Populations and Prey Selection of Wintering Raptors in Boulder County, Colorado

Wintering raptor populations were monitored between 1983 and 1988 in a 35 km2 study area centered 8 km north-northeast of the city of Boulder, Colorado. Raptors congregated around active prairie dog (Cynomys sp.) colonies. Golden eagles (Aquila chrysaetos) , ferruginous hawks (Buteo regalis), and red-tailed hawks (Buteo jamaicensis ) were observed hunting and capturing prairie dogs. Bald eagles (Haliaeetus leucocephalus) and northern harriers (Circus cyaneus) participated in the competition for captured prey. Thirteen occurrences were noted of bald eagles stealing captured prairie dogs from ferruginous hawks. A bubonic plague outbreak killed most of the prairie dogs within the study area in 1986, corresponding with a > 60% decline in numbers of wintering bald eagles, ferruginous hawks and red-tailed hawks. A bald eagle winter roost that had been occupied by 40 eagles prior to the plague outbreak was abandoned the following winter.