Habitat Associations and Population Trends of Two Hawks in an Urbanizing Grassland Region in Colorado

Despite increasing interest in the ecological effects of urbanization, relatively little is known about its effects in grasslands. We examined population trends and habitat associations of two predators, the rough-legged hawk (Buteo lagopus) and the red-tailed hawk (B. jamaicensis), in a rapidly urbanizing grassland region at the western edge of the North American Great Plains. Count data indicate that rough-legged hawk populations declined in the area by nearly 75% between 1971 and 2003, at the same time that numbers of red-tailed hawks more than tripled. These changes were not part of wider regional trends, nor were they buffered by development of an open space system in one of the urbanizing counties. While the human population grew steadily over the 33-year period, hawk numbers did not begin to change significantly until the early 1980s, suggesting landscape threshold responses to development. Rough-legged hawks remaining in the area between 1999 and 2002 avoided human settlements and hunted in places with tracts of treeless grassland. In contrast, red-tailed hawks selected relatively tall perches in trees or on utility poles from which to hunt, in areas closer to buildings and roads than randomly selected plots, and remained abundant in the mosaic of developed and rural agricultural lands. The failure of the grassland open space system to sustain the rough-legged hawk, and other bird species characteristic of treeless open prairie, illustrates the challenges of conserving fauna with apparent hypersensitivity to the three-dimensional habitat complexity that accompanies even modest amounts of development.