Egg Viability, Nest Predation, and the Adaptive Significance of Clutch Size in Prairie Ducks

Waterfowl begin incubating their eggs before the clutch is completed. No current hypotheses can explain this phenomenon. We show that egg viability declines during the period of egg laying if eggs remain unincubated, and we suggest that early nest attendance may slow the rate of decline in viability. We then develop the hypothesis that declining egg viability associated with delayed incubation plays an important role in determining the most productive clutch size in temperate-breeding waterfowl. The benefit of laying additional eggs is offset by the reduced value of the first-laid eggs, which suffer lowered viability and greater risk of predation. Our model combining egg viability and the risks of egg predation can account for most of the selection pressure determining clutch size in prairie ducks.