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The Role of Territoriality in the Social Organization of Harris' Hawks

James W. Dawson and R. William Mannan
The Auk
Vol. 108, No. 3 (Jul., 1991), pp. 661-672
DOI: 10.2307/4088106
Stable URL: http://www.jstor.org/stable/4088106
Page Count: 12
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The Role of Territoriality in the Social Organization of Harris' Hawks
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Abstract

We studied territoriality and sociality in a population of Harris' Hawks (Parabuteo unicinctus) in Arizona during breeding and nonbreeding periods from 1984 to 1986. Our study area contained 22 to 26 breeding groups and density of nests averaged 1/2.0 km2. The number of hawks in breeding groups ranged from 2 to 7 and averaged 3.8. Hawks in breeding groups rarely ranged beyond 0.8 km from active nests, except to visit sources of water, and we did not observe overlap of hunting ranges in any groups. Hawks from different groups formed social aggregations in zones between nesting areas. Aggregations formed only during nonbreeding periods (autumn and winter) and the frequency of aggregations peaked approximately 3 weeks before egg laying. Aggregations averaged 5.9 hawks (range = 4-11), and were composed of members of 2 or 3 adjoining groups and transient hawks. Aggregations may allow potential immigrants to assess and be assessed by group members, and may provide benefits to participants via cooperative hunting. All aggressive behaviors observed during the study, except supplanting (i.e. one hawk replacing another at a perch), were more common between individuals in aggregations than in groups. The most intense aggressive behaviors (e.g. chasing and foot grabbing) were never observed between group members. Resident hawks chased trepassing conspecifics out of the nest area in all incursions observed during breeding and nonbreeding periods. Residents also showed aggression toward a trained conspecific released at 13 of 14 active nests. Our evidence that Harris' Hawks are territorial contradicts part of the information that has been used to reject the habitat saturation model for the development of cooperative breeding in this species. We propose that water, an important resource during the summer, may represent an ecological constraint that favors group living in Harris' Hawks in the Sonoran Desert.

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