Breeding Season Diet of Northern Goshawks in Southeast Alaska with a Comparison of Techniques Used to Examine Raptor Diet

Publication Date

6-2001

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Raptor Biology

Department

Biology

Supervisory Committee Chair

Marc R. Fuller

Abstract

A variety of factors influence a species' viability and survival (Lack 1954, Newton 1979). The availability and abundance of food, nest and roost sites, and cover, as well as processes such as predation and competition, all can act to limit populations. Understanding the processes of species limitation and the integral factors involved, is not only central to the study of ecology (Newton 1998), but forms the basis for practical wildlife management (Errington 1935, Litvaitis et al. 1994). While each of these factors plays a role in a species' ecology, there can be little doubt as to the fundamental importance of diet and feeding ecology (Newton 1979). In areas where nesting habitat is readily available, lack of food is often the limiting factor (Kenward and Widén 1989, Newton 1994).

Populations of northern goshawks (Accipiter gentilis; hereafter goshawk) can be food limited during certain years or seasons (Widén 1989, Doyle and Smith 1994, Ward and Kennedy 1994, Dewey and Kennedy 2001). The link among goshawk prey species, the habitat they require, and the management of those habitats is recognized by the USDA Forest Service as a key element for conservation of goshawks and the biotic communities in which they occur (Reynolds et al. 1992, Iverson et al. 1996). Management guidelines for important prey of goshawks have been published for the southwestern region of the Forest Service (Reynolds et al. 1992) and other National Forest regions (e.g., Graham et al. 1999).

Goshawks are large raptors that occur throughout much of the forested Northern Hemisphere (Squires and Reynolds 1997) where they nest (Reynolds et al. 1982, Crocker-Bedford and Chaney 1988) and hunt (Bright-Smith and Mannan 1994, Beier and Drennan 1997) in mature to old growth forests. The viability of goshawk populations in the western United States has been the subject of much debate recently (Kennedy 1997, Crocker-Bedford 1998) because of petitions to list the northern goshawk as threatened under the Endangered Species Act (Federal Register 1992, 1997a, 1997b). Concerns focus on the effects of additional forest fragmentation from timber harvesting in mature to old-growth forests.

Within the temperate rainforests along the Pacific coast, two goshawk subspecies (A. g. atricapillus and A. g. laingi) are thought to exist. However, the exact ranges of these subspecies and the zones in which they integrate are unclear. Therefore, in this thesis I refer to the northern goshawk in southeast Alaska; I do not differentiate between the atricapillus and laingi subspecies.

Interest in the conservation status of goshawks in southeast Alaska began during the late 1980s. Industrial-scale timber harvesting had occurred in this region for more than 40 years (Kirchhoff 1997), and been associated with the productive old growth forests of the Tongass National Forest. Data on goshawk habitat use in this region revealed that goshawks selected moderately productive to very-highly productive (i.e., old-growth) forests while non-forest and early successional forests, notably clearcuts, were used little relative to their occurrence (Iverson et al. 1996). A petition to list the goshawk as endangered in this region was filed in 1995 by several conservation organizations (Federal Register 1995) and continues to be litigated in Federal court (T. Woods, personal communication).

The Tongass National Forest (hereafter Tongass), the nation's largest at 16.9 million acres, occupies roughly 80% of southeast Alaska and contains some of the largest remaining tracts of pristine temperate rainforest in the world (Iverson et al. 1996). The most contentious management issues of the Tongass focus on the medium to high timber volume tracts of coniferous forest, which are valuable to wildlife and the timber industry (Schoen et al. 1988). The continued fragmentation of these forests can affect goshawks adversely because foraging goshawks typically use areas with higher canopy closure, greater tree density, and more trees in larger size classes (Austin 1993, Bright-Smith and Mannan 1994, Hargis et al. 1994, Beier and Drennan 1997); all characteristics of mature or old growth forests.

In 1991, Alaska Department of Fish and Game (ADF&G) and the USDA Forest Service, Alaska Region began a cooperative effort to study the ecology of the goshawk in southeast Alaska. This cooperative goshawk study has delineated use areas, described goshawk habitat relations and movement patterns, and calculated survival and re- occupancy rates (ADF&G 1997a, 1997b). Peer review of this study identified the goshawks' diet as a priority for further study (K. Titus, personal communication), leading to the initiation of my research.

The goal of my study was to describe the breeding season diet of the goshawk in southeast Alaska. To accomplish this, I used three techniques to collect data on the diet, including a relatively new technique employing a video surveillance system to record prey deliveries. My thesis comprises three chapters. The first chapter describes in detail the video surveillance system I used to monitor goshawk nests and record prey deliveries. The second is a comparison of the results from the three techniques used to collect data about the goshawks' diet. Based on these results, my third chapter describes the breeding season diet of the goshawk in southeast Alaska.

I used the Boise State University guidelines for thesis formatting except where those guidelines did not provide information (e.g., Literature citations formatting), in which case I used the format of the journal to which I eventually plan on submitting each chapter. Chapter one follows guidelines for the Journal of Field Ornithology, chapter two for the Wildlife Society Bulletin, and chapter three for the Journal of Wildlife Management.

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