Characteristics And Behavior
Generally, hawks kill their prey with their claws, unlike the falcons, which catch prey with the claws but kill with a blow of their beak. However, despite their fierce reputations, some hawks are quiet and gentle. In addition to their familiar scream, hawks' vocalizations include a high plaintive whistle like the wood pewee (broad-shouldered hawk); a musical kee-you, kee-you (red-shouldered hawk);and a high-pitched squeal (short-tailed hawk).
Hawks are unusual among birds in that the female is generally larger than her mate. In some species, this difference—called sexual dimorphism—can be as great as the female being twice the size of the males, as in the accipiters. Some researchers have found a correlation between the size difference between the sexes and the diet of the species. For example, among Falconiformes like vultures, which eat carrion, the sexes are similarly sized. However, moving from there through the diets of insects, fish, mammals and birds, the sexual dimorphism increases. So many other factors correlate with sexual dimorphism, it is difficult to say which is the major contributing factor. For instance, another hypothesis holds that a larger female bird of prey is better equipped to protect herself during contact with the potentially dangerous and certainly well-armed male. Yet another theory suggests that size is related to the vulnerability of the prey pursued. That is, the more agile the prey, the less likely the success of each hunt. Or, perhaps the secret to sexual dimorphism lies in a simpler explanation: that the larger female is better at catching some prey, and the male is better at catching others.
Courtship among the hawks is among the most spectacular of all animals. In the case of the red-tailed hawk, for example, the pair soar, screaming at each other; then the male dives at the female, who may roll in the air to present her claws to him in mock combat. The male marsh hawk flies in a series of graceful U's over the marsh from where the female is watching. Hawks generally mate for life, and are strongly attached to their nesting territory; one pair of red-shouldered hawks (and their offspring) used the same area for 45 years.
Hawks usually build their nests high in trees. The nests are quite large, up to about 3-4 ft (0.9-1.2 m) across, and consist mostly of sticks, with twigs, bark, moss, and sprigs of evergreen. Nests are often used year to year, with the bird abandoning it only at death or when the nest has grown so large that it breaks the boughs it is built upon.
Generally, the pair will defend their territory against all who approach, but some species, including the ferruginous hawk, will abandon their nest if disturbed by humans. Some hawks will dive at humans who approach too near their nests, as in the case of a pair of red-tailed hawks nesting in a park in Boston, who injured several curious passers-by before park officials removed the raptors and their eggs to a more secluded spot. The territory defended can range from 650 ft (198 m) between nests in small hawks to up to 18.5 mi (29.8 km) in larger ones. Some species, including the kites, are more gregarious and nest in loose colonies of about 10 pairs.
Female hawks lay between two and five eggs. Depending on the species, the female either incubates them alone or with the help of her mate. Incubation lasts about 28 days. The young hawks fledge at about 40 days of age.
Some young hawks may remain with their parents for a while after fledging, and these family groups have been observed hunting as a team. Generally, hunting buteos circle high in the air, watching the ground for any movement of prey. They then fold their wings and dive upon their prey. Accipiters are more likely to pursue their avian prey on the wing, darting thickets and woods during the chase. Some accipiters are decried for their impact on the populations of songbirds; in fact, in the past some ornithologists considered the sharp-shinned hawk a "harmful" species because it preyed on "beneficial" songbirds. Such human prejudice is at the root of most human-raptor conflict.
After the breeding season ends, many hawk species conduct spectacular migrations. The most spectacular is that of the Swainson's hawk. Huge flocks of these birds will travel overland from their North American summer range to their wintering grounds in South America, a total distance of 11,000-17,000 mi (17,699-27,353 km) annually. The broad-winged hawk (B. platypterus) is also noted for its large migrations: in one day (September 14, 1979), 21,448 broad-winged hawks passed over Hawk Mountain, Pa. Besides Hawk Mountain, other good sites to watch hawk migrations include Cape May, NJ; Duluth, MN.; Port Credit and Amherstburg, Ont.; and Cedar Grove, WI.