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Ferruginous Hawk

North America’s Largest Hawk

By John Hughes

The Ferruginous Hawk, Buteo regalis, whose name means a kingly or royal hawk, is so named for its large size and eagle-like appearance. Its common name Ferruginous comes from the Latin word ferrugo, meaning rust, to describe the rusty brown color of the species’ light morph. While uncommon west of the continental divide, keep your eyes peeled for the occasional vagrant in the Flathead and Mission Valleys.

Ferruginous Hawks are large, heavy buteos with robust chests and long, broad, pointed wings. Length ranges from 20 to 27 inches and wing span from 48 to 60 inches with females noticeably larger than males. Their big, pale head has a dark cap and gray cheeks with a large bill and a long gape. The cere and mandible margins are yellow on an otherwise dark bill. Like Rough-legged Hawks, Ferruginous Hawks have feathers on their legs that extend to their yellow toes. Adult light morphs have a rufous-red (ferruginous) color on their back and shoulders. When viewed in flight, look for a dark V formed by rust-red leg feathers folded against a whitish body and tail. The upper surface of the wings is brownish gray with prominent white streaks in the primaries. The underside of the wings is light with varying degrees of rufous and gray flecking. The dark morph, less than 10 percent of the population, is various shades of brown and ferruginous on the upper and lower parts of its body but retains a white un-banded tail, white streaking in the upper primaries and white on the trailing edge of the underside of the wing feathers.

Arriving as early as mid to late March in some areas of the state and leaving in August to early September, Ferruginous Hawks are primarily breeding migrants in Montana. Look for Ferruginous Hawks in the wide-open spaces east of the continental divide, where they inhabit mixed grass prairie, shrub-grasslands, grass-sage complex and sagebrush steppe. The highest concentration of Ferruginous Hawks in Montana is at the Kevin Rim IBA, located south of the Alberta border and 20 miles northwest of Shelby.

Ninety percent of Ferruginous Hawks’ diet consists of prairie dogs, rabbits, hares and ground squirrels, but they will take the occasional bird, reptile, amphibian and insect. Low, rapid flight over an open area is most often used when hunting, but pouncing from a perch, swooping from above and hovering are tactics also employed to catch prey.

Prior to the elimination of bison on the plains, bison bones and wool were often found in Ferruginous Hawk nests. Today, from mid-April to mid-May, huge stick nests lined with turf or cow dung are constructed in lone trees, on cliff faces and rock outcrops, or on the ground if no elevated sites are available. Ferruginous Hawks will also build nests on man-made structures such as power poles, abandoned buildings, haystacks and artificial nest structures. Nest sites are fiercely defended against predators. Nest building is secretive; both male and female participate in the process and will often abandon sites if disturbed. Several sites, including old nest sites, will be explored before the final location is selected.

Ferruginous Hawks produce one clutch per season. Clutch size is prey density dependent averaging two to four eggs but can range between one and eight eggs. Females incubate the eggs much of the day and night with some early help from the male. For his part, the male patrols the area around the nest and brings food to his mate. Incubation is estimated to be 32-33 days. During brooding and after hatching, the female rarely leaves the nest until the chicks are older, and even then she doesn’t go far. The male continues to hunt and patrol the nest area. The young leave the nest at 38 to 50 days with the smaller males leaving first. Fledglings remain in the nest area about 28 days before striking out on their own.

Ferruginous Hawk populations are presently stable in Montana, but there are reasons for concern. These include fragmentation or loss of habitat associated with oil and gas exploration and with agricultural development. Due to prey specificity, the use of rodenticides to control prairie dog populations, along with other rodents, also poses a great threat to Ferruginous Hawks by increasing the risk of secondary poisoning and decreasing prey availability.

Equally troubling is the increased proliferation of wind farms. At the Kevin Rim IBA, Montana Audubon successfully lobbied NaturEner, a Spanish-owned company, to move 25 wind turbines one-half mile from nesting raptors to cut down on bird collisions. We can only hope that continued efforts to set aside large areas of land, eliminate the use of rodenticides and institute bird-safe wind programs will be effective.

In the meantime, take a closer look at any large, light-colored hawks you see as you drive around the valley. Who knows, you might be lucky enough to spy a Ferruginous Hawk.

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