Data in Wisconsin shows that pairs line up in correspondence with their size, i.e. larger female Cooper's hawks mate with large males and smaller females with smaller males. The data indicated that larger pairs tend to have earlier laying dates, larger broods and more recruits than smaller ones. Relatively large body mass may be a heritable trait. However, no correlation was found between the age of the pair and apparent breeding site quality and time of breeding or annual productivity (though older females may lay slightly earlier than yearlings in most cases). In Arizona, birds of each sex were found to usually pair with like-age individuals. Pairs frequently high circle together. Either sex or the pair together perform in courtship, sometimes over an open field. Courting usually occurs on bright, sunny days, in midmorning. During sky-dances by males, the wings are raised high over back in a wide arch with slow, rhythmic flapping, similar to the flight of a nighthawk, with exaggerated down strokes. Often, much like the northern goshawk, the displaying male flares his undertail coverts. Sometimes frequently for a month or so, the pair will perform as such. When perching together, the male usually keeps on a perch at least away from his unpredictable larger mate. The bowing display reported in Wisconsin, usually (but not always) done by the male, may be a sign to the other mate of their readiness to nest build. Breeding may begin as early as February in the southern part of range, but, for the most part, the breeding season is from April to July. In central New York, the male arrives in nesting woods by March, initially defending an area of or so. Both members of pair arrive by early March in Wisconsin and, in both Pennsylvania and Wisconsin, nest building and copulation is often complete within the month of March. Generally both members of a pair remain on the breeding ground vicinity year-around in Arizona, New Mexico and even in British Columbia.