Biological Characteristics

This review includes information covering many aspects of northern goshawk life history but focuses on those most relevant to fire. The information presented here relies heavily on syntheses material from several sources, including "The birds of North America Online" [48] species account for the northern goshawk and reviews of the northern goshawk's status and/or habitat needs throughout its range [10] and in the following regions: New England [15], Great Lakes [7,43], central Rocky Mountains [26], Rocky Mountains [23], western United States [3,10,24], the Pacific Northwest [17], and the Southwest [40,53].

Photo Credit

Photo by Jack Kirkley.

Adult northern goshawk.

Photo Credit

Photo by Jack Kirkley.

Juvenile northern goshawk. 

Physical Description

The northern goshawk is a large forest hawk with long, broad wings and a long, rounded tail. Females average 24 inches (61 cm) in length, 41 to 45 inches (105-115 cm) in wingspan, and 30 to 45 ounces (860-1,264 g) in mass, while the smaller males average 22 inches (55 cm) in length, 39 to 41 inches (98-104 cm) in wingspan, and 22 to 39 ounces (631-1,099 g) in mass [48].

Adult northern goshawks are brown-gray to slate-gray on top. The head has a black cap and a pronounced white superciliary line. Underparts are light gray with some black streaking. The tail is dark gray above with 3 to 5 inconspicuous broad, dark bands, and sometimes a thin white terminal band. Juveniles are generally brown on top and have brown streaking on the chest [48].

Life Span

Based on band recoveries at trapping sites, the maximum life span of wild northern goshawks is at least 11 years [48]. One review reports a captive northern goshawk living 19 years [26].

Development and Reproduction

Age at First Reproduction

Northern goshawks may breed as subadults (1-2 years old), young adults (2-3 years old), or adults (≥3 years old). Females are more likely than males to breed at a young age [48].

Mate Fidelity

Northern goshawks may show high mate fidelity. A study from northern California found that over 9 years, 72% of the adults located in subsequent years (18 of 25 instances) retained the mate from the previous year [18].

Nesting Phenology

Northern goshawk pairs occupy nesting areas from February to early April. Some pairs may remain in their nesting areas year-round. Nest construction may begin as soon as individuals return to their territories. Eggs are laid anywhere from mid-April to early May. Cold, wet springs may delay incubation. Incubation varies from 28 to 37 days. Nestlings move from nests to nearby branches when they are around 34 to 35 days of age. Their first flight from the nest tree ranges from 35 to 36 days for males and 40 to 42 days for females. They reach independence approximately 70 days from hatching. Most fledglings disperse from the nest area between 65 and 90 days after hatching, with females dispersing later than males [48].

Clutch Size

Northern goshawks usually produce one clutch per year. Clutch size is usually 2 to 4, but occasionally 1 or 5 eggs. Because northern goshawk chicks hatch asynchronously, older, larger nestlings may attack smaller, younger nestlings [48].

Photo Credit

Photo by Jack Kirkley.

Northern goshawk female with young. 

Nest Success and Productivity

Northern goshawk nest success and productivity vary and may be limited by prey availability [6,46,48], weather [26,48], predation [7,26], disease [26], habitat features [24], and disturbance from timber harvest or other human activities [26,48].

In North America, nest success usually ranges from 80% to 94%, with most successful nests producing an average of 2.0 to 2.8 fledglings. Unsuccessful nests usually fail early in the breeding season, before or soon after laying. Weather, particularly cold temperatures in the spring and exposure to low temperatures and rain, may cause egg and chick mortality. Once chicks reach 3 weeks of age, nests rarely fail. Productivity may differ between years in the same study area and among landscapes within a limited geographic area. The availability of prey strongly affects nest occupancy and productivity. The age of the female may also affect productivity; pairs with a younger female may produce fewer fledglings than pairs with an older female. If food resources are low, siblicide and cannibalism may occur [48].

Nest Description

The northern goshawk constructs a nest of thin sticks, forming a bowl lined with tree bark and greenery. Nests are usually placed on large horizontal limbs against the trunk, or occasionally on large limbs away from the bole. A variety of tree species is used for nesting [48].

Photo Credit

Photo by Jack Kirkley.

Northern goshawk nest in western Montana. 

Nest Site

Northern goshawks build nests in both deciduous and coniferous trees. They typically use the largest tree in a nest stand. Nest height varies by tree species and regional tree characteristics. The size and structure of a nest tree may be more important than species. Northern goshawks occasionally build nests on dwarf mistletoe (Arceuthobium spp.) clumps and rarely in dead trees [48]. Several reviews provide lists of specific tree species used by northern goshawks for nesting throughout their range [48] and in the Great Lakes region [7] and central Rocky Mountains [26].

Nest and Nest Site Fidelity

Northern goshawks may use the same nest for consecutive years, but they usually alternate between 2 or more nests within a nest area. They may maintain as many as 8 alternate nests within a nest area. It is thought that most nest sites are occupied from 1 to 3 years, though some may be occupied much longer. Though the importance of alternate nest maintenance is not completely understood, it is hypothesized that nest switching reduces exposure to diseases and parasites. This behavior complicates the determination of nest-site fidelity because it is difficult for biologists to locate all alternative nests [48]. A synthesis of 5 studies correlating nest occupancy with habitat features found a consistently positive relationship between closed-canopy forests with large trees and northern goshawk nest occupancy. Occupancy rates were reduced by removing forest cover in the home range, which thereby resulted in reduced productivity because there were fewer active breeding territories [24].

Area Requirements and Movements

Home Range

In North America, home range in the breeding season ranges from 1,400 to 8,600 acres (570-3,500 ha) [48]. Home range size varies depending on sex [26,48], season [48], local prey availability, climate [7], and habitat characteristics [7,26,48]. The male's home range is generally larger than the female's. Within a home range, individuals often have core-use areas that include the nest and primary foraging areas. Outside of a nesting area, the home range of a breeding pair may not be defended and may overlap with the home range of adjacent pairs. The shape of a home range may vary from circular to linear or may be discontinuous, depending on local habitat characteristics [48].

Dispersal

Natal dispersal of the northern goshawk had not been well studied as of this writing (2012). One review notes that very few (24 of 452) fledglings in an Arizona study were recruited into the local breeding population, and mean natal dispersal distance was 9.1 (SD 5.1) miles (14.7 (SD 8.2) km) (range 2.1-22.6 miles (3.4-36.3 km)) [26].

Migration

The northern goshawk is considered a partial migrant. Some individuals, particularly those that inhabit northern latitudes, may migrate long distances. Other individuals make short winter movements to lower elevations and/or more open plant communities. Food availability in the winter may influence the degree to which individuals or populations migrate [40,48].

Population Density and Dynamics

Northern goshawk populations occur at low densities compared to many bird species [43]. One review reports that regular territory dispersion is a consistent characteristic of northern goshawk populations that likely results from territorial behavior. In North America, mean nearest neighbor distances range from 1.9 to 3.5 miles (3.0-5.6 km) [26], and density estimates range from less than 1 to 11 pairs per 100 km². Densities in the range of 10 to 11 occupied nests per 100 km² were reported for 3 study areas in Arizona, California, and the Yukon. However, nest density across a landscape is difficult to determine and often based on either assumed censuses of breeding pairs or the distribution of nearest neighbor distances. Because most searches for nests are conducted in what is predetermined to be "suitable" habitat, reported densities may not accurately reflect the number of territories per unit area. Surveys may also be incomplete or inaccurate [26].

Factors limiting northern goshawk populations may include food availability [10,26,40,48], availability of nest sites [40], and territoriality [26]. Food availability is more of an issue in northern latitudes, where northern goshawks are more dependent on populations of few species (e.g., snowshoe hare (Lepus americanus)). There is less evidence of population fluctuations in response to food in lower latitudes, where a greater variety of prey species are available [40,43]. See Population status for more information on how stand and landscape characteristics may influence northern goshawk populations.

Predation and Mortality

Northern goshawks are vulnerable to predation from red-tailed hawks (Buteo jamaicensis), short-eared owls (Asio flammeus), great horned owls (Bubo virginianus), American martens (Martes americana) [48], fishers (M. pennanti) [7], wolverines (Gulo gulo) [48], coyotes (Canis latrans), bobcats (Lynx rufus), and northern raccoons (Procyon lotor) [40]. It is likely that other mammals prey on nestlings and/or adults [48]. In the Great Lakes region, great horned owls were the most common nest predator [7]. Other potential sources of northern goshawk mortality include starvation, disease, shooting, trapping, poisoning, and collisions with vehicles [48].

Photo Credit

Photo by Jack Kirkley.

Great gray owls (Strix nebulosa) using a nest formerly used by northern goshawks.

Interspecific Competition

Reduction and fragmentation of mature forest habitat may favor early-successional competitors such as red-tailed hawks and great horned owls and reduce occupancy of an area by northern goshawks [26]. One study from California found great horned owls, long-eared owls (Asio otus), spotted owls (Strix occidentalis), red-tailed hawks, and Cooper's hawks (Accipiter cooperii) occupying traditional northern goshawk nests or nest stands, but the territories were usually not abandoned entirely by northern goshawks. In 3 instances, however, northern goshawks moved out of their traditional nest stand after it was occupied by spotted owls [51].Diet

The northern goshawk opportunistically feeds on a wide diversity of prey items that varies by region, season, and availability. Though the list of potential prey species is extensive, a few taxa are particularly prevalent in most diets [48]. Diet options may be narrower in northerly latitudes, where fewer prey species are available, than in lower latitudes [40,43].

Regional diet summaries are available from the Great Lakes [7,43], South Dakota [47], the central Rocky Mountains [26], and eastern Oregon [17].

Prey Habitat and Availability

Managing for prey species is a major component of habitat recommendations for the northern goshawk (e.g., see [40]). Northern goshawk populations may experience reduced fitness and reproduction, greater interspecific competition for food, and greater susceptibility to predators when food resources are limited [26]. Several reviews emphasize the importance of both prey abundance and availability when determining suitable northern goshawk habitat. In other words, prey need to be both present and huntable, with availability determined by stand structure [24,26].

For information on habitat preferences of northern goshawk prey species, see the following reviews from the Southwest [10,40,53] and central Rocky Mountains [26] or FEIS reviews for the following species: Abert's squirrel, Townsend's ground squirrel, eastern cottontail, black-tailed jackrabbit, snowshoe hare, ruffed grouse, gray jay, and black-backed woodpecker.

Habitat

Northern goshawk habitat includes a variety of forest types and stand structures, depending on geographic location and life history activities. The northern goshawk's large home ranges and ability to move great distances mean that it may encounter a variety of habitats over a short period. In addition to nesting habitat, northern goshawks need foraging habitat in both the breeding and nonbreeding seasons and in postfledging areas where young learn to hunt but are protected from predators [43]. Habitat selection may be shaped by landscape structure and pattern and/or occupancy by other raptors [43]. In general, as the scale of analysis increases (i.e., from stand to landscape), northern goshawks use more diverse habitats and show less preference for specific habitat features [3,10,17,43]. Northern goshawks appear to use a wider range of habitats during the nonbreeding season than the breeding season [3].

In general, northern goshawks appear to prefer relatively dense forests [24,25,47] with large trees [3,24] and relatively high canopy closures [3,24,25,47]. A review noted that 9 of 12 radio telemetry studies from the western United States found northern goshawks selected stands with higher canopy closures, larger trees, and more large trees than found in random stands. But northern goshawks still used stands with a wide range of structural conditions [24]. The use of forests with relatively large trees and high canopy closures may be related to increased protection from predators, increased food availability, limited exposure to cold temperatures and precipitation early in the breeding season, limited exposure to high temperatures in the summer nestling period, high mobility due to a lack of understory structure, and less competition from other raptor species that inhabit more open habitats [3].

The relatively large body size and wingspan of the northern goshawks limit its use of young, dense forests where there is insufficient space in and below the canopy to facilitate flight and capture of prey. There are also few suitably large trees for nesting in young, dense forests [40].

Breeding Habitat

Northern goshawk habitat use may be most selective during the breeding season, mostly due to strong preferences for nest placement [43,48].

Nest Stand

Forest stands containing nests are often small, ranging from approximately 24 to 247 acres (10-100 ha) [48]. Tree species composition is highly variable among nest sites both within a region and a across the range of the northern goshawk [40]. Northern goshawks nests are often found in mature or late-successional forests [3,15,17,43,48] with high canopy closures [9,17,43,47,48] and large trees [43,48] but relatively open understories [26,43,48]. However, due to frequent bias in northern goshawk nest detection methods, the selection of mature forest over other forest successional stages has been demonstrated in only a few studies [43].

Photo Credit

Photo by Jack Kirkley.

Northern goshawks nested in this conifer forest in western Montana.

Though northern goshawks are most often documented nesting in late-successional forests, they sometimes nest in younger, more open forests. For example, in dry areas of the West such as the Great Basin, northern goshawks nested in high-elevation shrubsteppe habitats supporting small, highly fragmented stands of quaking aspen (Populus tremuloides) [52]. In a conifer plantation in western Washington, 3 northern goshawk pairs nested in younger, denser stands than previously reported for the region; nest sites were composed of 40- to 54-year-old, second-growth conifer stands with high live tree and snag densities [9].

Northern goshawk nest sites are often located near water [43,47,52], though some studies have shown no association between nest sites and water [26,43] and the presence of water is not considered a habitat requirement [48]. The function of open water during nesting is unknown [43].

Nest sites often are located close to forest openings or other open areas [12,15,26,43,47], which may increase nest access, serve as travel corridors, support open-habitat prey species, or reduce flight barriers to fledglings [48]. However, one study from west-central Montana noted that the number of young fledged per nest was negatively correlated with the size of the nonforested openings near the nest (P≤0.05) [12].

Slope and aspect may influence microclimate conditions important to northern goshawk nesting. Northern goshawk nests are often located at the base of moderate slopes [48] and tend to be on gentle rather than steep terrain [15]. However, there may be no relationship between nesting and slope in areas with low topographic relief, like the Great Lakes region [7]. One study from west-central Montana found that 82.6% of occupied nests were located on north slopes [12]. Preferred aspects may vary regionally; one review noted that in southern parts of the range, northern goshawks nest areas typically had northerly aspects, while nest areas in interior Alaska had southerly aspects [26].

Postfledging Family Areas

A postfledging area represents the area of concentrated use for a northern goshawk family from the time the young leave the nest until they are no longer dependent on the adults for food. Northern goshawks typically defend this area as a territory. Postfledging family areas provide hiding cover and prey for fledglings to develop hunting skills. They typically contain patches of dense trees, developed herbaceous and/or shrubby understories, and habitat attributes that support prey, such as snags, downed logs, and small openings. Postfledging family areas range in size from 300 to 600 acres (120-240 ha) [40].

Wintering Habitat

Northern goshawk breeding habitat has been studied much more intensively than nonbreeding habitat. In general, northern goshawks use a wider range of habitats during the nonbreeding season than during the breeding season [48]. One review reports that northern goshawks in northern Arizona may select winter foraging sites based on forest structure rather than prey abundance, similar to selection in the breeding season [26] (see Foraging habitat). In some regions, northern goshawks appear to remain near breeding areas throughout the year [3,7,43], though there is considerable annual variation and variation between sexes in nonbreeding habitat use [3]. In at least some landscapes, northern goshawks forage in late-successional forest habitats throughout the year [3,24]. However, some northern goshawks move to low-elevation, open plant communities (e.g., woodlands) in the winter [3,24].

Foraging Habitat

Northern goshawks forage by ambush and perching in vegetation to scan for prey items. They occasionally hunt by flying rapidly along forest edges and across openings [26]. Ideal foraging habitat includes space under the canopy to allow for flight, abundant trees perches, and available prey [53]. Preferred perches while hunting are low (usually <3 feet (1 m)), bent-over trees or saplings. Plucking perches where northern goshawks consume prey are usually located in dense vegetation below the main forest canopy and are often upslope and fairly close to the nest in the breeding season [48].

Northern goshawks forage over large areas and encounter a variety of forest structures [23,26] and plant communities [48] when foraging. In the breeding season, a foraging area may encompass 5,400 acres (2,200 ha) surrounding the postfledging family area [40]. Northern goshawks may rely heavily on mature forest while foraging [3,10,17] but may also forage in younger forests, edges, and openings [10,17,26,43]. An open understory may enhance the detection of potential prey [3,40,43].

Prey abundance may be an important feature of foraging habitat, but several sources stress the importance of prey availability [3,5,22,26,40,53], which is often linked to vegetative structure that allows northern goshawks to hunt successfully [3]. For example, over 2 breeding seasons in ponderosa pine (Pinus ponderosa) forests in northern Arizona, 20 adult northern goshawks did not select foraging sites based on prey abundance; abundance of some prey was lower in selected sites than what was generally available. Northern goshawks instead selected foraging sites that had higher canopy closure (P=0.006), greater tree density (P=0.001), and greater density of trees >16.0 inches (40.6 cm) DBH (P<0.0005) than what was generally available. The authors concluded that above a minimal prey threshold, northern goshawks may select sites with favorable structure over those with abundant prey. However, they also suggested that their results only apply to foraging habitat selection within an established home range. Prey abundance may be an important factor when northern goshawks initially establish a home range [5].

In the Great Lakes region, male northern goshawks primarily foraged in mature upland conifer and upland deciduous stands, but other stand types were used and may be important to prey production [7]. In lodgepole pine (P. contorta) and quaking aspen forest in south-central Wyoming, the kill sites of male northern goshawks in the breeding season were more related to stand structure and aspect than prey abundance. Males returned most often to sites with more mature forests (P=0.0), gentler slopes (P=0.011), lower ground cover of woody plants (P=0.023), and greater densities of trees (P<0.089) and conifers (P<0.14) ≥9 inches (23 cm) but ≤15 inches (38 cm) DBH. Average canopy closure at kill sites was 52.8%. Kill sites were often associated with small openings; average distance to the nearest open area was 152.2 feet (46.4 m). The author noted that several prey species were often associated with forest edges. The results of this study suggest that the high density of large trees allowed northern goshawks to approach prey unseen, while the low density of understory vegetation allowed northern goshawks to see potential prey items. At the landscape scale, male northern goshawks intensively used large areas of conifer forests interspersed with small openings in proximity to nests. They used a variety of habitats, from narrow patches of quaking aspen in drainages surrounded by sagebrush (Artemisia) and grassland to areas dominated by conifer forests [22].

Roosting Habitat

Northern goshawks roost alone in the tree canopy and may use several sites for roosting. In the early nesting phase, female northern goshawks roost on the nest while brooding young [48]. In California, roost tree species and roosting stand characteristics varied by season, which the authors hypothesized was in response to changes in prey abundance and availability [42].

Landscape Features

Northern goshawks use large landscapes for many life history activities, though it is difficult to make broad generalizations about the importance of landscape features to northern goshawk populations. Studies and reviews highlight the importance of landscape features such as the presence of large areas of mature forest [10,15,26,40], a mosaic of forest structural stages [17,25,26,40], limited forest fragmentation [26,51], and large patch sizes [9,43,51].