Anthropogenic effects on winter habitat use by ferruginous hawks in Colorado

Most raptors remain unstudied outside the nesting season, yet winter represents a significant portion of a raptor's annual cycle, and events in winter influence demographics throughout the year. Additionally, little is known regarding how anthropogenic pressures affect raptors, particularly while wintering. To address these information gaps, I studied wintering ferruginous hawks (Buteo regalis) from 1992-1995 in the Colorado Front Range. Wintering behavior and habitat use was studied in two areas with low and high levels of anthropogenic influence and habitat fragmentation; the Rocky Mountain Arsenal National Wildlife Refuge (RMANWR), and several Denver suburbs, respectively. Hawks (N = 36) spent 84% of the daylight interval perching, and 16% in flight. Hawks averaged 18 perches/day (range: 3-50), with perching events averaging 30 min (range: 1-482 min, SE = 0.82) in duration. Perch substrates were used with unequal frequency ({dollar}P <{dollar} 0.001); poles and other human-made structures were used more frequently than ground and deciduous tree perches. Duration of perch use differed ({dollar}P <{dollar} 0.0001) by substrate; tree perches were used for the longest mean duration. The mean daily Minimum Convex Polygon (MCP) home range of 36 hawks was 3.53 km{dollar}sp2 pm{dollar} 0.44 (range: 0.01-40.0 km{dollar}sp2{dollar}, N = 148 days). Black-tailed prairie dogs (Cynomys ludovicianus) were the most important prey species, and extant prairie dog colonies characterized winter habitat for hawks. Daily MCPs were not different (P = 0.28) for RMANWR (N = 25, x = 4.71 km{dollar}sp2{dollar}, {dollar}pm{dollar} 1.33) and suburban hawks (N = 13, x = 2.30 km{dollar}sp2{dollar}, {dollar}pm{dollar} 0.50). Although hawk activity levels between the sites were not different (P = 0.146), RMANWR hawks used more pole and ground perches, of longer sum duration, and comprising a greater proportion of the daily time budget ({dollar}P le{dollar} 0.05). Conversely, suburban hawks used more tree perches, of longer sum duration, and a greater portion of the daily time budget ({dollar}P <{dollar} 0.001). RMANWR hawks spent less time roosting after daylight began (x = 61 min) than did suburban hawks (x = 138 min, P = 0.004). Prey acquisition and competitive interactions did not differ ({dollar}P >{dollar} 0.05) between the sites. Relative hawk abundance differed by site and year ({dollar}P <{dollar} 0.0001), and reflected local availability of prairie dogs. Hawks modified perch use, time budgets, and roosting habits to exploit fragmented habitats, provided some foraging habitats with adequate populations of prairie dogs remained.