The Rough-legged Hawk's nest site is primarily high on a rock or dirt cliff on a ledge or in a recess. Eroded river banks, columnar rocks or mounds of rock or dirt will also be used. Occasionally man-made structures will be used and rarely the ground. Where cliffs are not available trees will be used. Whenever possible, the highest of all potential nest sites will be chosen. Unlike falcons who also use cliff faces, a stick nest is constructed. Twigs, bones or any transportable debris are also used where sticks are at a premium. The nest is lined with grasses, sedges, small twigs, feathers, down and rodent fur. The nest shape may vary depending upon constraints of the site, but will be at least 30 inches (76 centimeters) in diameter and 15 inches (38centimeters) in height with about a 10 inch (25 centimeters) bowl. Nests are refurbished each year and can become quite large but alternate nests may be built nearby (within 100 yards ([91 meters] or up to half a mile away [0.8 kilometers]). Favored sites are likely traditional and used for many years by the same pair unless they are displaced by another raptor. The male brings the sticks and the female builds the nest.
Initial breeding may not occur until age two or three. Clutch sizes vary with food supply but usually two to three and as many as five to seven eggs in a year of abundant rodents. Some areas are consistently three to five eggs and other places four to six eggs with seven being infrequent. The eggs are generally elliptical and average 2.2 inches (56 millimeters) long by 1.8 inches (46 millimeters) wide. The ground color is generally reported as white but it has been described as palish green or blue that bleaches to white. The markings are highly variable blotching and streaking of chestnut, brown or violet and may form somewhat of a ring around the larger end. Not all eggs in the clutch are marked to the same intensity but none are ever plain. The laying interval in the wild is probably 1 day, but in captivity, the interval has been as much as two or three days. Depending upon latitude and general climate, egg laying could start in late April but over most of the range, laying probably peaks from late May through June. Incubation is primarily by the female but the male may perform brief incubation duties in between his hunting forays. Incubation starts as the first egg is laid and averages 31 days.
The nestling period is about 40 days, but males are often able to fly at 36 days, and females around 40. Nestlings can be fed at about six hours and by 21 days, juvenile feathers appear as the young are able to feed themselves. Standing happens by day 28 and by 35 days, they are able to tear up large prey items. Fledglings will remain dependent upon the parents at least until migration begins but some birds are independent at about 30 days after fledging. Rough-leggeds are single brooded but if eggs are lost early enough in the season, a replacement clutch may be laid.
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