Cooperative Fish and Wildlife Research Units Program: Montana Wildlife
Education, Research and Technical Assistance for Managing Our Natural Resources

Mechanistic causes of evolved differences in clutch size among species remain debated. I propose a new theory (collection of hypotheses) that integrates aspects of prior hypotheses, while also including novel elements, to explain evolution of clutch size differences among species. I postulate that selection by nest predation on length of time that offspring remain in the nest and locomotor development at fledging determines mobility and extent of self-feeding of fledglings. Species with high nest predation risk produce fledglings with low mobility and slower development of self-feeding that require greater parental energy expenditure per offspring. The greater energy demands of less mobile fledglings constrain the number of offspring that parents can raise compared with species that have low nest predation risk. Differences in mobility also can potentially yield differences in fledgling survival among species and, thereby, influence the adaptive age of fledging. Finally, differences in adult mortality are proposed to favor evolution of differences in total parental energy expenditure among species to further influence clutch sizes. Data from the literature and from my Arizona site are used to provide initial, although indirect, support for several elements of this proposed theory, but more detailed tests are needed.