Regional Correspondence among Shrubsteppe Bird Habitats

Abstract

Habitats of Brewer’s Sparrows (Spizella breweri) and Green-tailed Towhees (Pipilo chlorurus) were compared (as biological replicates) in Colorado, Nevada, and Utah to identify regional commonalities in vegetation species, structure, and vigor among habitats of shrubsteppe passerines. Sparrow habitats were in landscapes dominated by sagebrush (Artemisia spp.) whereas towhee habitats were better described as ecotones between sagebrush and other shrub species. Individuals of both species selected bushes of comparable vegetative vigor across the three states even though species populations were allopatric in two of the states. For each species, measures of vegetation structure and vigor were compared between breeding habitats in Colorado and Nevada (near the longitudinal extremes of the continental distribution of each species at 42”N latitude) in 1983. Sparrow habitats in the two states were similar in four measures of vigor but differed in all structural variables measured. Towhee habitats were similar between Colorado and Nevada for three measures of vigor and three measures of structure. Independent tests of the Colorado/Nevada conclusions were conducted near the center of the species’ ranges (Utah) in the subsequent breeding season (1984). Those tests confirmed the validity of all measures of vegetation vigor as descriptors of sparrow habitat and one measure of vigor plus two of vegetation structure as descriptors of towhee habitat. Thus, sparrow habitats were best characterized regionally by measures of sagebrush vigor, whereas towhee habitats were characterized by shrub vigor and structure. Our tests and conclusions generally support recent, correlational studies that identify vegetative physiognomy as the primary descriptor of passerine habitats at a regional scale and floristic composition as the primary descriptor at a local scale. We add that individuals of these two species selected habitats from a mosaic of patches of varying vegetation vigor (supplemented by structural information for towhees) within a locale. Measures of vegetation vigor are rarely used to described passerine habitats, although vigor surely correlates better with food abundance and cover for nests than vegetation structure. These observations suggest an hierarchical approach for viewing habitat selection by shrubsteppe birds.