In level flight, The Rough-legged Hawk tends to utilize steady wingbeats with less of the "pump and glide" type of flight that other buteos use. The wingbeats have been described as "unhurried and methodical" with the overall flight called "effortless, graceful and purposeful." Soaring flight is used and the wings are held in a pronounced dihedral. The wings extend up from the body and then flatten out. This species seems less dependent upon the use of thermals than other buteos. Hunting occurs during the day but often until much later than in other buteos and can be grouped into the following tactics.
Perch and Wait
- perching with an erect stance on elevated objects such as lone trees, power poles, and building tops is used as an energy saving method
Quartering Flight
- birds will fly slowly, alternately flapping and sailing to cover ground much like a Northern Harrier
Hovering
- using rapid wing beats, birds will hang motionless in mid-air, searching visually and possibly with its ears for prey. This usually occurs at altitudes of 50 to 100 feet (15.5 to 31 meters) but at times in excess of 200 feet (62 meters).
Fishing
- birds have been seen catching fish with Osprey-like plunges into water. They have also been seen wading in water to catch dying fish.
Piracy
- although often the victim of birds like Common Ravens, the Rough-leggeded Hawk will attack other raptors and cause them to drop their kill which is then consumed. They will also rob other Rough-leggeded Hawks
Carrion Eating
- wintering Rough-legged Hawks have fed on dead fish washed ashore in freshwater environments as well as on mammal carcasses
Strike, Kill, and Consumption
- the tarsi may be lowered well prior to pouncing on its prey such as when in hovering flight and prey is spotted. The bird will drop with extended legs and if there is nothing to seize, it will glide a short distance and pull up into a hovering position again. Prey are seized with the talons as in other buteos. Prey are typically beheaded and larger victims are torn apart.
The Rough-legged Hawk is quite trusting and docile around humans, allowing close approach which can put them at some risk. In captivity, at rehabilitation centers, it is usually quite calm and unassertive. Around the nest however, it is a fierce defender, engaging in close swoops and chases in order to drive off intruders. However, it is not as aggressive as raptors like the Peregrine Falcon and may well lose out in skirmishes over the ownership of a potential nest site. Response to human intruders at nests varies from limited to aggressive and body contact is not unknown. Territorial conflicts have been reported with Peregrine Falcons, Gyrfalcons, and Golden Eagles as well as aggressive interactions over prey with other Rough-legged Hawks, Northern Harriers, Red-tailed Hawks, and Common Ravens. Communal roosting at night, on some wintering ranges, has been documented in association with high rodent populations. The Rough-legged Hawk is mostly solitary or perhaps paired in migration but it is common to see several perched within a half mile (0.8 kilometer) of each other and hundreds have passed some key migration points in a day in loosely aggregated flocks.
Nesting density probably varies with the availability of nest sites and food supply and it is quite variable across the species' range. Density estimates for one pair of birds range from two to 31 square miles ( 5.2 to 7.8 square kilometers) with a suggested average of one nesting pair for every three to four square miles (7.8 to 10.4 square kilometers). Hunting territories probably range from two to three square miles (5.2 to 7.8 square kilometers) and increase to at least four square miles (10.4 square kilometers) after the young have hatched. Internest distances range from 100 yards (91.4 meters) to 14 miles (22.4 kilometers) with averages closer to two miles (3.2 kilometers) in parts of Alaska. Home ranges on winter territories cover four to six square. miles (10.7 to 15.5 square kilometers).
Some observers believe that pair bonding occurs on the wintering ground or during migration as at least some birds may be mated before arrival at the nest site. "High - circling" has been reported on the wintering grounds and also on the breeding grounds. This consists of paired birds circling with the wings and tail spread and occasionally passing close to each other and uttering a two syllable call. The "sky-dance" pattern typical of buteos consists of the male folding its wings tightly in flight and hurtling towards earth. It checks its descent, climbs steeply, stalls with the body held vertically and then pitches forward into another plunge, repeating the whole sequence several times. This display is used on the nesting territory but for only a short duration after arrival. Nesting material may be offered to the female before copulation which starts at least two weeks before the first egg is laid. Pre-laying copulation frequency averaged once or twice per day in a pair of captive birds. The duration of copulation is 8 to 10 seconds. Territorial defense consists of close swoops and chasing flight with the occasional physical contact.
The average life span of 48 dead birds recovered in one study was about 21 months. The oldest bird was 18 years. Collisions with automobiles on the wintering grounds are common as this species frequently hunts roadways. Shooting has historically been a visible cause of mortality but recent figures are not available. As with all predators, the availability of an adequate food supply is critical and starvation on wintering grounds subject to deeper snows is likely higher than in low or no snow areas. Predation on nestlings has been noted by Golden Eagles but this is likely an unimportant source of mortality. On the wintering grounds, birds have died after consuming rodents that had been eating grain treated with Dieldrin. This is an uncertain source of mortality.
ADAPTATIONS
For a buteo, its ability to feed on carrion or hunt for fish is somewhat novel. It will also hover in lighter winds than heavier buteos.
