Are Microhabitat Preferences of Coexisting Species Under Selection and Adaptive?

Microhabitat preferences are assumed to be adaptive, suggesting that fitness is higher in preferred microhabitats and potentially reflecting natural selection on habitat choices. I examined microhabitat preferences and adaptiveness of preferences for seven bird species coexisting in high elevation snowmelt drainages based on study of microhabitat and survival of 1556 nests. Habitat features in the nest patch differed from both random and non-use (sites centered on the same plant species as used for the nest) sites within each bird species, indicating nonrandom nest site choice. Bird species within a nesting guild (ground, shrub) also differed from each other based on the same vegetation features that differentiated nest sites from non-use and random sites, and this vegetation feature dominated the microhabitat type that was used most frequently by each species. In short, frequency of use of dominant vegetation types, comparisons of nest vs. random and nonuse sites, and comparisons among species were concordant in their indications of microhabitat preferences. The frequency in use of microhabitats was taken as an unambiguous measure of microhabitat preference within this study system: vegetation varied along a short microclimate gradient in the study system and territories of birds encompassed the entire gradient, thereby making all microhabitats available within the territory of each individual, such that use reflected a clear choice among alternatives. Microhabitat preferences differed among species and reflected differing positions on the microclimate gradient. Thus, species partitioned either microhabitat or microclimate within each nesting guild. Nest success was greater at preferred than at nonpreferred microhabitats for all seven species, indicating that preferences were adaptive. Examination of cubic spline curves and standardized directional selection differentials (s) and selection gradients (b) indicated that preferences had positive directional selection coefficients. These selection coefficients suggested that selection might be acting to favor preferences, but information on genetic bases of habitat choices is needed before selection can be ascertained. Advances in understanding evolution of habitat preferences depend on an individual-level examination of habitat choices and their fitness consequences, and also examination of the phenotypic traits and mechanisms that underlie habitat-induced variation in fitness components.