TIME, CONDITION, AND THE SEASONAL DECLINE OF AVIAN CLUTCH SIZE

A seasonal decline in clutch size is typical of bird populations. This phenomenon may result from a conflict between the advantages of early breeding (greater offspring value) and the advantages of delay (greater accumulated condition and hence clutch size). We construct a dynamic model for adaptive seasonal decline in clutch size on the basis of these premises. The model requires a small number of well-supported assumptions; it is formulated and analyzed in both analytical and graphical forms. We outline some novel predictions and suggest tests of our conclusions. Initial comparisons of our predictions with results available from wild birds in the field are favorable. We briefly extend the model to consider the evolution of multiple clutches within a season and the effects of between-season costs of reproduction. Although this work focuses on avian clutch size, the analysis provides a general framework for studies of condition-dependent transitions in life histories. Such transitions between stages (e.g., maturation, metamorphosis, or reproduction) characterize the ontogeny of organisms. We expect that such conflicts between the advantages of early and late transitions are common to life-history decisions.