Evolutionary determinants of clutch size in cavity-nesting birds: nest predation or limited breeding opportunities?

Large clutch sizes of cavity-nesting birds have been considered a classic example of the consequences of low nest predation for almost half a century. I show that reduced nest predation cannot explain large clutch sizes in cavity-nesting birds. Instead, clutch sizes of cavity-nesting species in North America and Europe are more closely related to the propensity to excavate nest holes. Propensity to excavate is determined by skull and bill morphology of species. Species with weaker excavating morphology depend on existing holes more and have larger clutches and more broods per year than species that excavate new holes. These results cannot be explained by energy costs of excavation. Possible effects of nonreproductive mortality among adults cannot be evaluated. The results support a new hypothesis that the more limited availability of nest sites for species that depend on existing holes favors greater reproductive effort.