The Harris' Hawk nests atop saguaro cacti, in short desert trees, and occasionally in pines and on transmission towers. In Arizona, the nest is commonly situated just below 19.3 feet (6 meters) in a saguaro branch or in the branches of a paloverde. Heights average between 10.2 and 20.2 feet (4.4 and 6.3 meters). Cottonwoods, mesquite and ironwood are also used in Arizona. In Texas and New Mexico, nests are constructed in mesquite, hackberry, Spanish Dagger, and also in stands of live oaks with persimmon.
The bulky nest is constructed of twigs with finer twigs, grass, Spanish moss, bark, and elm shoots as lining. Birds add green sprigs continually while eggs or nestlings are in the nest. The nest is often built atop a mistletoe mass which partially conceals it from below. The Harris' Hawk will refurbish old nests and sometimes use extra nests as feeding platforms.
Clutch size ranges from one to five eggs, but the average is only three to four eggs. The egg averages 2.1 inches (53 millimeters) in length and 1.7 inches (42 millimeters) in breadth. It is of a short subelliptical shape and is smooth without gloss. It is very pale bluish, fading to white during incubation, plain, or lightly dotted or spotted with lavender or light brown.
The Harris' Hawk commonly breeds in groups, the composition of which may vary within an area and from one region to the next. In Arizona, a common pattern is for two males and a female to form a polyandrous trio. Both males mate with the female and all trio members defend the nest and feed the young. Usually a number of "helpers", often juveniles, immatures, and adult plumaged birds from previous broods, bring food to the core group. Occasionally "immigrants" from other families will join a new group. The core group can vary from a monogamous pair to two males and two females mating with each other. Group breeding has also been found in New Mexico but not Texas.
Generally the female does most of the incubation while the male(s) feed her. Incubation lasts 33 to 36 days with an average of 35 days (34 in captivity). The female broods, shades, and feeds the young with food brought by the male (and food brought to the core group by the helpers). Little aggression has been reported between nestling siblings. The young become 'branchers' at about 40 days and begin short flights soon thereafter. Immatures may associate with the breeding group for months or years after fledging.
The Harris' Hawk commonly has more than one brood a year, sometimes up to three. Helpers aid in provisioning the young so that in some cases eggs of a second brood can be laid while the young of the first brood are still in the nest. Renesting after the failure of a nesting attempt is also common.
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