Among 90 independent pairings of Cooper's Hawks (Accipiter cooperii) in Tucson, Arizona, fewer adult-subadult pairings and more adult-adult and subadult-subadult pairings occurred than was expected. On average, hatching occurred at nests of adult pairs five days earlier than subadult male-adult female pairs, 12 days earlier than adult male-subadult female pairs, and 19 days earlier than subadult pairs. Brood sizes were largest among adult pairs, lower among mixed-age pairs, and smallest among subadult pairs. Adult female Cooper's Hawks nested earlier and had larger broods than subadult females, but brood size did not differ between subadult females paired with adult or subadult males. Adult pairs fledged significantly more young per year than mixed-age and subadult pairs. Differences in phenology between adult and subadult Cooper's Hawks may be associated with adults being preferred as mates. Differences in productivity, however, may be more closely associated with age-related constraints on reproductive physiology of females.
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Vol. 103 • No. 2