Bird Associations with Floristics and Physiognomy Differ across Five Biogeographic Subregions of the Great Basin, USA

The majority of management plans for birds on public lands across the western United States do not recognize the geographic variation in a given species’ habitat. We examined associations of plant species and functional groups with occupancy of 19 bird species across 5 biogeographic subregions of the Great Basin (central, western, Sierra Nevada, northern, and eastern), USA. We hypothesized that occupancy was associated with floristics (individual plant species) within subregions, and with physiognomy (characterized by functional groups) across the Great Basin. We used two methods to evaluate bird-vegetation associations within and across subregions. First, we examined which covariates of floristics and physiognomy were significantly associated with occupancy in each subregion. Second, for each bird species, we compared covariate estimates between each of the 10 pairs of subregions. We classified the effects of covariates on occupancy in 2 subregions as significantly different if {$<$}5% of their posterior distributions overlapped. The plant species and functional groups that were associated significantly with occupancy varied considerably among subregions. Twenty-four percent of bird-plant associations that were significant at the Great Basin level were not significant in any subregion. Associations between occupancy and floristics differed the most between the Sierra Nevada and central or western subregions, and the least between the eastern and western subregions. Associations between occupancy and physiognomy differed the most between the Sierra Nevada and western and central subregions, and the least between the northern and western subregions. These differences and similarities may reflect variations in climate or bird communities or differences in sampling effort. In addition, the number and strength of associations between occupancy and floristic or physiognomic covariates varied substantially among bird species and subregions. We recommend that the management of birds across the Great Basin or other large ecoregions evaluate and account for geographic variation in environmental attributes associated with occupancy, and not assume bird-plant relations are consistent across the Great Basin.