THE LOWER COLORADO RIVER: We were successful at breeding Harris’s hawks (Parabuteo unicinctus) early in the life of the Santa Cruz Predatory Bird Research Group and initiated a program of releases in the spring of 1979. We worked closely with California Department of Fish and Game, Arizona Game and Fish, U.S. Fish and Wildlife Service, and Bureau of Land Management biologists to locate safe release sites for the hawks between Yuma, Arizona and Blythe, California.
Harris’s hawks thrived historically along the river corridor but the population disappeared due to the effects of DDT, human persecution, and habitat alteration caused by new dams during the early and mid 1900s. With habitat stabilized and mesquite, willow, and cottonwood re-vegetation efforts underway by the Bureau of Land Management, we began the process of Harris’s hawk population recovery through the release of captive-reared birds.
Early in the project, we found that summer releases were too stressful for fledgling aged birds because of extreme heat. Releases were scheduled for fall and large cohorts of juveniles were released successfully from large hack box enclosures by taking advantage of the Harris’s hawks’ natural tendency to socialize and hunt in extended family groups. We observed that hawks released a year or more earlier would help the new birds in their transition toward independence by serving as role models and sentries at the hack site.
A breeding captive pair of Harris’s hawks were released in 1981 and nested near the release site to produce two young. The first recorded nesting by Harris’s hawks released as juveniles occurred in 1985 near Imperial Dam. There have been anecdotal reports of as many as nine Harris’s hawk nests within and adjacent to the Cibola and Imperial National Wildlife Refuges.