The role of predation in the evolution of clutch size

Abstract

There has been considerable discussion about the adaptive nature of clutch size and whether or not the average clutch can be related to the maximum number of young that the parents can successfully raise (Lack, 1954; Wynne-Edwards, 1962). In nidicolous species, those which feed their young in the nest, it is possible to see at least some of the advantages to a bird of being conservative in the number of eggs that it lays. Although parent birds tend to bring more food to larger than to smaller broods, the increase in the amount of food brought is not proportional to the increase in the number of mouths to be fed. As a result, the individual young in large broods receive less food than those in smaller broods and leave the nest lighter in weight. In at least a few species the probability of survival has been correlated with the weight at fledging (Perrins, 1965). Thus the lowered survival rates of the young in large broods may be sufficient to outweigh the initial advantage that the large brood had in terms of greater numbers of young. In such circumstances, parents with broods of average size may be at least as productive as, and sometimes more productive than those with larger broods (Perrins and Moss, 1975).

Dr C. M. Perrins has worked on the breeding biology of a number of birds round Oxford and on seabirds in Wales. At present he is at the Edward Grey Institute of Field Ornithology where his main research is centred on a long-term study of the population dynamics of the Great tit.