Habitat Use and Habitat Overlap of Riparian Birds in Three Elevational Zones

Abstract

I examined patterns of variance in habitat use and habitat overlap in 20 breeding bird species found along a riparian vegetational gradient in southeastern Wyoming to test whether habitat use in species differed (1) from availability of random habitat resources, (2) among elevation zones, and (3) between species that inhabited only one zone and species that occupied multiple zones. I sampled habitat features in bird territories and at random stations on 10 8.1—ha study grids distributed over an elevational gradient of 940 m. Principal components analysis was performed on both randomly sampled and territory—centered habitat data to examine the habitat use of each bird species in relation to the random centroid in n—dimensional habitat space. Using this transformed data set, I computed habitat size of each species, defined as degree of specialization in habitat use; species—habitat position, defined as use of common or scarce habitat; habitat overlap among species; and sum of variation in structure of the available habitat. Habitat size was significantly smaller for bird species found in subalpine riparian shrublands than for those found in foothill woodlands, but this result was partly a reflection of variation among elevational zones in structural diversity of the available habitat. The null hypothesis that species used habitat as it occurred randomly was rejected for all three subalpine bird species examined, 6 of 10 dominant mid—elevation species, and 1 of 10 lowland species. Habitat overlap among species did not vary among zones, but habitats used by lowland species were closer to the random expectation than habitats used by mid—elevation or subalpine species. In general, species restricted in distribution to one zone had smaller habitat sizes than species occupying multiple zones, but some zone—dependent species were generalists within their zones. Vegetation of lowland riparian habitats was structurally more complex than that of riparian habitats at higher elevations, and bird species richness and bird abundance were greatest in riparian lowlands. Because most lowland species were generalized in habitat use and occupied habitats that were similar among species, I concluded that bird species diversity was greater in lowlands than in mid—elevation and subalpine shrublands merely because woodlands were more heterogeneous. The large proportion of generalists in deciduous woodlands of the central Rocky Mountains may be explained by geographic dispersal patterns. Woodland species may have been historically prevented from dispersal by the ecological barrier of the Great Plains grasslands. Recent establishment of widespread riparian forest permitted colonization of woodland bird species, with generalists colonizing first.