Habitat Occupancy Patterns of North American Shrubsteppe Birds: The Effects of Spatial Scale

Understanding the processes that underlie how birds select and use habitats depends on an accurate representation of the patterns of habitat occupancy. These patterns are sensitive to the spatial scale on which they are viewed. Densities of several shrubsteppe bird species in North America varied in relation to features of habitat structure at a biogeographic scale, but these associations disappeared at a regional scale within the shrubsteppe. In another regional comparison involving a different array of shrubsteppe plots and sites, densities of both sage trashers (Oreoscoptes montanus) and sage sparrows (Amphispiza belli) varied with habitat features in quite different ways than in the other regional analysis. A consideration of the pattens of distribution of the bird species in a multivariate habitat space created by Principal Components Analysis of the regional habitat data revealed several clear patterns, but these relationships generally failed to hold when the spatial scale was further reduced, to consider differences between plots at the same location. At this scale other bird-habitat relationships were apparent, but these patterns differed for populations of the same species at different sites. Consideration of habitat differences between areas within occupied territories versus unoccupied areas within plots revealed still other patterns of habitat occupancy. Some of these were consistent with those obtained in analyses at other scales, but many were not. How one characterizes the habitat occupancy of a species is thus dependent on the spatial scale used. We explore several factors that complicate bird-habitat analyses at different spatial scales, and conclude that these problems of scale are most likely to be circumvented by conducting studies at several hierarchically nested scales arranged on clearly defined environmental gradients over a reasonably long period of time.