Much debate remains about the factors ultimately limiting clutch size and causing its seasonal decline in waterfowl. Our review suggests that the physical condition of the laying female is a primary proximate determinant in some species.
Condition is in turn affected by food supplies, age, and other variables.
Genotypic variation has also been demonstrated, and parasitism, where it occurs, has an additional effect. Survival of young hatched from larger eggs may be enhanced in certain situations, but the determinants of egg size are unclear.