March Nature Almanac: Frigid Conditions Push Some Hawks Southward
Stephen Jones and Ruth Carol Cushman
March 2025
For those of us prone to tell out-of-town visitors that Boulder generally experiences only a couple of weeks of winter, the first seven weeks of 2025 were an anomaly. One arctic cold front after another pushed frigid air up against the foothills, bringing us seven nights with below-zero temperatures and only a handful of days when the highs got out of the twenties and thirties.
January, with a mean daily temperature of 27.9° F, was the second coldest since 2000. The first three weeks of February were not much better, with mean daily temperatures running about 5° below average.
One benefit of these frigid, snowy days is the opportunity to get close to Ferruginous and Rough-legged Hawks. When temperatures are hovering near zero, raptors would rather stay put on their perches than fly away. This makes it relatively easy to creep up to them while driving back roads in northern Boulder County so long as you don’t get out of your vehicle. In January and early February, we enjoyed photographing young Ferruginous Hawks at Boulder Reservoir and near Potato Hill north of Niwot Road, and wintering raptor survey volunteers reported seeing several south of Rabbit Mountain.
A subadult ferruginous hawk perches on a telephone pole near Potato Hill southeast of Lyons. Note the rusty-colored feathers on its breast and legs. Photo by Steve Jones.
During the 1980s and early 1990s, we experienced some winters when Ferruginous Hawks were more numerous than Red-tailed Hawks on the plains of Boulder County. They appear to have been lured here by our abundant prairie dog colonies and expanses of protected open space.
During those years, it was not unusual to see a half-dozen Ferruginous Hawks and Bald Eagles gathered around a single prairie dog carcass. During the winters of 1984-89, up to 40 Bald Eagles, apparently lured by the prairie dog bounty, roosted in a single cottonwood grove below Haystack Mountain.
Suburban development, along with more consistent removal and poisoning of prairie dogs by ranchers and open space agencies, appear to have reduced numbers of Ferruginous Hawks wintering in Boulder County (Figure 1). Meanwhile, Rough-legged Hawks have nearly vanished. Red-Tailed Hawks, urban-adapted generalists that prey on a variety of rodents, lagamorphs, reptiles, and snakes, have increased as our native grasslands have been replaced by riparian woodlands, golf courses, and other semi-wooded areas.
Figure 1. Mean number of each species of Buteo observed per kilometer on the Boulder Reservoir Winter Hawk Watch survey route.
These open-country raptors nest on the ground in the arctic region, where they feed predominantly on mice and voles. Once early winter snows cover the ground, they wander southward, typically settling into snow-free areas with abundant populations of these small rodents. As climate change has pushed the line of permanent winter snow northward, many of “our” Rough-legged Hawks appear to have ceased migrating as far south as central Colorado.
A juvenile Rough-legged Hawk takes off after a mouse or meadow vole. Note the white tail with a prominent black band and the developing black “wrist” spots at the crook in the wings. Young Rough-Legged Hawks can be surprisingly approachable, possibly because on the arctic plain where they’re born, they have little experience with people and vehicles. Photo by Steve Jones.
We suspected that this winter’s January-February snowpack on the plains of Boulder County, along with cold conditions in the northern Great Plains, would push them farther south. The February 18 satellite map showed continuous snow cover still extending from Canada south to southern Wyoming, central Colorado, and southern Kansas.
But we found only a handful of Boulder County Rough-legged Hawk observations reported on the internet during January and early February, and most of our wintering raptor survey teams saw none during that period.
In addition to following the snow line southward, Rough-legged Hawks take advantage of periodic peaks in vole populations, and an abundance of voles in the Dakotas and Wyoming, combined with a relatively shallow snowpack, could have kept them north of Colorado this winter.
For whatever reason, the numbers of Rough-legged Hawks seen on our wintering raptor surveys have plummeted over the past 25 years.
Now that milder temperatures have finally arrived, our Ferruginous Hawks will disperse northward and eastward to nesting sites amid shallow cliffs and in deciduous trees on the prairie. We’ll have to wait another year to see if significant numbers of Rough-legged Hawks ever return to Boulder County.
Find out more about the Boulder County Winter Raptor Surveys on the Boulder County Nature Association website.
Other March Events
Mountain, Western, and a few Eastern Bluebirds arrive on the plains and in the foothills.
Bald Eagles brood recently-hatched young on nests located primarily in cottonwoods on the plains.
Red fox and coyote young poke their noses out of dens located in plains and foothills ravines.
Early easter-daisies continue to bloom on exposed shales along US 36 north of Boulder.
Nature Almanac is a monthly series by Stephen R. Jones and R. Carol Cushman, along with other guest contributors. Ruth Carol Cushman and Stephen Jones are authors of A Field Guide to The North American Prairie (Peterson Field Guides) and Wild Boulder County: A Seasonal Guide to the Natural World.