In late autumn, cold air masses come barreling down from the Canadian Arctic like an atmospheric avalanche, bringing with them a visitor from the North: the rough-legged hawk (Buteo lagopus). Its arrival is not as heralded as that of another arctic invader – the snowy owl – but the rough-legged hawk’s southern sojourns are generally more regular than the snowy owl’s irruptive ventures. Rough-legged hawks (known as “roughies” by birders) breed on the arctic and subarctic tundra, feeding on the seasonal bonanza of small mammals. Because they cannot hunt in the dark, they need to leave their northerly locations in the winter as they chase the light. When they head south, they seek out habitats similar to the tundra – open marshes (both salt and freshwater), grasslands, and fallow agricultural fields – where they can hunt for their favorite winter prey, voles.
Rough-legged hawks like to forage in windy areas where they use the rapidly moving air to generate lift and help keep them aloft as they hover. In this regard, they resemble the much smaller American kestrel, floating at a fixed point in the air as they scan the ground for prey. When not hovering over a field, soaring high overhead, or catching a windy updraft along a ridge, these hawks will settle on exposed perches like telephone poles and dead snags. Vantage points that afford a good view of the open habitat where their prey is likely to occur are greatly preferred.
Researchers think that rough-legged hawks (and kestrels) have a special ability that helps them find prey: they can see the urine trails left behind by small mammals. This unusual superpower is a product of two factors: the hawks (like most birds) have a fourth color-related cone in their eyes that allows them to see UV light (in addition to the red-blue-green cones we have), and small-mammal urine reflects UV light. When rough-legged hawks survey a field, the vole urine creates little UV arrows pointing to the grass tunnels that the small mammals frequent.
Rough-legged hawks get their name from feathers that extend down their tarsi to their toes, providing a rough-looking layer of insulation for their legs. They are similar to, but slightly smaller than, our more familiar red-tailed hawks. Rough-legged hawks can be distinguished by their longer and narrower wings, which have a dark trailing edge, dark tips, and a dark wrist patch. In addition, the tail has a white base and a dark outer band. The caveat is that these hawks exhibit a high degree of plumage variation: there are “light morphs” and “dark morphs” as well as age-related differences. This variability can lead to difficulty in identifying rough-legged hawks, but at least some of the attributes highlighted above are present in all individuals.
When these northern hawks arrive at wintering grounds, some individuals remain in the same area for the duration of the season, while others display more nomadic movements. Prey availability and the number of competitors influence the duration of time a given bird will remain in one place. Snowpack may also impact a rough-legged hawk’s decision, with birds opting to seek out areas with reduced snow cover to more easily locate their prey. In the winter, rough-legged hawks supplement their vole-heavy diet with other small mammals, birds, and carrion.
When they head back to their breeding grounds in the spring, their diet is dominated by lemmings and voles, which they bring back to their cliff nests to feed their young. Studies of their diets suggest that it takes about 26 pounds of prey to raise two chicks from hatching to fledging – a mass equal to approximately 100 lemmings for each chick over the span of 30 to 40 days. When the small mammal populations on their breeding grounds crash (which they do every few years), rough-legged hawks will hunt other prey, including ground squirrels, ptarmigan, waterfowl, hares, and even fish, frogs, and insects.
To find these cold-hardy visitors you need to be a bit cold-hardy (or perhaps fool-hardy) yourself. Most of my encounters with these raptors have occurred in open, windswept areas where the cold is driven into you by the wind, sapping the warmth from your body like a heat-hungry parasite. Personally, I find it a toll I am willing to pay to watch these visiting winter hawks.