A note on eggshell porosity, nest humidity and the effects of DDE in the grey heron (Ardea cinerea)

Water vapour conductance (GH2O) was determined for 25 grey heron Ardea cinerea eggs in the laboratory, and in nests during natural incubation at two Scottish colonies. The mean GH2O of eggs measured in the nest which successfully hatched was 9.0 mgH2O/mmHg/day and the mean water vapour pressure gradient between egg and nest (delta PH2O), measured using "calibrated" duck eggs, averaged at 31 mmHg (4.13 kPa). Based on eggshell porosity results, from the eggs which hatched, such a gradient would result in a loss of water from the eggs during incubation equivalent to 11% of their fresh weight. Shell thickness, the number of pores/cm2 of eggshell and DDE content were also determined for the 25 eggs measured in the laboratory. Eggs containing high levels of DDE had thinner shells, more pores in the eggshell and a higher overall eggshell porosity. The main problem posed by a high level of DDE would appear, however, not to be an excessive water loss from the egg during incubation, but rather eggshell thinning leading to a loss of the egg due to breakage in the nest.