NORTHERN GOSHAWK REPRODUCTIVE HABITAT - fRI Research NORTHERN GOSHAWK REPRODUCTIVE HABITAT HABITAT...
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NORTHERN GOSHAWK REPRODUCTIVE HABITAT
HABITAT SUITABILITY INDEX MODEL VERSION 3
Last Modified: 21 October 1999
Warren Schaffer, 5 Gillivray Ave., Toronto, Ontario. M5M 2X9. Barbara Beck, Department of Renewable Resources, University of Alberta, Edmonton, Alberta. T6G 2H1. James Beck, Department of Renewable Resources, University of Alberta, Edmonton, Alberta. T6G 2H1. Richard Bonar1, Weldwood of Canada, 760 Switzer Drive, Hinton, Alberta. T7V 1V7. Laurie Hunt, Environmental Training Centre, 1176 Switzer Drive, Hinton, Alberta, T7V 1V3.
1. INTRODUCTION Habitat Suitability Index (HSI) models predict the suitability of habitat for a species based on an assessment of habitat attributes such as habitat structure, habitat type and spatial arrangements between habitat features. This HSI model for the northern goshawk (Accipiter gentilis atricapillus) applies to habitats of the Foothills Model Forest (FMF) in west-central Alberta. The intended use is to predict habitat suitability at landscape scales and over long- time periods. The model will be used to determine potential changes in northern goshawk habitat area and carrying capacity throughout an entire forest management cycle (200 years). This model was developed based on original data gathered by the first author as part of a M.Sc. degree at the University of Alberta, literature review, and expert opinion.
2. SPECIES DESCRIPTION AND DISTRIBUTION The northern goshawk is the largest North American member of the genus Accipiter, which includes sharp-shinned hawks (A. striatus) and Cooper’s hawks (A. cooperii). Northern goshawks are the size of a common raven (Corvus corax) ( Jones 1979). They have long tails (> 30 cm) and relatively short rounded wings (adult male wing length: 30-33 cm, adult female wing length: 33-37 cm; Jones 1979, Godfrey 1986). Northern goshawks are known for their rapid flight over short distances and their pursuit of prey into dense cover (Jones 1979, Johnsgard 1990). A reverse sexual dimorphism is exhibited with an average adult male mass of 0.9 kg and an average female mass of 1.1 kg (P. Duncan, and D. A. Kirk. Status report on the northern goshawk Accipiter gentilis in Canada, unpublished manuscript, 1994).
The breeding range extends from north-western Alaska, through to Labrador and Newfoundland, south to California, across the continental U.S. to Maryland and into Mexico. The winter range extends from western and central Alaska and British Columbia and southern Newfoundland to southern California, Mexico, Tennessee and the Virginias (Jones 1979). In Alberta, the northern goshawk breeds in northern and western Alberta and is most common in the Boreal, Foothills, and Rocky Mountain regions (Semenchuk 1992). Northern goshawks in Alberta are considered a sensitive species that are not currently at risk but may require special management (Wildlife Management Division 1996).
In years with sufficient availability of prey, northern goshawks are year-round residents in the northern parts of their range (including the boreal forests of west-central Alberta; P. Duncan, and D. A. Kirk. Status report on the northern goshawk Accipiter gentilis in Canada, unpublished manuscript, 1994). Large southward migrations of northern goshawk populations occur approximately every 10 years (Keith et al. 1977, Mueller et al. 1977). Other movements in response to low prey numbers include the movement from subalpine forests to low elevation woodlands, riparian areas, and scrubland during winter (Reynolds et al. 1992).
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Northern goshawk breeding can be disturbed by human-related activities. Logging and vehicular traffic in the immediate vicinity of the nest can reduce the attentiveness of adult females (Grier and Fyfe 1988, Richardson and Miller 1997). Changes to nesting (Reynolds et al. 1982, Moore and Henny 1983, Kennedy 1988, McCarthy et al. 1989, Reynolds 1989, Crocker-Bedford 1990, Bosakowski and Speiser 1994) or foraging (Kirk 1995, Beier and Drennan 1997) habitats can make these habitats unsuitable.
3. FOOD Northern goshawks require 120-150 grams of food per day or about 10-15% of their body weight (Brown and Amadon 1968). From birth to fledging, two nestling northern goshawks require about 13 kg of food. Depending on the size of prey taken, adults have to capture between 100-250 prey items to achieve this mass (Schnell 1958).
Northern goshawk prey in west-central Alberta was mainly small mammals and birds (Table 1; Schaffer 1998) which is consistent with other investigations (Henderson 1924, Meng 1959, Storer 1966, Palmer 1988, Widen 1989, Bull and Hohmann 1994, Doyle and Smith 1994). Throughout the North American range, mammals accounted for 86-94% (by biomass; Table 2), and birds accounted for 6-13% of the diet (by biomass; Table 2).
Table 1. Diet of northern goshawks in west-central Alberta as determined by direct observation of northern goshawk feeding behaviour (Schaffer 1998). Frequency of Prey Species Diet Composition
Prey Species Direct Observation Percent of Total Biomass
MAMMALS 25 88.6
Snowshoe Hare (Lepus americanus) 6 63.97
Red Squirrel (Tamiasciurus hudsonicus) 13 20.94
Bushy-tailed Woodrat (Neotoma cinerea) 1 2.49
Meadow Vole (Microtus pennsylvanicus) 3 0.69
Unidentified Microtine 2 0.46
BIRDS 8 11.4
Ruffed Grouse (Bonasa umbellus) 1 4.28
Pileated Woodpecker (Dryocopus pileatus) 2 4.26
Northern Flicker (Colaptes auratus) 2 1.65
Three-toed Woodpecker (Picoides tridactylus) 1 0.49
Gray Jay (Perisoreus canadensis) 1 0.53
Hermit Thrush (Catharus guttatus) 1 0.23
TOTAL PREY 33 100
Table 2. Percent composition of birds and mammals (by biomass) of northern goshawk diet. Location Mammals Birds Source
Alaska 90 10 Zachel 19851
Alberta 89 11 Schaffer 19982
Arizona 94 6 Boal and Mannan 19942
Yukon 86 15 Doyle and Smith 19943 1 - Pellet analysis. 3 - Prey remain analysis. 2 - Direct observation.
4. COVER Cover for nesting and foraging is typically in mature to old deciduous-dominated mixedwood stands, with well- developed canopies and open understories (Shuster 1980, Reynolds et al. 1982, Moore and Henny 1983, Reynolds et al. 1992, Schaffer 1998). Nest trees within these areas are generally large diameter/tall deciduous species with a suitable nest holding location and high surrounding cover for protection against extreme weather and predation.
4.1 FORAGING COVER
Open understory conditions beneath a dense canopy offer a good flight corridor for foraging (Reynolds et al. 1982, Hayward and Escano 1989). Stands with open canopies tend to have dense understories which may diminish the success of foraging due to reduced visibility, restricted flight access to prey, and greater escape cover for prey (Widen 1989; P. Duncan, and D. A. Kirk. Status report on the northern goshawk Accipiter gentilis in Canada, unpublished manuscript, 1994). Northern goshawks have a fairly large body size and wing span, and are not suited for hunting in dense immature forests (Reynolds et al. 1992; P. Duncan, and D. A. Kirk. Status report on the northern goshawk Accipiter gentilis in Canada, unpublished manuscript, 1994). Prey use data gathered in west- central Alberta revealed that prey of northern goshawks were usually in the lower forest strata (Table 3; Schaffer 1998). Mature forest seems to represent the best compromise between maneuverability and attack, and concealment of its approach (Widen 1989). Trees that predominate older stands are tall with large diameters. Tree breakage results in standing and fallen woody material, which provides perching and plucking posts for northern goshawks (Schnell 1958, Reynolds et al. 1992).
Table 3. Diet of northern goshawks on the western study area, as detected by the direct sampling technique, according to their foraging zone (Schaffer 1998).
Vegetation Zonea Biomass of Prey (%)
(1) Ground-Shrub 67.84
(2) Shrub-Canopy 0
(3) Canopy 4.75
(4) Aerial 0
(5) Generalist 27.41
TOTAL 100.00 a - vegetation categories from Reynolds and Meslow (1984), based on information found in Banfield (1974), Kaufman (1996).
4.2 NESTING COVER
Northern goshawk nesting areas in central Alberta exhibited a variety of attributes (i.e. multiple canopy layers, canopy gaps, large standing and fallen dead trees, multiple ages of trees; Schaffer 1998) which characterize the mature seral stage of deciduous-dominated mixedwood forest (Perala 1990, Mehl 1992, Moir 1992). Other North American studies have also concluded that older seral stages are important for northern goshawk nesting (Reynolds et al. 1982, Moore and Henny 1983, Speiser and Bosakowski 1987, Kennedy 1988, Hayward and Escano 1989, Squires and Ruggiero 1996). Overstory vegetation in nesting areas in central Alberta was dominated by trembling aspen (Populus tremuloides), balsam poplar (Populus balsamifera), and white birch (Betula papyrifera), but also had some conifers (white spruce Picea glauca, lodgepole pine Pinus contorta, black spruce Picea mariana). Coniferous trees may be an important component of the nesting stand because they protect the nest from extreme conditions early in the nesting season (Moore and Henny 1983, Speiser and Bosakowski 1987, Schaffer 1998). Across North America, northern goshawks preferred mixedwood areas for nesting, while deciduous areas were of secondary importance, and pure conifer forests were the least used hab