Of a Feather: Buteo variations

By BILL CHAISSON
By Bill Chaisson

During my week in Colorado earlier this month I saw relatively few raptors, which I found worrying. I have noticed this lack in the Midwest on previous road trips across the country and attributed it to the miles and miles of industrial agriculture doused or infused with pesticides. But while I saw almost no agriculture during my time in the Rockies, soaring hawks were few and far between.

When I finally did see a raptor it was a red-tailed hawk, which was a little disappointing, as it looked quite a bit like the ones that we see around here. The eastern and western “morphs” of Buteo jamaicaensis may be similar, but there are four other morphs of red-tailed hawk, and one of them doesn’t even have a red tail.

Buteos are the squarely-built soaring hawks, and there are 12 species found in North America. Six of them have regional color morphs, and there is much intergradation from one to the next. This phenomenon would appear to be a plesiomorphy (ancestral trait) for this subfamily (Buteoninae) of hawks, because is not really seen in other subfamilies of family Accipitridae, which includes most diurnal raptors aside from ospreys and falcons. For example, although they have as extensive a range as the red-tailed hawk, accipiters like the Cooper’s hawk and eagles like the bald eagle look very much the same wherever you find them.

Within the buteonines, the more sedentary species demonstrate the most variation. Migrants like the Swainson’s and the broad-winged hawk gather together to fly from North to South America for the winter, which apparently mixes the gene pool effectively. Both of these species have dark morphs, but they constitute small percentages (<10%) of the population and are found in the western portions of their respective ranges. Several buteonine species have a distribution that extends up into the southwestern U.S., but the bulk of the range is in Latin America, and all of them withdraw there in the winter to mix with the permanent residents. In all of these species, it has been found that individuals of the same coloration tend to seek each other out as breeding partners, although this is not consistently the case. The day after I saw the relatively “normal” looking red-tail by the Colorado roadside, I saw another buteo sitting on a utility pole, trying to keep its footing in the wind. This one appeared to be entirely black or a very dark brown. I have seen these dark morphs only on previous cross-country trips; they occur only rarely in the northeastern U.S. On one trip with my brother back in the 1990s we were both defeated when we tried to identify a dark hawk on a wire along a two-lane highway in rural western Nebraska. We thought we got a pretty good look at it, but neither of our field guides were much help. That prompted him to buy William S. Clark and Brian K. Wheeler’s contribution to the “Peterson Field Guide” series, appropriately called “Hawks.” If you plan to travel around the western states, this is an essential reference book to bring along. He also gave me Sibley’s guide when it came out in 2000, and Sibley’s illustrations for the buteos are impressively complete. There are two dark morphs of the red-tailed hawk. The one I saw in August in Colorado could not have been Harlan’s because it breeds only in Alaska and the Yukon. It is the red-tail morph that lacks a red tail. The other dark morph constitutes 10-20% of the western population. It retains the red tail, though the rest of the body is mostly chocolate brown. The bird I saw did not appear to have any red on the tail. The ferruginous hawk has a dark morph that is less than 10% the population, but this western species has noticeable washes of rusty feathers on the chest and wing coverts. I didn’t see my bird in flight, but that would have clinched it, as the undersides of the primary and secondary feathers of the dark ferruginous hawk are white. Of species that are found in Colorado in the summer, that leaves Swainson’s hawk. Again, the dark morph makes up less than 10% of the population, but these birds are almost entirely dark. At rest only the undertail coverts show a deep rust and are barred thinly with white. Had I seen it fly I might have caught the rusty patch on the leading edge of the wing between the shoulder and wrist. The Swainson’s or “grasshopper” hawk is relatively common in the grasslands and prairies of the Great Plains and intermontane West. It went through a sharp decline in the 1970s and ‘80s because in Argentina, where it spends the winter, farmers were poisoning grasshoppers with DDT. The hawks were gorging on the dying insects. U.S. conservationists worked with the Argentines to end the practice and the hawk recovered. It is still struggling in the western coastal U.S. because agribusiness practices there have reduced insect and rodent populations drastically. The light morph western red-tail I saw had more rusty red on the chest and belly than an eastern bird. The western birds are overall redder and darker, lacking the speckled white V on the upper back that is common on eastern birds. In much of the continental U.S. the red tail is a resident year-round. So, while the dark morphs are often scattered through the western populations in the migrating species, the red-tail has four light morphs with distinct regional distributions. The Fuertes’ population lives in the desert Southwest and the Krider’s is found on the northern Great Plains. The eastern and western forms split the country between them with many intermediate forms. Bill Chaisson has been a birdwatcher from the age of 10. He is a former managing editor of the Eagle Times and now lives and works in the town of Wilmot. Contact him at [email protected].