Reexamining Cooperative Hunting in Harris's Hawk (Parabuteo unicinctus): Large Prey or Challenging Habitats?

Abstract

— 548 — The Auk, Vol. 130, Number 3, pages 548−552. ISSN 0004-8038, electronic ISSN 1938-4254.  2013 by The American Ornithologists’ Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals. com/reprintInfo.asp. DOI: 10.1525/auk.2013.120063 3Present address: 64340 Fogg Lane, Pearl River, Louisiana 70452, USA. E-mail: jacoulson@aol.com In group-living carnivores, cooperative hunting may be a cause or a consequence of cooperative breeding. Harris’s Hawk (Parabuteo unicinctus), a raptor that hunts and nests in cooperative groups of 3 to 7 hawks (Mader 1975a, b; Bednarz 1987; Dawson and Mannan 1989), provides an opportunity to examine benefits and possible causes of sociality because its cooperation is facultative and the species is relatively well studied. Attempts to demonstrate factors that promoted the evolution of cooperative breeding in Harris’s Hawks have mostly been unsuccessful. For example, the reproductive performance of groups did not differ from that of pairs (Mader 1975b, Bednarz 1987). And, for the most part, researchers did not find differences in the habitat quality or the resources available in territories occupied by groups versus pairs (Bednarz and Ligon 1988, Dawson and Mannan 1991b). However, Bednarz (1988) demonstrated net energetic benefits for hunting parties of 5 or 6 hawks and proposed that cooperative hunting led to social living in Harris’s Hawk. Here, we support and extend this argument with a reinterpretation of data from the published literature, including an overlooked finding of significance. We hypothesize that one benefit of cooperative hunting is to increase success in habitats that make the prey difficult to catch (e.g., a high density of hiding places). We think that this benefit could be as important as capturing and overwhelming large prey, a benefit proposed by Bednarz (1988). We propose the “challenging habitats hypothesis” (CHH), similar to an idea proposed by Dawson (1988), as an additional or alternative explanation for the maintenance and, possibly, the evolution of cooperative hunting in Harris’s Hawk. As falconers, we developed this hypothesis after many years of hunting with single and cooperative groups of 2 to 8 Harris’s Hawks in a variety of habitats. We hope to rekindle scientific inquiry into cooperation in this species. In cooperatively breeding birds, helpers are usually offspring that delay natal dispersal. Factors proposed to explain delayed dispersal include a lack of suitable breeding territories and other ecological constraints such as a lack of available mates (Selander 1964, Emlen 1982); increased probability of survival and of inheriting the natal territory (Stacey and Ligon 1987, 1991); and high-survival, K-selected life-history traits, and low turnover of territories (Brown 1974, Ricklefs 1974, Arnold and Owens 1998). Kin selection may maintain helping in relatives, whereas helping in non-kin is maintained by reciprocity, intraspecific mutualism, or manipulation (Trivers 1971; Clutton-Brock 2002, 2009). These hypotheses are not mutually exclusive, and some or all probably help explain cooperative breeding in many cases (Hatchwell and Komdeur 2000, Pen and Weissing 2000).