With a length of 560-690 millimeters and a weight of 970-2500 grams, this is without question the largest and heaviest of North American Buteos. Among North American raptor species; only the Bald Eagle, the Golden Eagle, and the heaviest female Snowy Owls weigh more than the Ferruginous Hawk.
At a distance during flight, the best field marker is the presence of a rough triangle formed by three areas where the pure white feather bases show at the base of the tail and the base of the primaries of each wing. This is the same as that which distinguishes first year golden eagles. However due to the fact that the undersurface of the entire wing of the Ferruginous Hawk is white, this characteristic shows best when specimens are viewed from above. There is a dark “V” formed by the red thighs of adult light phase birds. This characteristic allows for positive identification when specimens are viewed in flight from below.
Immature, or first year birds; are most difficult to identify at a distance, especially while perched. This is even more true for males. Light phase individuals bare a remarkable resemblance to pale Krider’s Red-Tailed Hawks. The dark phase Ferruginous Hawks often closely resemble other dark phase individuals of other Buteo species; such as the Rough-legged hawk, Swainson’s hawk, or even the darkest sub-species of the Red-Tailed Hawk (known as the Harlan’s Hawk). To make matters even more difficult, all of the species possessing these color phases occur regularly within the Ferruginous Hawk’s winter range. However to the discerning eye; during flight the Ferruginous Hawk has a somewhat narrower and bent wing shape when compared to other Buteo species. When not soaring, the Ferruginous Hawk flies much more swiftly on slightly angled wings. This is reminiscent of an osprey. The Ferruginous Hawk alights right on the ground, even on perfectly level ground; far more than other hawks. In fact this species prefers to perch on the ground even when elevated perches are available.
There are three color phases of this species. The pale, or light phase; occurs most commonly, comprising approximately two thirds of the entire population. During the light phase the head is creamy white and the crown is a darker pale brown streaked with dark sepia. The nape of the neck is pale brown streaked with dark sepia and white. This coloration becomes rufous (reddish-brown) on the back. The back, shoulders, and leading edge of the wing when the wings are spread are also rufous and streaked with sepia. The breast is a creamy white, with a few dark shaft lines at the sides of the breast. These broaden to become rufous bars on the flanks and belly. The Thighs are a dark, rich rufous, barred with dark to almost black sepia.
The tail of the Ferruginous Hawk is nearly white toward it’s base, gradually darkening to a marbled grayish or reddish brown at it’s tip. The upper surface of the wings, the primaries and secondaries; are a bluish gray barred with black. The tips of the wings are black. The under surface of the wings is white, except for the wing tips and a few shadowy gray bars on the secondaries. The bases of all dark plumage on the entire bird is pure white, and can only be seen when these areas are exposed.
When viewed from above during flight, these white areas show up as a distinctively white “V” at the base of the tail. A a white area at the base of the primaries on each wing can also be seen. When viewed from below during flight, light phase adults can also be distinguished by the sharply defined “V” of the dark colored thighs against the belly plumage, which is otherwise pale to nearly white.
With Red phase individuals, the plumage is generally much more rufous, with the exception of the wings, which are the same as in light phase specimens. The tail feathers tend toward being marbled with a rich rufous brown, rather than the grayish brown or sepia found in the pale plumage phase.
In the much rarer dark or black phase, the general plumage is entirely a dark sepia brown except for the wings and tail. These areas remain as described above for the light phase.
The Immature, or first year plumage; generally resembles that of the adult. However, it has a much more brown appearance because it is entirely without the creamy over wash and reddish brown barred thighs. Immatures therefore appear nearly pure white when viewed from below. The dorsal plumage, or mantle; is a pale sandy brown with dark streaks.
According to many sources, the Ferruginous Hawk is more active and aggressive than other Buteo species. This species soars well. However; having a narrower wing with a higher wing-loading than other North American Buteo or eagle species, the Ferruginous Hawk must compensate by maintaining a higher flight speed. Thus when soaring, the Ferruginous Hawk's flight has less of a stalling and floating quality to it than other species.
It is interesting to note that the tarsi of the Ferruginous Hawk, like those of the golden eagle; are feathered right down to the toes. This is a characteristic usually only associated with Booted eagles. The cere, gape skin, and feet are a bright yellow. The eye is a pale yellow in first year or immature birds, darkening to light amber or even brown in the adults. This hawk has evolved a huge mouth. The gape extends to well below the eye. With this large gape, the Ferruginous Hawk is capable of easily and instantly swallowing entire rodents up to the size of pocket gophers, and even the smaller ground squirrels! This unique and very distinctive characteristic probably evolved as a way of reducing the chances of the Ferruginous hawk being the victim of raptor piracy from larger more aggressive species, such as the Golden eagle.
The Ferruginous Hawk has evolved as a species of open desert and prairie landscapes. Therefore, it is extremely unlikely to be found in any other habitat. The Ferruginous Hawk is confined to the plains and deserts of western North America. It can be found from California, Arizona and Texas north, to extreme southwestern Manitoba, southern Saskatchewan, and Alberta. The range of this species is entirely west of the Mississippi, with only a very tiny fragment extending beyond the borders of the United States into Canada. With the exception of extreme southern California and northern Baja, this is strictly an inland species. There is some southward movement in the more northern parts of the Ferruginous hawk’s range in the autumn. However, some individuals also spend the winter slightly farther north than the Ferruginous Hawk has ever been recorded as breeding. Therefore as a species, this hawk must be considered as essentially non-migratory.
This species shows a unique hunting specialization for a Buteo. Prey is seldom attacked directly from high aloft in the usual Buteo fashion. This species shows a marked preference for a low, skimming approach when attacking prey. However, such surprise attacks are often to quarries that have first been observed and their position marked while soaring from above.
When perched the Ferruginous hawk pays sharp attention to the flight pattern of any other birds seen flying. If a bird continues flying until it passes over the horizon, or disappears over a hill, no attempt is made to hunt it. However; should such a bird perch within sight, the Ferruginous hawk suddenly shows great interest by stretching as tall as possible, head held high in a most comical manner, thus marking the spot and the distance with remarkable accuracy.
The Ferruginous hawk then launches itself into a swift, low to the ground flight. Depending largely on the nature and contours of the intervening land, the flight path chosen for attack may be quite direct or even somewhat indirect. Either way, the attack is always launched with the last seen position of a given quarry kept in mind. Thus, by carefully keeping the quarry’s view of the hawk’s approach blocked behind any ridge or low obstacle, the Ferruginous Hawk is able to deftly surprise it. Skillfully accounting for any of the quarry’s movement, often as much as three to ten meters from where it was originally seen; the hawk then depends on its excellent vision and quick reflexes to discern, veer, and strike all in a split second sequence! Only once such an attack is missed, is a long aerial chase, usually for hares or game birds; likely to ensue.
From April to August, when ground squirrels are active; this species feeds on little else. However, during the late autumn and winter months, this food source changes. It is thought that the presence of large populations of raptors which are specialized for small rodent hunting; such as Northern Harriers, Rough-Legged Hawks, and Short-Eared Owls, has resulted in the Ferruginous Hawk evolving towards hunting larger and more active prey in order to avoid competition.
At such times, it hunts the white, as well as the black-tailed jackrabbits. The Ferruginous Hawk also hunts birds; particularly large, upland game birds. All of the indigenous species and introduced species of game birds are hunted. It is only when the Ferruginous Hawk is seen in direct aerial pursuit of one of these quarrys that the full speed and determination of this big hawk is so impressive. At these times the Ferruginous Hawk flies almost like a falcon, on swept wings, with strong and regular wing beats. Holding a position behind the quarry by three to four meters without losing ground or gaining it. Instead, pacing it exactly, the Ferruginous hawk follows until the quarry tires and then makes a desperate attempt to get down into some ground cover. The instant that the quarry touches ground, the hawk accelerates and strikes. Ferruginous Hawks also hunt small birds, particularly horned larks and meadow larks; in about the same way as they hunt their summer quarry.
The Ferruginous Hawk is strictly non migratory. As such, this species remains in its winter habitat throughout the year. Almost without exception, the nest site is located near a colony of one of several species of western ground squirrels. During the earliest part of their annual courtship, consisting of soaring and diving flight displays; these ground squirrel populations are not yet available as food for the young. The hawks spend this time repairing and adding to the nest, with the female doing some hunting.
This species builds the largest nest structure of any North American bird. The nest is even larger than the large nests of such phenomenal nest builders as ospreys, and even Bald and Golden Eagles. This species prefers to use trees where they are available. In the northern plains states, and in southern Alberta and Saskatchewan, abandoned shelter-belt plantings around farms are well suited to this species requirements. However, in treeless ranch country broken by coulees, low mesas, and hill scarps, this species nests right on the ground, or if possible on a cliff ledge. Though typically they nest right at the upper edge of a low escarpment or a coolee.
In this instance, the nest is placed in such a way that the rim of it is only slightly below, or level with the scarp edge. It is these nests, that after many years of use; reach the most incredible proportions. Nests of this nature have been known to measure as much as 2-1/2 meters in diameter, and twice that from the down slope base to the rim. The female Ferruginous hawk ceases all hunting activity once their summer food supply of ground squirrels break hibernation between mid march and mid April and become available. At this time all food is supplied entirely by the male. It is thought that this sudden reduced physical activity by the female is necessary to allow the female’s body to devote energy to egg production.
This species in unusual among large raptors, in that it produces large clutches of eggs. It shares this infrequent characteristic with two other species, namely the Snowy Owl, and the Rough-Legged Hawk. In all these instances, the clutch size is directly related to the abundance of food brought to the nest by the male. In years when ground squirrels are abundant immediately prior to the period of egg production, this species can raise large broods of up to five or six. However, in years of prey scarcity, brood size will be dramatically reduced, or breeding will fail to occur at all.
The eggs are predominately white in coloration. They are blotched to varying degrees with light to dark brown, and in some cases even with a purplish shade to them. The first egg is usually the largest and most heavily blotched, but as the clutch is laid the eggs become whiter, until the last egg laid, which is often almost completely white. The average egg measurement for this species is 6.2 X 8.6 mm (Brown and Amadon) The incubation period is between 28 and 32 days. The young remain at the nest for approximately 60 days. This species requires unusually low humidity in order for its eggs to incubate successfully. In some cases the relative humidity around the eggs must be as low as 12%.
This species is known to be extremely aggressive in defense of the nest site, as the nest being located on or near the ground is especially vulnerable to predation by mammalian predators such as coyotes and wolves. Even when it comes to human intrusion at the nest site, this species is aggressive to the extent that nests of this species are best avoided by man. Intruders are often attacked as much as a kilometer from the nest, and in some cases they can be knocked off their feet. Early accounts of mountain men and trappers attacks by large hawks are likely attributable to this species. Ferruginous Hawks are very sensitive to intrusion at any time during their breeding season, particularly during the production of eggs or the onset of incubation, or when the young are newly hatched. Human disturbance at these times usually results in complete abandonment of breeding activity for the year.
Like most open landscape raptorial birds, young Ferruginous Hawks remain at the nest until they are flying well. Their first flights are often quite strong. They spend a lot of time exercising their wing muscles by holding onto the edge of the nest with their feet while facing into the wind and flapping vigorously like eagles. Even after leaving the nest, they return regularly for several weeks. The young usually attain independence through abandonment by the adults, which forces the young to leave the parent’s territory and wander extensively in search of food.
Among Buteos, the Ferruginous hawk is among the least vocal. The call most commonly heard is made by the adults during times when they are angered by intruders at the nesting site. It is a harsh “kree-a-ah”, beginning high and sharp and descending down the scale to a gull like gargle. Occasionally this call is followed by one or two shorter “kraa-kraa” calls, also very gull like in tone.
The Ferruginous hawk has a markedly restricted geographical range, indicating specialization. This is due in part to the fact that it only nests where its nesting requirements are met, and also by the fact that large areas of is former habitat have been consumed by human agriculture and development needs.
Ferruginous Hawks also face severe depletion of their necessary breeding season quarry, ground squirrel species and prairie dogs; due to their decimation through continual poisoning by humans wishing to eradicate such rodent pests from their land. As a result this species probably has the fewest number of living individuals in the wild of any of the diurnal raptorial birds of North America. Although the Ferruginous hawk is currently listed by CITES (the convention on the international trade in endangered species of wild fauna and flora) as Appendix II (threatened), it is a species very likely to have this listing ‘upgraded’ to Appendix I (endangered) in the very near future!
This species is one of the number of very unique life forms that have evolved only in the special habitat of the western North American plains and deserts, and like a number of it’s congeners, such as the burrowing owl, prairie falcon, prairie dog, black-footed ferret, prong-horn antelope and kangaroo rat; occurs nowhere else in the world. Many observers who have studied this species in detail have noted its close resemblance in morphology, plumage, markings and even hunting habits to the Golden eagle. It has been suggested repeatedly that this species would be as well, or better classified as a second North American species of Aquilla (Booted eagle), than as a Buteo!
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