Skip to Main Content

Younk, James V. – Breeding ecology of the Northern Goshawk in relation to surface gold mining in naturally fragmented aspen forests of northern Nevada

Northern goshawk on branchThe northern goshawk (Accipiter gentilis) is a large forest-dwelling accipiter found throughout the northern hemisphere from sea level to the limits of trees (Palmer 1988, Johnsgard 1990). In eastern North America, it breeds from the northern tree limit south to 40 N latitude. In western North America, its breeding range extends from Alaska southward through mountainous areas and higher elevation forests into Mexico (Brown and Amadon 1968). Goshawks prey on birds up to the size of ring-necked pheasants (Phasianus colchicus) and small mammals as large as hares (Lepus spp.), and are typically the dominant diurnal avian predator in mature forests (Bent 1937, Schnell 1958, Mannan and Boal 1990).

Goshawks are forest habitat generalists that use a wide variety of forest types (Brown and Amadon 1968). Like any other raptor, the availability of suitable breeding habitat plays a principle role in determining its density and distribution (Newton 1979, 1989, 1991, Village 1990). Studies in the western U.S. show that suitable goshawk breeding habitat can be identified by its unique topography and vegetative structure. It is typically associated with mature conifer forests, and nest stands are either on slopes with northern exposures or in drainages and canyon bottoms protected by these slopes. Nest stands have mixed species contents, high tree densities, and high canopy covers (Bartelt 1974, McGowan 1975, Hennessy 1978, Shuster 1980, Reynolds et al. 1982, Saunders 1982, Moore and Henny 1983, Hall 1984, Crocker-Bedford and Chaney 1988, Kennedy 1988, and Hayward and Escano 1989).

Goshawks also occur in a number of less typical habitats such as those found in the relatively dry mountains of the northern Great Basin in Nevada (Herron 1985). Unlike most mountains in the west, these ranges contain little forested habitat, and shrubsteppe is the most common habitat type (Loope 1969). Because trees are limited to high elevations and riparian areas along drainages, the limited availability of suitable nesting areas may play a key role in determining the density and distribution of goshawk populations in these areas.

Prior to the 1970′s, little information existed on the breeding biology of the goshawk in North America. Historically, studies concentrated on its diet and its effect on economically valuable small game mammals and birds (Bent 1937). Little emphasis was placed on habitat selection or nesting requirements of goshawks. Reynolds (1971, 1978, 1982, 1983, 1984) described the distribution and density, nesting habitat, and diet of nesting accipiters in conifer forests of Oregon, as well as management techniques required to preserve adequate goshawk nesting habitat. He stated that goshawks required a 25 acre (10 ha) nesting stand, and all logging activity should be stopped within 0.5 miles (0.7 km) of an occupied goshawk nest during the nesting season.

McGowan (1975) found that young female goshawks in interior Alaska breed during years of high snowshoe hare (Lepus Americanus) abundance, but only 1 of 17 territories was occupied when the hare population crashed.

By the mid 1980′s, several authors began to address the impacts of timber harvesting on breeding populations of goshawks on National Forest lands in the west (Bloom et al. 1986, Crocker-Bedford and Chaney 1988, Kennedy 1988). Modeling techniques for identifying essential goshawk habitat were proposed (Fowler 1988). Several studies conducted in-depth analyses of goshawk nesting habitat (Schuster 1980, Reynolds et al. 1982, Moore and Henny 1982, Bartelt 1974, Hennessy 1978, Sanders 1982, Hall 1984). These studies found that goshawks nested in forest stands consisting of large, mature trees with an open understory. They were near water in relatively flat areas. A north-or east-facing slope and small forest openings were also used.

Declines in the breeding population of goshawks in the Kaibab National Forest in northern Arizona have been suggested by Crocker-Bedford (1990). Since 1982, the goshawk has been used as an indicator species for old-growth forests by the U.S. Forest Service (Silver 1991). Although a petition to place the goshawk on the endangered species list in 1992 was unsuccessful, there continues to be concern over reported declining populations in areas of the western U.S. (Reynolds et. al. 1992).

In the northeastern U.S., the goshawk may actually be increasing its range southward. Speiser and Bosakowski (1987) found that the goshawk has returned to New Jersey as a breeding bird in newly maturing forests. Although they are composed of deciduous species, these forests have an open understory and a closed canopy similar to goshawk nesting habitat in the west. The authors felt that as forests matured and clearing activities such as mining and logging were no longer practiced, goshawks are moving back into these areas as the habitat recovers.

There is very little forested habitat and no timber harvesting in the Independence and Bull Run Mountains of northern Nevada, but surface gold mining does occur. The Humboldt National Forest and Nevada Department of Wildlife raised concerns about the possible affects of mining activity on breeding goshawks in this area.

The goals of this study were (1) to describe aspects of goshawk breeding ecology including its home range size, productivity, habitat use, diet, and nesting chronology in naturally fragmented forest habitat and (2) to assess these parameters in relation to disturbances resulting from surface mining and mining exploration activities. A preliminary field season in 1991 focused on occupancy and productivity of goshawk territories, and vegetative and topographical characteristics of nest stands. Similar data were collected in 1992 and 1993 with observations on adult nesting behavior, diet, habitat use, and home range size.

Thesis Abstract