Habitat selection by nesting Swainson's hawks : a hierarchical approach

Graduation date: 1992The habitat relationships of Swainson's hawks in\ud northern California were studied at several levels of\ud resolution: 1) distribution of hawk populations among three\ud adjacent geographic areas, 2) density of hawk territories\ud among landscapes within a single geographic area, 3)\ud habitats associated with individual nesting territories, and\ud 4) selection of foraging habitats and prey resources by\ud individual hawks. Patterns of habitat selection observed at\ud each level were compared to results obtained at other\ud levels, and to temporal variations in habitat quality.\ud Individual Swainson's hawks exhibited strong selection\ud for specific habitats wherein prey availability was highest,\ud however these habitats were not found in increased\ud proportions in Swainson's hawk territories. Similarly,\ud densities of Swainson's hawk territories among different\ud landscapes did not reflect the availability of high-quality\ud habitat; many pairs nested in marginal habitats and\ud exhibited low reproductive success and large areas of\ud apparently suitable habitat were unoccupied. At a larger\ud scale, the distribution of Swainson's hawk populations was\ud markedly dissimilar among three adjacent geographic areas\ud with similar habitat composition.\ud Lack of concordance among levels of habitat selection\ud by Swainson's hawks was explained by behavioral and\ud environmental factors that constrain optimal habitat\ud selection. I suggest that high temporal variability of\ud habitats in agricultural environments, combined with site\ud tenacity and natal site philopatry by nesting hawks,\ud prevented the density and distribution of the hawk\ud population from closely tracking the available habitat.\ud Reproductive fitness, however, was strongly associated with\ud habitat quality