Northern Goshawk Habitat Model
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Northern goshawk, Accipiter gentilis
Use of Study Area Resources:
Reproduction; Nesting is confirmed throughout the study area (Breeding Bird Atlas data). In North America northern goshawks breed and winter from Alaska and the Canadian Maritime Provinces, south through the Midwest. They prey on birds and small mammals, sometimes hunting in the open but more typically in forested areas (Squires and Reynolds 1997).
Cover: Goshawks nest in interiors of extensive, remote, mature and old-growth forests dominated by large trees, high canopy closure, on moderate slope, with an open understory (Falk 1990, Speiser and Bosakowski 1987, Squires and Reynolds 1997). In areas with temperate climates there seems to be an avoidance of southerly aspect, perhaps to avoid high nest temperatures (Falk 1990, Speiser and Bosakowski 1987, Bosakowski and Speiser 1994, Squires and Reynolds 1997). Forest cover types used for nesting include deciduous, conifer and mixed (Squires and Reynolds 1997). Occupancy increases dramatically in stands > 70% canopy closure (Squires and Reynolds 1997). In eastern deciduous forests, goshawks prefer nesting in mature, mixed hardwood-hemlock stands characterized by birch (Betula sp.), beech (Fagus sp.), maple (Acer sp.), and eastern hemlock (Tsuga canadensis; Speiser and Bosakowski 1987). Here goshawk nest sites are in extensively forested areas, at higher elevations and in close proximity to water and swamps (Speiser and Bosakowski 1987). Nests are in the largest trees in the forest stand and the territory often contains alternate nest trees, usually within a distance of < 0.7 km (Squires and Reynolds 1997).
Food/Forage. Goshawks are opportunistic, taking a wide variety of prey. Main foods include rabbits, hares, squirrels, large passerines, woodpeckers, game birds, and corvids. Less often they will feed on reptiles or carrion. Goshawks are "reckless" when hunting, crashing through vegetation, chasing ducks into water, or poultry into buildings (Squires and Reynolds 1997). They typically forage on the wing in mature forests characterized by a high canopy and open understory (Squires and Reynolds 1997).
Area: Forest stands containing nest trees are typically 10-100 ha in size (Squires and Reynolds 1997). Graham et al. (1994) note that nesting patches of large trees are 8 to 12 ha. In northern California, forest stands < 10 ha occasionally contained 1-2 nests, whereas larger stands (> 20 ha) were more consistently occupied (Squires and Reynolds 1997). Large habitat patches (> 40 ha) of mature forest were preferred during winter (Squires and Reynolds 1997). Post-fledging habitat is a mosaic of about 170 ha of forest, downed trees, and openings, while the surrounding foraging area is a mosaic of about 2000 ha of similar covers (U.S. Forest Service; Graham et al. 1994).
Water and Wetlands. Speiser and Bosakowski (1987) found an association of goshawk nest with water and swamps. Squires and Reynolds (1997) also mention the frequent occurrence of "free water" near nests. Based on 8 goshawk occurrences for our study area (see below), and landcover information, we found that areas used all had ponds or shrub swamps within 500 meters. Goshawks are known to bathe in water, and Squires and Reynolds (1997) suggest that although the reason is unknown, it may be related to maintaining humidity of the eggs. Falk (1990) suggests that the association between nest sites and wetlands may be indicative of greater diversity and prey and remoteness from human disturbance.
Disturbance: Goshawks nest further from areas of human habitation than would be expected by chance (Speiser and Bosakowski 1987; Bosakowski and Speiser 1994, Bosakowski and Smith 1997, Falk 1990). Bosakowski and Speiser (1994) found that the average distance to human habitation in New Jersey sites was about 1050 m (s.d. = +/- 635), with the distance to the nearest paved road about 1170 m, s.d. = +/- 650).
Several models were developed and the resulting habitat maps were compared to known goshawk occurrences in the study area. The best fit was achieved by regarding goshawk habitat as patches of forest, of adequate size, and which include areas likely to be suitable for nesting.
Goshawk nesting areas were identified as: 1) having suitable forest cover (see table, below); 2) situated at least 460 m from development (based on the Bosakowski and Speiser, 1994, average distance of nests from roads and dwellings of 1100 m, minus 1 s.d. of 640 m); 3) situated within 500 m of a pond or shrub swamp, and; 4) having north, northeast, northwest, or flat aspect. These areas were scored 1.0. Other contiguous forest within 500 m of the nesting habitat was scored 0.5. The sizes of the habitat patches then was computed, and only clusters having at least some nesting habitat and an area of 8 ha or larger were retained.
|Cover Types||Cover Suitability
(0 - 1 scale)
|Upland deciduous forest||1.0*|
|Upland coniferous forest||1.0*|
|Upland mixed forest||1.0*|
|PEM, L2EM||Lake/pond, emergent vegetation|
|PFOcon||Palustrine forest, conifer||1.0*|
|PFOdec||Palustrine forest, deciduous||1.0*|
|PSSdec||Palustrine scrub shrub, deciduous|
|PSScon||Palustrine scrub shrub, conifer|
|PAB, L2AB||Lake/pond, aquatic vegetation|
|L1UB, PUB||Lake/pond, unconsolidated bottom|
|L2US||Lake, unconsolidated shore|
|L2RS||Lake, rocky shore|
|R1UB||Riverine subtidal unconsolidated|
|E1AB||Estuarine subtidal vegetated|
|E1UB||Estuarine subtidal unconsolidated bottom|
|E2AB||Estuarine intertidal algae|
|E2EM||Estuarine intertidal emergent|
|E2RS, R1RS||Estuarine, tidal river rocky shore|
|E2SS||Estuarine intertidal shrub|
|E2US||Estuarine intertidal unconsolidated shore|
|M1AB||Marine subtidal vegetated|
|M1UB||Marine subtidal unconsolidated bottom|
|M2AB||Marine intertidal algae|
|M2RS||Marine intertidal rocky shore|
|M2US||Marine intertidal unconsolidated shore|
|NOTES||* score if > 460 m from development, within 500 m
of a pond or shrub swamp, having NE, N, NW, or flat
aspect, and minimum of 8 ha area; otherwise scored 0.
Model Testing: Five goshawk occurrences in Pierson et al. (1996) were mapped from the narrative descriptions, three occurrences were obtained from the Breeding Bird Surveys (http://www.mp2-pwrc.usgs.gov/bbs/), and 107 records with confirmed or probable breeding from the Breeding Bird Atlas (BBA) for Maine, New Hampshire and Massachusetts (provided by Sean McFaden, BRD COOP Unit, University of Vermont). The BBA data were in 7.5 minute blocks in Maine, and 1/6 this size in Massachusetts and New Hampshire.
Testing was limited by the paucity of goshawk occurrence data, the generality of the model, and the abundance of forest cover within the study area. We observed that 106 of 107 BBA blocks with occurrences had mapped habitat, while a random selection of 629 BBA blocks had habitat in 605. A Chi-square examination of this finding indicated the difference in these proportions was not significant.
Bosakowski, T. and D.G. Smith. 1997. Distribution and species richness of a forest raptor community in relation to urbanization. J. Raptor Res. 31(1):26-33.
Bosakowski, T. and R. Speiser. 1994. Macrohabitat selection by nesting northern goshawks: implications for managing eastern forests. Studies in Avian Biology (16):46-49.
Falk, J.A. 1990. Landscape Level Raptor Habitat Associations in Northwest Connecticut. M.S. Thesis, Virginia Polytechnic Inst. 127 pp.
Graham, R.T., R.T. Reynolds, M.H. Reiser, R.L. Bassett and D.A. Boyce. 1994. Sustaining forest habitat for the northern goshawk: a question of scale. Studies in Avian Biology 16:12-17.
Pierson, E.C., J E. Pierson and P.D. Vickery. 1996. A Birders Guide to Maine. Down East Books, Camden, ME.
Speiser, R. and T. Bosakowski. 1987. Nest site selection by northern goshawks in northern New Jersey and southeastern New York. Condor 89:387-394.
Squires, J.R. and R.T. Reynolds. 1997. Northern Goshawk, No. 298 in Birds of North America series, c/o Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA. 31 pp.
USFS website: http://www.fs.fed.us/database/feis/animals/bird/