Regulation of Incubation Water Loss in Eggs of Seven Species of Terns

Water loss during incubation in the eggs of seven species of terns are reported. The nesting sites ranged from a relatively cold region, southern Alaska, to a tropical climate in the equatorial Pacific Ocean. In addition, measurements of various physical parameters of the eggs and eggshells were made, including the water-vapor conductance of the fresh egg, which allows one to calculate the total effective pore area of the shell. The rate of water loss during incubation was proportional to egg weight (9-39 g) but inversely proportional to incubation period (21-36 days); the eggs of all species lost about the same fractional amount of water, namely, 14% of their initial weight. The rate of water loss is determined by a species-specific water-vapor conductance or pore geometry of the shell, provided that a water-vapor pressure difference of about 27 torr is maintained between the egg and the microenvironment of the nest. Since water-vapor pressure in the incubating egg is determined by egg temperature and is about 47 torr, the microclimate of the nest must be maintained at a vapor pressure of about 20 torr. Therefore, the bird must achieve a nest ventilation which is a function of the water loss of the egg and brood patch and the difference between the water-vapor content of the ambient air and that in the microclimate of the nest.