The Ferruginous Hawk
(Buteo regalis) is one of three Buteos or
"soaring hawks" that regularly breed in Alberta's grasslands. The
species is currently listed as
endangered under the Alberta Wildlife Act and, Canada-wide, is considered
vulnerable by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC 1999).
The Ferruginous Hawk has a restricted prairie distribution, and compared to other prairie hawks
is frequently described as a hawk of the "open country". The species ranges exclusively within the Great Plains of North America, occupying flat rolling terrain in grassland or shrub-steppe regions. Ecoregions in which the Ferruginous Hawk can be found in Alberta include Dry Mixed-grass, Mixed-grass, Fescue and Aspen Parkland. Generally, the Ferruginous Hawk occurs in desert shrub and grassland regions, west and east of the
Rocky Mountains. The Ferruginous Hawk nests where grazing is the dominant land use or where the open landscape is otherwise relatively wild.
Although the Ferruginous Hawk may coexist with the Red-tailed and Swainson's Hawk, each of these three species of
Buteo is specialized to exist without one or the other in a portion of their range. The Ferruginous Hawk tends to exist without the Red-tailed or Swainson's Hawk in the treeless and
grasslands or desert habitats where it can nest on eroded banks and even relatively level ground. In contrast, the Red-tailed Hawk is strongly associated with woodlands, and on the dry plains relies on wooded river valleys. The Swainson's Hawk is adaptable, occupying the interface between the two
habitats using shrubs for nesting when trees are not available. The Swainson's Hawk also has adapted to agricultural activity and can be abundant in intensively cultivated regions, in marked contrast to the Ferruginous Hawk.
Ferruginous Hawk density within prairie subregions is dependent on the amount of remaining native vegetation. Ferruginous Hawk nesting density decreases consistently as cultivation increases and nesting hawks occur in significantly higher densities in areas
that contain at least half native grassland. Landscapes with sufficient natural grassland for the Ferruginous Hawk tend to exist where cattle or sheep grazing is the dominant land use. However, Ferruginous Hawk density increases with small amounts of cultivation. This could reflect that the hawks benefit through food chain effects arising from the agricultural inputs that elevate productivity, or, that lands that now remain in continuous grasslands, without natural disturbances such as wildfires, are inherently lower in productivity.
Ferruginous Hawks are usually monogamous and have only one mate for one to several breeding seasons. There are no identified subspecies or races recognized. Major differences in appearance of individuals result from variation in plumage. A dark morph, which occurs in approximately 10% of individuals, appears black on belly, back and wings from a distance. This contrasts with the more common light morph, where
the feathers on back, legs and wings are a mix of rusty
"ferruginous" brown, grey and white. Both morphs have a grayish tail.
These large hawks arrive in Alberta in late March to early April. Pairing takes place prior to or soon after arrival to the nesting grounds. The Ferruginous Hawk may use a variety of
places for nesting such as trees, abandoned farmsteads or level ground. Ground nesting was likely more prevalent in the past before trees became more common due to
post-settlement fire suppression, and the planting of shelterbelts. On average, hawks produce more young in elevated nests than ground nests likely because nests are further out of reach of predators and disturbance. Ferruginous Hawks are also more prone to initiate breeding when suitable nests are available and nests on near level ground appear to be used only under extreme nest shortages and superabundance of food. Nest building starts in April and females lay one to five eggs in April or early May.
Only the female Ferruginous Hawk has a brood patch for incubation, although the male may shelter warm eggs briefly. The male is the main provider of food, especially until the fledging of young. The youngest recorded breeders were two years old.
Fledglings and adults generally remain near their nest for one month, or may move a short distance to favorite hunting areas within the territory. In August, young drift away and begin their migration. Adults follow later, remaining on the nesting area as late as mid-October.
Throughout its range, the Ferruginous Hawk primarily relies on only two families of mammals for the majority of its food;
Leporidae (rabbits and hares) and Sciuridae (ground squirrels and prairie dogs). In contrast to the desert-shrub
ecosystems in the western United States, however, white-tailed Jack rabbits
(Lepus townsendii) play a minor role in the diet of the Ferruginous Hawk in Alberta where the species likely has an overwhelming reliance on ground squirrel prey.
Much evidence exists from Alberta suggesting a strong link between Ferruginous Hawk density and/or reproductive success and ground squirrel abundance. Not only are ground squirrels (largely Richardson's ground
squirrels) the most important prey item for the Ferruginous Hawk overall, but during the nestling period ground squirrels made up 89% of prey items. Like other raptors that prefer large rodent prey, the Ferruginous Hawk produces larger clutches than other
Buteo hawks that occur at the same latitude. Buteos that rely the most on rodent prey also show the greatest fluctuations in reproduction depending on rodent prey density. Their specialization on ground squirrel prey may be a substantial reason why the Ferruginous Hawk reproduces so well in years of high ground squirrel numbers.
The possibility of reduced genetic variability in the Ferruginous Hawk has been suggested based on the extensive habitat loss and isolation of some Ferruginous Hawk populations in Canada, and given a possible population decline (bottleneck) during the drought and low ground squirrel populations of the 1930s.
Reprinted from Alberta Wildlife Status Report No. 18
with permission from Alberta
Sustainable Resource Development.