9707598 Martin Habitat selection among coexisting species is generally thought to reflect choices that minimize ecological costs and maximize fitness. Ecological processes among coexisting species are often emphasized as a potential explanation for differences in microhabitat choices. Yet, processes and the mechanisms underlying processes are rarely directly tested. Predation can favor microhabitat differences among coexisting species and is a process that can be tested directly. However, habitat selection also may be constrained by physiological tolerances as a function of the evolutionary history of a species and different evolutionary histories or physiological tolerances may provide an alternative explanation for differences in resource use among species. Such alternative actions of biotic versus abiotic influences are particularly highlighted along microclimate gradients because species commonly segregate along microclimate gradients. This study will address the question, "Does the distribution of species along an environmental gradient reflect physiological constraints or responses to ecological costs such as nest predation or vegetation preferences?" Continued long term study of this system and nest site shifts is important because such shifts provide an unprecedented opportunity to examine the potential role of biotic versus abiotic influences on habitat selection and species coexistence based on dynamic changes in potential environmental influences. Continuation of this long-term work will advance our understanding of mechanisms underlying nest predation as a process influencing habitat selection, species coexistence, and parental activity patterns. These data will provide important information for long-term conservation of species; habitat requirements for successful reproduction are critical for conservation and data being collected under this project is currently being used by the US Forest Service to affect land management decisions on the forests being studied.
