Species Abundance and Habitat Relations of an Insular Montane Avifauna

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Abstract
The summer birds of the Sweetgrass Hills, an isolated mountain range in the northern Great Plains region of Montana [USA] were investigated. Of the 100 breeding spp. reported, 18 are restricted to montane habitats during the breeding season. Data are presented on local distribution, sample abundances, distribution among major habitats, summer altitudinal range, and evidence of breeding for each species observed. Sample abundance data were used to construct a species curve with which to test thoroughness of avifaunal sampling. While montane habitats supported fewer breeding bird species than the more extensive steppe habitats, they provided a slightly greater representation of breeding-season guilds. The number of breeding montane species inhabiting montane islands was significantly correlated with island area, forested area, distance from mainland, and several measures of habitat diversity. Only species numbers of sedentary or weakly migratory montane species were correlated with barrier width. The relations between species number and community structure is discussed; while no evidence of competitive exclusion was noted in this largely migratory avifauna, evidence of niche expansion and abundance compensation was found, and may be reflected in differences in the species-abundance relation between mainland and insular avifaunas. Philopatry and chance may be the best explanation for the absence of species not clearly related to lack of suitable habitat or inability to colonize.