On the Significance of Clutch Size in Nidifugous Birds

The average survival of young Semipalmated Sandpipers (Calidris pusilla) from experimental 5—young broods was lower than that of young from natural 4—young broods. This is predicted by a theoretical model for the determination of clutch size in nudifugous birds: precocial young are vulnerable to predation when they move around, and nudifuguous birds actively protect their young. The capacity of parents to provide protection is limited, and their protective effort is proportional to the product of the number of young times the movement rate of each young. But since the latter is inversely related to the distances between prey items, an increase in clutch size is advantageous only if accompanied by an increase in food density. However, hunting efficiency does not increase linearly with increase in food density, and above a certain level a further increase in density has a negligible effect on efficiency. Thus, the upper limit of clutch size of nudifugous birds may be set by the fact that the protection required for more young exceeds that which the parents can offer.