Clutch Size Variation of Birds in Relation to Nest Predation: On the Cost of Reproduction

Abstract

First broods of the great tit were experimentally manipulated to contain either fewer (3-4) or more (10-11) young after hatching than the usual mean number. When the young were 15 days old both nests and young were removed. Those parent birds which reared few nestlings renested sooner than those which reared greater number of hatchlings. The former birds also reared a greater number of fledglings in the renesting attempt than the latter (by 69%) because fewer complete nesting failures occurred. The advantages of laying a small-sized 1st clutch were also seen in the case of the 1st brood itself compared with the enlarged broods; few or none of the nestlings were lost (5% and 38%, respectively) and body weights of the fledglings of the former broods were much greater, indicating that their survival rate on leaving the nest would be much higher. Breeding success of an attempted renesting depends on the degree of parental effort already involved in an initial attempt. Birds which are subjected to a high rate of nesting failure may therefore be expected to lay clutches which contain fewer eggs than those of birds which suffer little degree of nesting failure.