returning to normal.