LANDSCAPE CHARACTERISTICS OF FRAGMENTED SHRUBSTEPPE HABITATS AND BREEDING PASSERINE BIRDS

We examined the influence of local and landscape-level attributes of f ragmented habitats in shrub-steppe habitats on the breeding distributi ons of Sage (Amphispiza belli) and Brewer's (Spizella breweri) Sparrow s, Sage Thrashers (Oreoscoptes montanus) Horned Larks (Eremophila alpe stris), and Western Meadowlarks (Sturnella neglecta) in the Snake Rive r Plains of southwestern Idaho. We developed habitat (resource) select ion models for each species by combining bird counts conducted from 19 91 through 1933 with local vegetation characteristics and landscape at tributes derived from satellite imagery. Site selection by shrubsteppe species (Sage and Brewer's Sparrows and Sage Thrashers) depended on l ocal vegetation cover and landscape features, such as the patch size o f shrub habitats or the spatial similarity of sites. Marginal sites fo r these species (with species present in one of three years) were inte rmediate between unoccupied (never present) and occupied sites along e nvironmental gradients characterized by increasing size of shrub habit at patches and total shrub cover and by decreasing disturbance. Horned Larks and Western Meadowlarks, typical grassland species, were not se nsitive to landscape features, and their occupancy depended on the amo unt of grassland or shrub cozier. In contrast to shrubsteppe species, sites that varied by occupancy rates of Western Meadowlarks did not si gnificantly differ in vegetation or landscape components. Our results demonstrate that fragmentation of shrubsteppe significantly influenced the presence of shrub-obligate species. Because of restoration diffic ulties, the disturbance of semiarid shrubsteppe may cause irreversible loss of habitat and significant long-term consequences for the conser vation of shrub-obligate birds.