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I describe the breeding pattern and system of the Harris' Hawk (Perebuteo unicinctus) and test several hypotheses that have been proposed to explain avian cooperative breeding systems. Individual Harris' Hawks were color-marked and monitored for a 40-month period in southeastern New Mexico. Between 29% and 70% of Harris' Hawk social units made a second breeding attempt in either late summer or autumn within each year, often following successful spring breeding. Clutches were initiated between 17 February and 1 November. The interbrood period was positively correlated with the size of the first brood. The number of second nests initiated was correlated with the number of lagomorphs, but not with precipitation. Harris' Hawks did not time nesting to coincide with the peak population levels of their principal prey; rather, this hawk has a flexible and prolonged breeding season that may have evolved in part to minimize the impact of stochastic climatic conditions that increase the probability of nest failure.

The average size of breeding units was 2.7 individuals, with pairs being most common (49%; n = 61). Groups of more than two hawks (n = 23) included both adult- (39%) and immature-plumaged (83%) supernumeraries that were primarily offspring of the same social unit. Immatures rarely provided food to nestlings, whereas adult supernumeraries commonly did so. Preliminary electrophoretic analysis of two groups did not suggest polyandry. Pairs and groups showed no differences in clutch size, number of young produced per successful nest, or number of offspring fledged per year. Given the overall lack of correlation between reproductive output and group size, I suggest that kin selection has not been a major influence in the evolution of the Harris' Hawk breeding system.

Univariate and multivariate analyses of 21 habitat variables revealed no differences (P > 0.1) between the nest areas of pairs and groups. However, Harris' Hawks tended to exploit habitats with greater densities of larger mesquites, less grass cover, and more exposed ground than Swainson's Hawks (Buteo swainsoni). The range of habitat used by both species was similar.

The density of lagomorph fecal pellets (an index of the abundance of the primary prey species) was slightly greater in Harris' Hawk group areas than in pair areas in summer, but the pattern was reversed in winter. Pair ranges had significantly fewer woodrat (Neotoma, a prey of minor importance) nests than group areas. Patterns of habitat variation and resource abundance were not consistent with predictions of either the habitat saturation model or the extended polygyny threshold model of cooperative breeding. Neither was frequency of groups in the hawk population negatively correlated with resource levels, as predicted by the fluctuating environmental constraints model.

My results fail to support any of the hypotheses examined. Alternatively, I suggest that a limited time period of peak prey vulnerability and/or reliance on relatively large prey may have favored cooperative hunting. The benefits of cooperative foraging may be particularly important to juvenile hawks in insuring them an adequate portion of food on a predictable basis. Thus young hawks may pursue a survival maximization strategy early in life that is achieved in part by remaining on their natal range and cooperating with their parents or other current breeders.

Project Sponsors

The fieldwork was partially supported by the Department of Energy (Waste Isolation Pilot Plant studies), The Frank M. Chapman Memorial Fund, The University of New Mexico (UNM) Graduate Student Research Allocation Committee, and the Graduate Research Allocation Committee of the Biology Department, UNM.



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UNM Biology Department

First Committee Member (Chair)

J. David Ligon

Second Committee Member

Manuel C. Molles

Third Committee Member

John A. Weins

Fourth Committee Member

William R. Rice

Fifth Committee Member

John Faaborg


Sixth Committee Member: Edward J. Bedrick

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