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The rough-legged hawk is one of the best birds on my backyard list. To qualify for the list, the bird doesn't need to actually enter the yard, but it does have to be visible from the yard.

I saw my rough-leg twice within three days. The first time, it was quite far off, apparently flying along the river. The second time, I just caught a glimpse of it as it flew between two houses a couple doors south. (Note that I am assuming I saw the same bird twice. One rough-legged hawk in the neighborhood seems reasonable; two seems unlikely. However, for reasons I will explain later, I may be wrong about that.) My only other sighting in the city was of a bird soaring high over the comer of Irving Park and Southport.

Sightings of rough-legs in the city are unusual. The rough-legged hawk is a bird of open places, so usually you have to get out in the country to find one. Cornfields, especially if they are bordered by a tree line, are good places to look. One fairly reliable place near town is along Irving Park Road at the south edge of O'Hare field. You can often find one sitting in one of the small trees on airport property, surveying the weedy grasslands in search of mice and other small animals.

We see rough-legged hawks in the Chicago area only in winter. They nest in the higher latitudes, hunting mostly lemmings on the tundra from the Northwest Territories of Canada to Siberia. Many northern animals, from polar bears to loons, show this kind of circumpolar distribution. Tundra is tundra it seems, whether it is in northern Scandinavia or northern Alaska; and with only a tiny space separating Eurasia from North America at the Bering Strait, even land dwellers, can move easily from one hemisphere to the other.

Rough-legged hawks begin arriving here in October. They are common from mid-November through the end of March, and we usually hear of some sightings in April and early May. The record high count for the Chicago region comes from Berrien County Michigan, where 200 birds were seen passing in a single day in November 1971. Some of the birds in such large, migrating flocks are mated pairs that seem to stay together through the winter, which is why I say that there could have been two rough-legged hawks in my neighborhood at once.

The rough-legged hawk is a big bird with a wingspan reaching four-and-a-half feet. In flight, it has the broad-winged, wide-tailed look of the buteos, the soaring hawks that are built like small eagles. The only winter bird you might confuse it with around Chicago is the red-tailed hawk, a year-round resident of approximately the same size and general outline.

The plumage of rough-legged hawks is quite variable. Dark-phase birds, as they're called, have dark brown to black bodies. Light-phase birds are variously mottled and streaked, with the mottling and streaking interrupted by a solid dark band across their bellies.

The most distinctive and easily noticed field mark on birds of either color phase is the white tail with its dark terminal band. Seen from above, it gives the bird a white-rumped look. The wings on light-phase birds, viewed from below, show a dark marking at the wrist, or bend, of the wing. Dark-phase birds show solid black contour feathers and white flight feathers tipped, or barred, with black. Both types typically show a white patch at the base of the primaries--the flight feathers at the tip of the wings--when seen from above.

These variations in colors and patterns seem to occur within populations. They do not mark different geographical races. A two-year census taken in De Kalb County revealed that 52 percent of the wintering population were light-phase birds; 31 percent were dark-phase; and the remaining 17 percent fell somewhere between these two extremes. It is likely that similar proportions would hold for Chicago-area birds.

The scientific name of the rough-legged hawk is Buteo lagopus. "Buteo" tells us that this is a soaring hawk. The literal translation of "lagopus" is "rabbit-footed." The name has nothing to do with luck. It refers to the feathering on the birds' tarsi, which extends all the way to the toes. The feathers reminded the taxonomist who named this bird of the fur on a rabbit's foot.

The tarsus is the portion of a bird's leg below the drumstick. On most birds, the tarsus is scaly and unfeathered, but a few arctic species--snowy owls, ptarmigans, and rough-legged hawks among them--have feathered tarsi. Ptarmigans even grow feathers on their toes. These extra feathers are presumably an adaptation to the extreme cold these animals face. The feathered tarsi on the roughlegged hawk are also the source of the bird's common name.

A really experienced eye might notice that the wings of a roughlegged hawk are slightly more slender than the wings of a red-tail, and that both the wings and the tail are somewhat longer in relation to the body size. These differences may be related to the way of life of the rough-legged hawk.

Rough-legs sometimes soar so high that they practically disappear into the empyrean, but their usual hunting method is to fly low and level, crisscrossing over open fields, alternately flapping and gliding, searching the grass for mice. Sometimes they hover over one spot with rapid wing beats, searching the ground below.

Red-tailed hawks are unlikely to do either of these things. But marsh hawks--or harriers, to give them the name the American Ornithologists Union has recently applied--do both. Harriers also have very long wings and long tails, and the wings are somewhat more slender than the wings of buteos. Perhaps selection pressure has equipped both rough-legs and harriers with this configuration to give them more control during their low-level hunting flights.

Rough-legged hawks, for all their large size, have small, relatively weak feet. Birds of prey depend on their feet. Their hooked beaks may administer the coup de grace and tear meat from bone, but their talons are their principal weapons, the tools they use to capture and subdue their prey.

The rough-legs' small, weak feet put a limit on the size and ferocity of prey they can handle. They do sometimes take rabbits and even weasels, but their staple here is mice. Lemmings, their specialty in the northern nesting grounds, are also small animals. Red-tails, with larger and more powerful talons, are far more likely to eat rabbits, or squirrels, or even skunks.

Shooting rough-legged hawks was a popular winter sport before protection laws, and I expect a few retrograde souls still knock off a few when they can. Animals from the arctic are often very trusting of human beings. They probably never saw one in their tundra home, so they had no opportunity to learn to be afraid--especially of a human standing 100 feet away.

Rough-legs follow the melting snow north in the spring, since the removal of the snow cover reveals the trails of mice. When the rough-legs reach the tundra, they seek out cliffs as nesting sites. They prefer a ledge just below the crest of a bluff, a place that will be sheltered from the weather and yet high above the reach of ground-dwelling predators. There they will raise as many as six young in years when lemmings are plentiful.

Rough-legged hawks are no longer the only large birds of prey to come to the Chicago area for the winter. Thanks to the banning of DDT, almost the only good thing Nixon did during his six years in office, we are now getting reports of wintering bald eagles. This year, birds have been reported from the Palos area and other locations south of the city and from Illinois Beach State Park, north of Waukegan.

The appearance of these birds does not come as a total surprise. The estimates of the numbers of breeding eagles in the lower 48 states have climbed from about 500 pairs in the early 60s to almost 1,800 pairs in 1985. Winter censuses in Illinois showed only 63 eagles in 1960, but by January 1986, the total was up to 1,217.

Most of the Illinois birds were found along the Mississippi, but almost 400 of them wintered on the Illinois River. It seems reasonable to surmise that our local sightings represent birds that came north from the Illinois.

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