THE INFLUENCE OF HABITAT, PREY ABUNDANCE, SEX, AND BREEDING SUCCESS ON THE RANGING BEHAVIOR OF PRAIRIE FALCONS

We studied the ranging behavior and habitat selection of radio-tagged Prairie Falcons (Falco mexicanus) during the breeding season in southw estern Idaho. The distribution and numbers of Townsend's ground squirr els (Spermophilus townsendii), the primary prey of Prairie Falcons in our study area, varied in response to drought during the study period. Prairie Falcons ranged over large areas (ca. 300 km(2)) and increased their foraging ranges in response to declining ground squirrels. Rept iles and birds were preyed upon most frequently when squirrels were ra re. Males and females differed little in their use of space. Successfu l pairs ranged over smaller areas than non-nesters and unsuccessful pa irs. Falcons nesting near habitat most suitable for ground squirrels r anged over smaller areas than those nesting farther from such habitat. Home ranges contained significantly more winterfat (Ceratoides lanata ) and native perennial grasses (especially Poa secunda), and significa ntly less salt desert shrubs and exotic annual grasses than expected b ased on availability. Salt desert shrubs were found less than expected , based on availability in core areas within home ranges. Selection fo r winterfat and bluegrass in core areas was contingent upon selection at the larger scale of the home range; falcons with home ranges contai ning more winterfat and bluegrass than expected based on availability were less selective in their placement of core areas with respect to t hese habitats. We believe salient features of Prairie Falcon home rang es result largely from patchy distribution of landscape features assoc iated with different densities and availabilities of Townsend's ground squirrels.