Incubation Water Loss in Eggs of Heermann's and Western Gulls

Abstract

We have studied various factors which govern the constant rate of water loss of gull eggs (Larus heermanni and L. occidentalis livens) during natural incubation in the relatively warm and dry climate of the Gulf of California, Mexico. The water vapor conductance of the eggshell in the former species is fixed by approximately 5,700 pores averaging 8.6 and 270 μm in radius and length, respectively. The constant daily rate of water loss by diffusion across these pores (300 mg H₂O · day⁻¹) is therefore controlled by maintenance of a water vapor pressure gradient across the eggshell averaging 26 torr. The vapor pressure in the air spaces of the egg is determined by the egg temperature, but the vapor pressure in the nest's microclimate is controlled by parental behavior serving to ventilate the nest air. The resultant transport of the accumulating water vapor from the nest to the atmosphere allows maintenance of a relatively constant nest humidity.