The Water Balance of Bird Eggs Incubated in Water

Abstract

Eared grebes (Podiceps nigricollis) and American coots (Fulica americana) lay eggs in nests that float on water. Eggs of most grebes come into contact with liquid water during incubation, whereas those of coots do not. The rate of change in the mass of grebe eggs incubated in natural nests was not significantly different from that of coot eggs, even though the water-vapor conductance of grebe eggs is much greater. When eggs of coots and of grey Leghorn chickens (Gallus gallus) were incubated in contact with liquid water in the laboratory, they increased in mass, whereas grebe eggs, incubated under the same conditions, decreased in mass. When eggs of grebes and chickens were placed in contact with a solution of methylene blue, the contents of the chicken eggs were stained blue, whereas the contents of the grebe eggs were not stained. When chicken eggs were incubated in nests of eared grebes, they increased in mass and rotted; eared grebe eggs in the same nests declined in mass and did not rot. These data suggest that eared grebe eggshells possess a barrier to the diffusion of liquid water that nevertheless permits a rapid exchange of water vapor.