Clutch Size in Birds: A Predation Perspective

Abstract

A simple model of clutch size in nidicolous birds is developed in which the cost of reproduction is the risk of predation to both the parent and its dependent young. An analysis of the model shows that (1) conventional ideas of food limitation, though sufficient, are not necessary for existence of an optimal clutch size; (2) predation as the sole cost of reproduction is adequate for the existence of an optimal clutch size; and (3) one need not expect a clutch—size—dependent physiological cost to a parent (leading to increased mortality) to be a major determinant of clutch size. In addition, several experimental and observational results that purport to demonstrate food limitation in breeding birds are also consistent with the idea of predation risk as the major cost of reproduction. Current ideas of clutch size determination and the costs of reproduction are considered in light of the above results, and possible syntheses are suggested. A predation—risk perspective may offer considerable insight into the evolution of clutch size in birds.