Northern Goshawk (Accipiter Gentilis) Population Analysis and Food Habits Study in the Independence and Bull Run Mountains, Nevada
Type of Culminating Activity
Master of Science in Raptor Biology
Marc J. Bechard
The Northern Goshawk (Accipiter gentilis, henceforth goshawk) is the largest North American member of the genus Accipiter. Like other accipiters, the goshawk has short, broad wings and a long tail and is well adapted to hunting in woodlands (Palmer 1988). It is a sit-and-wait predator that frequently switches perches and often uses stealth and cover while hunting, and will chase prey through vegetation (Squires and Reynolds 1997). Despite its use of trees for nesting, the goshawk will hunt in open habitats. A variety of prey items is taken, including medium-sized passerines (e.g., American Robin, Turdus migratorius, and Northern Flicker, Colaptes auratus; Younk and Bechard 1994), tree and ground squirrels (e.g., red squirrel, Tamiasciurus hudsonicus, and Belding's ground squirrel, Spermophilus beldingi; Reynolds et al. 1994, Younk and Bechard 1994), galliformes (e.g., Ruffed Grouse, Bonasa umbellus; Bosakowski and Smith 1992), and lagomorphs (e.g., snowshoe hare, Lepus americanus; Doyle and Smith 1994, Watson et al. 1998).
Typically associated with mature and old-growth conifer forests (Squires and Reynolds 1997), goshawks have been found breeding in a variety of habitats including conifer forests of the Pacific Northwest (Reynolds et al. 1982, Finn et al. 2002); ponderosa pine (Pinus ponderosa) forests of the Kaibab Plateau in northern Arizona (Reynolds and Joy 1998); mixed conifer forests of east-central Arizona (Ingraldi 1998); hardwood and conifer forests of New York, New Jersey, and Wisconsin (Speiser and Bosakowski 1987, Rosenfield et al. 1998); and tundra in Alaska (Swem and Adams 1992). In northern Nevada, goshawks breed in naturally-fragmented, high-elevation aspen (Populus tremuloides) stands that are typically surrounded by sagebrush steppe habitat (Younk and Bechard 1994). Several studies have addressed goshawk nesting habitat and generally agree that goshawks prefer larger diameter, mature trees for nesting, and nest stands typically have a relatively high degree of canopy closure and an open understory with low shrub cover (Squires and Reynolds 1997). Sizes of nest stands vary depending on location, but can be ≤1 ha (Herron et al. 1985, Younk 1996, Squires and Ruggiero 1996).
Goshawk movements are not well understood in the United States, but evidence indicates that migration occurs in some populations, but can vary from year to year (see Squires and Reynolds 1997 for a review). A few studies suggest that movements in northern populations may be related to food availability. Goshawks at Kluane, Yukon Territory, Canada exhibited increasingly nomadic behavior of unreported distances in response to declining snowshoe hare populations, being year- round residents when hare densities were high and almost completely absent during winters of low hare abundance (Doyle and Smith 1994). Goshawk populations in Wyoming exhibited local and/or altitudinal movements in winter (Squires and Ruggiero 1995). HawkWatch International migration count sites in northern Utah, eastern Nevada, and central New Mexico regularly record goshawks during fall migration through the Intermountain and Rocky Mountain west. Annual numbers range from three to >200 individuals, depending on year and location (Smith and Hoffman 1997).
In the late 1980s and early 1990s, concern increased over reported declining goshawk populations (Bloom et al. 1986, Crocker-Bedford 1990, Reynolds et al. 1992). Attempts to list the goshawk under the Endangered Species Act in both the southwestern (Federal Register 1992a) and western United States (Federal Register 1992b) failed (see Kennedy 1997 for a summary). The USDI Fish and Wildlife Service classified the goshawk as a Category 2 species until the elimination of that category in 1996, and currently the goshawk is considered a Sensitive Species in regions 3, 4, and 5 of the USDA Forest Service (Kennedy 1997).
In northern Nevada, the Nevada Department of Wildlife and Humboldt-Toiyabe National Forest voiced concern over potential impacts of surface gold mining on goshawk breeding. Because the ecological effects of gold mining on goshawks were unknown and goshawk ecology in sagebrush steppe habitat was poorly understood, a cooperative study began in 1991 in the Independence and Bull Run Mountain ranges (Humboldt-Toiyabe National Forest) to document aspects of goshawk breeding ecology, habitat use, and food habits in this habitat type. Cooperators included Nevada Department of Wildlife, Independence Mining Company (now AngloGold- Meridian Jerritt Canyon Joint Venture), USDA Forest Service, and Boise State University.
Although the initial study ended in 1993 (Younk 1996), researchers recognized that the population data were important because previous to the study little information was known about goshawks breeding in sagebrush steppe habitats. Therefore, Boise State University graduate students continued to collect goshawk reproduction data annually through 2002. This project is the longest-running population study of goshawks breeding in sagebrush steppe habitat.
I analyzed goshawk reproductive, demographic, band recovery, and diet data from 1992 - 2002, using data I collected in 2001 and 2002, and data collected by Boise State University graduate students James V. Younk (1992 - 1994), Michael S. Shipman (1995 - 1997), and B. Heath Smith (1998 - 2000). My research goals were to (1) document long-term trends in reproductive statistics as they related to gold mining, (2) examine the relationship between weather and goshawk reproduction, (3) document natal and breeding dispersal, and (4) describe goshawk food habits in the study area. Because Younk (1996) considered 1991 a pilot study, I did not include that year in my analyses.
Fairhurst, Graham David, "Northern Goshawk (Accipiter Gentilis) Population Analysis and Food Habits Study in the Independence and Bull Run Mountains, Nevada" (2004). Boise State University Theses and Dissertations. 439.