9527318 Martin Habitat selection and species coexistence are generally thought to reflect choice of environmental conditions that minimize ecological costs and maximize fitness. Biotic interactions are often particularly emphasized in theories. Yet, ecological influences may be constrained by physiological tolerances as a function of evolutionary history of a species and different evolutionary histories or physiological tolerances may provide an alternative explanation of differences in resource use among coexisting species. Such alternative actions of biotic versus abiotic influences are particularly highlighted along microclimate gradients because species commonly segregate along microclimate gradients. This research will document a microclimate gradient and show that seven bird species are segregated along this gradient. Such segregation raises an interesting question: Does segregation reflect physiological constraints or responses to ecological costs? Recent studies indicate that the ecological cost of nest predation may favor choice of nest sites that minimize risk of predation and favor coexistence of species that differ in nest site selection. Changes in microclimate from weather changes among years should cause dynamic shifts in habitat selection if physiological costs of microclimate are important. Yet, nest site shifts could increase ecological costs if shifts exceed ecological optima. Data collected over the previous six years show that nest sites do shift in response to weather changes. 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.
