The Ferruginous Hawk is the most adaptable of all the buteos in selecting nest sites. The sites vary from trees, flat ground, rock/dirt outcrops, cliffs, utility poles, transmission towers, buildings, artificial nest platforms, and haystacks. Within some broad categories such as cliffs, the variety includes clay, dirt and rock substrates. Tree nests are typically in isolated trees or isolated clumps of trees in exposed locations. Authors differ as to whether ground nests are more successful than tree nests, but they are more susceptible to mammalian predation. Nest locations are reused frequently, but several nests may be built in an area. Typically, one or two alternate nests may exist but up to eight have been found on some territories.
The nests are made of ground debris such as sticks, branches, cattails and earlier, of old bison bones, usually in combination. Old nests will be refurbished, or nests of other species may be taken over and refurbished with sticks being added on top of the old nests. Odd items such as paper, rubbish, barbed wire, cornstalks, plastic and steel cable have been incorporated into nests. Bark from trees and shrubs will be used for lining along with grasses and cow dung. Bits and pieces of greenery are often added to the nest.
Clutch size varies from one to eight and is likely linked to food supply. The average clutch is three to four eggs, each 2.5 inches (6.35 centimeters) long and 2 inches (5.08 centimeters) wide. They are smooth, non-glossy and whitish in color, irregularly spotted or speckled and blotched with reddish - brown markings. There may be a concentration of darker pigments at the small end of the egg. Occasionally, the eggs are almost unmarked or have faint scribblings on them.
Both sexes are involved with building the nests and bringing materials, but the male seems to be more involved in retrieving materials while the female arranges them in the structure. Copulation occurs during and after nest building. The egg-laying period varies with latitude, weather, and possibly food supply. In the Canadian parts of the range, laying occurs during the latter part of April through late June, whereas farther south laying starts from about March 20 through mid May. The earliest recorded clutch was in January in Utah and laying could occur as late as July 3 in Canada. Egg- laying occurs at two-day intervals with incubation starting when the first egg is laid. Incubation is shared by both sexes with each taking approximately the same number of shifts during the 32 day average incubation period. Replacement clutches following failure appears to be rare.
The nestling period varies from 38 to 50 days with brooding primarily by the female. Males fledge at 38 to 40 days and the females as late as 50 days after hatching, or 10 days later than their male siblings as they take longer to develop. Nestlings lie or sit for the first two weeks, stand at about three weeks and walk soon after. By 16 or 18 days, they are able to feed on their own. Wing flapping starts about day 23 and by day 33 the young are capable of vigorous flapping and "flap jumps." The nestlings are sensitive to high temperatures and seek shade however possible in the nest. Initial movement out of the nest is felt to be a response to heat stress as the young quickly move towards shade. The initial flight for the males is taken at 38 to 40 days while the slower-developing females fly about 10 days later. Post-fledging dependency upon the parents may last for several weeks. During the first four weeks after fledging, the young patrol increasingly large areas around the nest as they learn to hunt. Young hawks have killed prey as early as four days after fledging.
The Ferruginous Hawk is single-brooded and like so many raptors, the number of young reared is tied closely to food supply. In areas where jackrabbit populations are the principal food source, the initial clutch sizes and the number of reared young vary closely with variations in the number of jackrabbits. Fifty percent loss of young has been reported in low jackrabbit years and production of young to flight age has been 15 times as high as in years when prey was scarce. Fledging rates of 2.7 to 3.6 young per nest have been reported during years of abundant food supply. The high potential clutch size allows for a quick response to increases in the prey base.
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