Heavy October and November snows, along with frigid temperatures in the Northern Plains, have contributed to a modest influx of ferruginous and rough-legged hawks into Boulder County. Throughout November, we saw several of these large grassland hawks hunting around Lagerman Reservoir, Boulder Reservoir and in the grasslands north of Rocky Flats.
During the 1980s and 1990s, wintering ferruginous and rough-legged hawks were considered fairly common on the plains of Boulder County. Since then, urbanization, reduced numbers of jackrabbits and prairie dogs, and global warming have contributed to a steep decline in reported numbers of these open-country specialists.
Ferruginous hawks, named for the rusty feathers on their legs and shoulders, typically nest on broken cliffs in grasslands and prey on large rodents. We’ve seen adults huddled over just-captured prairie dogs and then screaming as bald eagles swoop down to steal their prey.
North American nesting populations have declined in areas where grasslands have been fragmented by highways, gas and oil drilling, and non-native vegetation.
Rough-legged hawks nest in the Arctic and typically prey on small mammals such as mice and voles. They migrate south in winter, searching for snow-free areas where they can hunt these smaller prey. Global warming has pushed the winter snow line northward, stimulating hawks who once wintered in Boulder County to winter in Nebraska and Wyoming instead.
For 35 years, more than 100 Boulder County Nature Association volunteers driving survey routes throughout the plains have tracked numbers of wintering raptors. These surveys have documented a local doubling in numbers of red-tailed hawks, which thrive around human settlements, and a more than 90 percent decline in numbers of ferruginous and rough-legged hawks.
Numbers of wintering golden eagles also have declined. Golden eagle pairs inhabit Boulder County year-round, nesting primarily on foothills cliffs. While there’s no evidence yet of a rash of nest failures, decreasing observations of golden eagles in winter suggest that resident pairs are having to fly farther afield to find large rodents and other prey.
These trends illustrate the striking loss of native prairies in Boulder County. More and more, our remaining grasslands are filled with invasive trees, trails, encroaching subdivisions, and the roar of traffic. White-tailed jackrabbits appear to have disappeared completely. Lark buntings, once considered our most common summer-resident songbird, haven’t been reported nesting in several years.
Sadly, this winter season may offer one of our last opportunities to see good numbers of our magnificent grassland hawks before nearly all of them head for greener and less fragmented pastures.
Stephen Jones and Ruth Carol Cushman are authors of Wild Boulder County and The North American Prairie.
Other December Events
- Look and listen for flocks of red crossbills and Clark’s nutcrackers in foothills forests. These “irruptive seedeaters” have descended to lower elevations in response to bumper conifer seed crops. Also be on the lookout for both eastern and western bluebirds.
- Early Easter daisies continue blooming in Bear Canyon, below the Mesa Trail, where shale barrens provide sun-warmed micro-habitats.
- The Longmont and Boulder Christmas bird counts are held on December 14 and 15. These counts continue a 120-year tradition begun by the National Audubon Society in 1900. New volunteers are appreciated: www.boulderaudubon.org.