Niche and Habitat Relations in Subalpine Bird Communities of the White Mountains of New Hampshire

The populations, foraging behavior, and habitat selection of 20 insectivorous species of subalpine birds were studied during three breeding seasons in the white Mountains, New Hampshire, USA. The object was to clarify how an elevation gradient of environmental adversity affected diversity, dominance, niche, habitat, and species biology. Population densities and territory sizes were measured from census plots. Territory sizes correlated with species body mass, probably in response to size—related food requirements. Lower elevations had more even distributions of densities among species and less steep dominance—diversity curves than did higher elevations. To show habitat selection patterns in the subalpine vegetation, principal components analysis (PCA) was used to develop a model that emphasized elevation and broadleaf—to—coniferous foliage as the two major gradients. Bird species diversity (BSD) reached a maximum in well—developed preclimax red spruce—balsam fir forests with some broadleaf crowns present. Individual species responses were mapped using the model as a base chart, grouping together: (1) five species living at altitudes above 1100 m with small differences in their specific patterns, (2) five other species widely distributed through the mid—subalpine zone (800—1100 m), (3) six more mid—subalpine species segregating along the broadleaf/conifer foliage gradient, and (4) three species found near water. Niche relations were studied using detrended correspondence analysis, a modified version of reciprocal averaging. Resource use was divided by (1) terrestrial vs. arboreal position, (2) a gradient from fine distal (leaves) to coarse proximal (trunk) substrates, and (3) different specializations for food acquisition. BSD was greater in the resource categories requiring less energy and less specialized morphological adaptations to exploit, or having greater insect resource bases. To investigate the relations between habitat and niche, a matrix of the correlations between the niche and habitat variables, using species means for data, was analyzed by PCA. Three relations emerged: (1) the elevation/forest structure gradient was correlated with the terrestrial/arboreal foraging segregation, (2) the niche substrate gradient responded to vegetation structure along the broad—leaf/conifer gradient, and (3) in both cases the response of foraging behavior to habitat structure involved many variables acting together. Niche and habitat were thus closely linked as aspects of species ecotopes; species responses were illustrated for the Blackpoll Warbler and other wood warblers. Food was inferred to limit populations occasionally, so that interspecific competition plausibly underlies niche differentiation. Thermoregulation is hypothesized to affect habitat selection.