A model of clutch size determination in birds

Summary This paper tests the hypothesis that the timing and the size of bird clutches are determined by (1) a tendency to delay reproduction so that the female can store enough internal resources for laying a large clutch; (2) a tendency to breed early because survival of offspring late in the breeding season might be lower than early. These factors are approximated by linear functions in a model. The model predicts that there will be a correlation between early breeding females and those that have large clutches. This agrees with published data for several bird species. Predictions from the model are also made of clutch size and the impact on final clutch-size if one egg is removed or added during the laying. These predictions were compared with data from a crow (Corvus cornix) population and were supported if it is assumed that the formation of an egg is a relatively costly process. More precisely, the model is supported if the formation of an extra egg requires a delay of about four or five days before laying can begin.