DISTRIBUTION AND NESTING SUCCESS OF FERRUGINOUS HAWKS AND SWAINSON'S HAWKS ON AN AGRICULTURAL LANDSCAPE IN THE GREAT PLAINS

摘要

We studied land-cover associations at nest sites and reproductive success of two Buteo species of conservation concern on the southern Great Plains, USA. The study area was in Cimarron County, Oklahoma, where land use is dominated by row-crop agriculture, livestock grazing, and grasslands enrolled in the Conservation Reserve Program (CRP). Ferruginous hawks (B. regalis) were uncommon and nested primarily in and around the Rita Blanca National Grassland. Swainson's hawks (B. swainsoni) were common and nested throughout the study area. Territories of ferruginous hawks contained more sandsage (Artemisia filifolia) habitat and less cropland and CRP lands than random sites, whereas territories of Swainson's hawks mirrored proportions of available landcover. Our results suggest that proportion of nearby sandsage habitat is an important factor in determining Swainson's hawk reproductive success. In addition, ferruginous hawk nest sites were located in areas that contained significantly more sandsage habitat than randomly selected sites in the study area. Nest-site availability may have constrained the distribution of buteos in our study area and is probably a major factor limiting nesting density of ferruginous hawks. Ferruginous hawks typically nest on man-made platforms (e. g., nest platforms and windmills) that in the study area were most common in and around the Rita Blanca National Grassland. Our results suggest that conversion of native grasslands to cropland may have negative consequences for ferruginous and Swainson's hawks. This relationship has been previously demonstrated in several studies of ferruginous hawks, but not for Swainson's hawks. In particular, loss of sandsage habitat on the southern Great Plains may have contributed to range declines in ferruginous hawks and decreased breeding success for Swainson's hawks.