A Comparative Nesting Study of Red-tailed Hawks and Harris’ Hawks in Southern Arizona

Abstract I studied the nesting of Red-tailed Hawks in the Sonoran Desert of Arizona in 1969-1976, emphasizing its ecology relative to the Harris' Hawk, a species that I have found to be sometimes polyandrous and have nest helpers. Red-tails averaged 2.32 eggs per clutch (59 nests) and Harris' Hawks 2.96 (50 nests). The clutch size distributions of the two species were significantly different. Nest success was 81% for Red-tails and 68% for Harris' Hawks, but may actually be slightly lower because a nest was considered successful if a chick was raised to an age of at least 28 days but not always to fledging. For Red-tails, the average number of chicks produced was 1.55 per nest attempt (42 nests) and for Harris' Hawks 1.60 (50 nests). Red-tails nested exclusively in saguaro cacti, but the more agile Harris' Hawks used not only easily accessible saguaro sites but enclosed tree sites as well. Nest site availability seemed to have no significant role in population regulation. The nesting cycle of the Red-tailed Hawk, from egg laying to fledging, was from February to June; for Harris' Hawks it was from February to early December (because of second broods). Optimum habitat and greater hunting versatility in the Harris' Hawk may allow it a less restricted breeding season in saguaro-palo verde desert than the Red-tail by providing essential food resources. These two factors, and nest helping if it is common, probably contribute to maximizing the productivity of Harris' Hawks in an arid environment with variable food resources.