The Swainson's hawk, is a larg soaring hawk that was named after the British Naturalist William Swainson.
When viewed from the front of breast of the bird, Swainson's Hawks are usually easily identified by the small white patch of feathers immediately beneath its lower mandible, which is followed by a wide rich brown or chestnut coloured to dark brown band that colours the top half of the breast of the bird. There is also a dark morph of Swainson's Hawk, of which only about 10% of the wild population consists of. Therefore, very dark individuals are very difficult to distinguish from similar dark morphs of the Red-Tailed or Harlan's hawk. The exception being that the Swainson's hawk has no rusty colour to the top surface of the tail feathers, whereas Red-Tailed hawks, even the dark morphs; do.
This bird is a summer breeding resident of the western Grasslands and North American Prairies. In winter, this species migrates to the grasslands of Argentina. Their migration is the longest and farthest of any North American raptor. Migrations can last as long as two months, and the birds are known to travel distances of up to 23,000 km (14,000 miles) during their annual migration to Argentina. During migrations, the vast majority of the worlds Swainson's Hawks soar over the the Isthmus of Panama. Therefore, Panama City, Balboa, and Ancon Hill are notable locations to witness this spectacular natural phenomenon. Some individuals have been recorded as overwintering in Florida.
Swainson's Hawks typically feed upon relatively smaller prey than would be expected, considering the size of this bird. Small mammals such a field mice and voles are consumed primarily during the breeding season, but the Swainson's Hawk is distinctive for its preference for preying on huge numbers of grasshoppers and locusts, which are caught either while in flight or upon the ground at other times of year. Small birds and even American Kestrels are known prey items, as are small reptiles and amphibians.
Nesting begins a week or two after the end of the spring migration, usually in late March or April. Pairs have been recorded as returning to the same nesting sites. This species is monagamous and very territorial. New nests are made, or old ones are repaired in about two weeks time. Abandoned nests of Corvids have been used as a foundation for nest construction as well. The nests are usually constructed from small twigs and grass by the males. Swainson's Hawks prefer that their nest be located within isolated trees, or at least within small groups of trees. This species has been known on rare occasions to build a nest upon the ground, or upon a cliff ledge. Nests are usually located about 2.7 to 4.6 meters (8 to 16 feet) from the ground. A shaded canopy, near the top of a tree is preferred. Swainson's Hawks build rather fragile nests that are smaller than the nests of the Red-Tailed Hawk, and nothing like the enormous nests of the Ferruginous Hawk. Therefore, Swainson's Hawks nests often do not last long after the breeding season ends.
The eggs are laid in a clutch of one egg to as many as four. The usual number of eggs laid is two or three. Eggs of this species are aproximately 57mm (2.25 inches) long and 46mm (1.8 inches) wide. This species eggs, though marked with tiny white fleck, soon arrear a dull white after a short amount of time during incubation.
The incubation period lasts about 33 to 35 days, with the female doing almost all the incubating, while the male does most of the hunting and provides the food. The chicks are fed almost exclusively small mammals. Fledging occurs at about five to six weeks of age, and the fledglings remain dependent upon their parents for approximately another month. This species is not known to lay a second clutch if the first is lost.
At the Centre we have found that the Swainson's hawk can go many years producing only infertile eggs, despite their best efforts at creating a family. Then suddenly the same pair will breed successfully. There seems to be no discernible pattern or explainable reason for this odd behaviour. Its entirely possible that this is the result of built up toxins from prior exposure to pesticides in their systems which has done irreversible damage. We have also found that Swainson's Hawks have a profound ability to demonstrate extreme will power, and as such can be positively the most stubborn of all the species of Buteos that the Centre has ever worked with.
Once they decide that they like or do not like something, (a food item, a favourite perch, a certain nest ledge, a rock, a particular way in which food is provided to them etc.) there is no convincing them otherwise. When they make up their minds, nothing will sway them. They would sooner starve than change their minds. This extreme strength of will power must be respected, and ways must be found to cope with their odd choices under these circumstances. Nothing will change their opinions once their minds are made up. Once this aspect of their nature is understood and watched for, they will breed in captivity very well (when they manage to produce fertile eggs), and they are very dedicated parents.
Electrocution from contact with high tension power lines, collisions with traffic, illegal shooting and severe prairie weather like hailstorms all cause Swainson's Hawk mortality. Swainson's hawks are also very easily disturbed at the nest. The nests are usually relatively low to the ground, and often such disturbances cause them to abandon their nesting efforts completely.
The chief danger posed for this species is the usage of pesticides such as DDT and monocrotophos. Given that a large portion of their diet consists of grasshoppers and locusts, this species is affected profoundly by pesticides. This being even more evident in their wintering grounds of Argentina, where pesticide usage has resulted in entire fields littered with the bodies of dead Swainson's Hawks that were found with their crops filled with the poisoned insects. Fortunately, the U.S. educated Argentine farmers to stop the slaughter. Otherwise pesticide usage would have quickly resulted in the decimation of this species. Swainson's Hawks may still be affected by the long term effects of their exposure to pesticides and herbicides which are known to cause permanent chromosomal damage. Effects that have yet to be documented.
In 1986 the Swainson's Hawk was placed on the National Audubon's List of Special Concern. It is listed by the United States Fish and Wildlife Service as a Category 3C candidate. The Swainson's Hawk still remains listed as a threatened species by the California Department of Fish and Game, but was removed from the active US federal list only because it was discovered to be more abundant than was previously known. It is not considered to be a threatened species by the International Union for Conservation of Nature.
Nevertheless, western populations of this species have declined at an alarming rate. This is due to modern agricultural practices and habitat loss, and the resulting loss of their summer prey species.
All these factors together take their toll on the Swainson's Hawks wild populations.
If you have found an injured bird of prey (hawk, falcon, owl, etc.), contact the Centre and our experienced staff can assist in determining what steps should be taken to ensure the bird receives the best possible care.
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