Martin 9407587 Differences among species in mortality of food limitation are generally assumed to cause differences in fecundity. This study examines a new hypothesis: fecundity increases, even at the cost of reduced survival, as breeding opportunities become more limited. A quantitative model shows that even small reductions in breeding opportunities (e.g., 10% reduction) can have large effects on fecundity and these effects are independent of any mortality effects. Moreover, such small reductions in breeding opportunities can commonly exist across any taxonomic group through limited availability of nesting sites, oviposition sites, or social behavior restricting access to mates. Cavity-nesting birds that excavate their nest holes (excavators) provide a model system for testing this hypothesis because they are so easy to study in their natural nests, they include closely-related species that cover nearly the entire range of fecundities found among small land birds, they vary in abilities to excavate nest holes and potentially obtain breeding opportunities, and clutch size is correlated with this variation in excavation ability. The proposed study will include experimental analyses of 4 species to test the hypothesis. The proposed work challenges long-standing theories of underlying causes of differences in clutch size and fecundity among species. Hence, this work will advance our understanding of factors affecting demography of populations, and will provide new information on life histories of several previously unstudied bird species. In addition, this study will provide critical information that is pertinent to conservation of cavity-nesting species; land management agencies have explicit recommendations for maintenance of snag (nest site) densities that may be insufficient. variety of studies indicate that nest sites are commonly limiting for cavity-nesting birds. Such limitations can affect populations in additional subtle ways such as influencing fecundity, survival and hence population fluctuations of such species. This study will advance understanding of the extent to which nest sites are limiting and affecting population dynamics and population size.
