It’s a rough (legged) life

By BILL CHAISSON
Of a Feather
Here in the southern half of New Hampshire, we are a bit below the preferred winter range of the rough-legged hawk, but they do occur here in low numbers between November and March. You need to look for them in open country with scattered trees, where they will spend the next several months hunting for mice and voles in their distinct manner. 

Like the finches of the boreal forests that visit our feeders and the tundra birds that flutter over our empty field, the rough-legged hawk is a northerner that we see only in the winter. These raptors breed in the Arctic, not only in North America, but clear around the globe through Europe and Asia as well. They are buteos, which are called buzzards in the Old World, which are large hawks that often hunt by soaring on nearly motionless wings. The most common local resident is the red-tailed hawk and it is therefore productive to describe the rough-legged in the all the ways that it is different from the familiar red-tail.

Although it is slightly larger on average than a red-tail, the rough-legged is a rangier, more lightly built buteo with longer (wingspan to 56 inches versus 52 inches), more narrow wings and a longer tail. As befits this build, the rough-legged soars less than a red-tail and instead will face into the wind and hover in one place on beating wings like the much smaller kestrel or the larger osprey. When they perch, rough-leggeds have the funny habit of standing on very small twigs near the tops of trees, while the red-tailed hawks generally select something a bit sturdier, like the top of a pole or a power line. In addition to their fondness for precarious perches, when they are not in flight you can see that the legs of rough-legged hawks are heavily feather right to the feet. This is not surprising in an Arctic bird.

In flight, its lankier appearance is one distinguishing feature, but two good field marks also distinguish it from the red-tailed hawks. First, the base of the tail is white and the tips of its feathers are black. The width of this black band varies, but the adult red-tails live up to their name and the juveniles have tails with alternating dark and light bands. Second, the rough-leggeds have squarish black “wrist” marks at the forward bends in their wings. These are present in both juvenile and adult plumages.

Like many buteos, the rough-legged has more than one “phase.” Red-tails are distributed across the entire continent and have a bewildering number of plumages, especially in the Great Plains and montane West. The rough-legged hawks simply have dark and light phases, although there is some variation within each of these. In the eastern U.S. you are unlikely to see any other large dark hawk. Both the red-tail and Swainson’s hawk have dark phases, but they found from the Midwest westward. The ferruginous hawk is dark at rest, but has a white tailed and white secondary feathers. There is a dark phased gyrfalcon, but this Arctic bird very rarely comes down to our latitude and has the pointed wings and rowing flight motion of a falcon. Finally, golden eagles are extremely rare in the eastern U.s. and much, much larger.

In most buteos the males and females are nearly identical, except that, as with most raptors, the females are about 20 percent larger. The genders of the rough-legged hawk are more distinguishable than any other buteo. Compared to the males, the females have much darker bellies and sides and their heads and chests are relatively lighter colored.

One of the ecogeographic rules that is generally taught in high school biology and college ecology classes is Allen’s rule. Formulated by Joel Asaph Allen in 1877, it holds that in endothermic (warm-blooded) animals the appendages should be smaller in colder climates. This is an adaptation to preserving body heat by decreasing the surface-to-volume ratio. Shorter, smaller appendages in effect make an animal tend toward a sphere, which has the smallest surface-to-volume ratio of any geometric shape. Bears are often used as an an example of this phenomenon; the brown bear and polar bear are both larger than more southern bear species (Bergmann’s rule), but the polar bear’s head is relatively smaller, as it is the arctic species, while brown bears are subarctic.

The rough-legged hawk has a relatively small beak and small feet compared to buteos of comparable body size. As these are the two unfeathered parts of their bodies, they are places where the most heat is lost, so this represents are good example of Allen’s rule.

These rules were originally formulated for comparing related species, but they have also been put to work looking at populations within wide-ranging species. It would be interesting to look at populations of the red-tailed hawk, for example, which ranges across the continent and over a wide range of latitude. Do the more northern populations have relatively larger beaks and feet than the southern ones? 

Allen’s rule should apply to change of temperature not just across space but also through time. With reference to the rough-legged hawk, is it responding to changing Arctic climate? If someone looked at museum collections at birds collected 50 and 75 years ago, would the beaks and feet be relatively smaller than those of individuals living in the now significantly warmer Arctic?

If this research has already been done and you were thinking of pursuing graduate work in ornithology or ecology, then doing more research to confirm or refute the earlier findings would be a decent master’s thesis. If the work has not yet been done and you wanted to test the hypothesis, it would be a worth doctoral dissertation.