ARE MICROHABITAT PREFERENCES OF COEXISTING SPECIES UNDER SELECTION AND ADAPTIVE

Microhabitat preferences are assumed to be adaptive, suggesting that f itness is higher in preferred microhabitats and potentially reflecting natural selection on habitat choices. I examined microhabitat prefere nces and adaptiveness of preferences for seven bird species coexisting in high elevation snowmelt drainages based on study of microhabitat a nd survival of 1556 nests. Habitat features in the nest patch differed from both random and non-use (sites centered on the same plant specie s as used for the nest) sites within each bird species, indicating non random nest site choice. Bird species within a nesting guild (ground, shrub) also differed from each other based on the same vegetation feat ures 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 domina nt 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 ta ken as an unambiguous measure of microhabitat preference within this s tudy system: vegetation varied along a short microclimate gradient in the study system and territories of birds encompassed the entire gradi ent, thereby making all microhabitats available within the territory o f each individual, such that use reflected a clear choice among altern atives. Microhabitat preferences differed among species and reflected differing positions on the microclimate gradient. Thus, species partit ioned either microhabitat or microclimate within each nesting guild. N est success was greater at preferred than at nonpreferred microhabitat s for all seven species, indicating that preferences were adaptive. Ex amination of cubic spline curves and standardized directional selectio n differentials (s) and selection gradients (beta) indicated that pref erences had positive directional selection coefficients. These selecti on coefficients suggested that selection might be acting to favor pref erences, but information on genetic bases of habitat choices is needed before selection can be ascertained. Advances in understanding evolut ion of habitat preferences depend on an individual-level examination o f habitat choices and their fitness consequences, and also examination of the phenotypic traits and mechanisms that underlie habitat-induced variation in fitness components.