ABUNDANCE OF DIURNAL RAPTORS ON OPEN SPACE GRASSLANDS IN AN URBANIZEDLANDSCAPE

We conducted point counts of diurnal raptors on Boulder, Colorado, gra sslands for three winters and summers, and compared results to landsca pe features of the count areas. Four wintering species were scarce on plots that included significant amounts of urban habitat, with a criti cal landscape threshold at about 5-7% urbanization: Bald Eagle (Haliae etus leucocephalus), Ferruginous Hawk (Buteo regalis), Rough-legged Ha wk (B. lagopus), and Prairie Falcon (Falco mexicanus). Counts of the f irst three species also were positively correlated with proximity of t he count plots to the nearest colony of black-tailed prairie dogs (Cyn omys ludovicianus). Two breeding species, the Red-tailed Hawk (B. jama icensis) and Swainson's Hawk (B. swainsoni), were more abundant on plo ts dominated by lowland hayfields and tallgrass prairies, as opposed t o upland mixed and shortgrass prairies. They, along with the ubiquitou s American Kestrel (Falco sparverius), were not sensitive to the amoun ts of urbanization (up to 30%) that occurred in the landscapes sampled . Results of this study suggest that urban open space grasslands can s upport sizable populations of most diurnal raptors, as long as prey po pulations persist, but that some species are highly sensitive to lands cape urbanization.