Skip booklet index and go to page content

COSEWIC Assessment and Update Status on the Northern Goshawk (2000)

Limiting Factors

Food

Goshawk populations are regulated by several factors but are probably strongly regulated by food availability (McGowan 1975; Linden and Wikman 1983; Widen 1989; Doyle and Smith 1994; Crocker-Bedford 1998; Ethier In prep). Prey availability (forest structure for hunting) is often more important than prey abundance (Widen 1989: Beier and Drennan 1997; Crocker-Bedford 1998). This has elevated the idea of management of habitat for goshawk prey species to compliment the management for nesting habitat (Iverson et al. 1996; Widen 1997). This approach is favoured by Crocker-Bedford (1990a), who speculated that reduced re-occupancy at protected nest stands following the removal of trees from the surrounding landscape was attributable to a reduction in suitable prey habitat and accessibility, and hence a reduction in foraging opportunity. Breeding pair density may depend on the amount of habitat where suitable prey abundance is above some threshold level, and is accessible enough that the chance of successful capture in the habitat is worth the time and energy expended (Crocker-Bedford 1998).

Logging in important foraging habitat likely has effects disproportionate to the size of the habitat; on the other hand, logging that misses important foraging habitat may have little or no effect on home range size or breeding density (Crocker-Bedford 1998). It may be possible, in managed forests with certain forest types, to improve habitat for goshawk prey species, and nesting habitat, through careful treatments using standard forest harvesting techniques (Reynolds et al. 1992); but such treatments are currently at the discretion of the forest licencee, private landowner, or Ministry of Forests (J. Deal pers. comm.).

Habitat

Breeding habitat loss or fragmentation, and its effect on prey availability and nesting habitat, is the single most significant threat to the long-term viability of the Queen Charlotte Goshawk in British Columbia (Cooper and Stevens 2000). Although data on large-scale population trends are equivocal, many studies have concluded that logging activities, especially clearcut logging, can adversely affect goshawks (Hennessy 1978; Reynolds and Wight 1978; Reynolds et al. 1982, 1992; Moore and Henny 1983; Hall 1984; Mannan and Meslow 1984; Crocker-Bedford and Chaney 1988; Reynolds 1989; Crocker-Bedford 1990a, 1994, 1998; Patla 1990, 1991; Marshall 1992; Austin 1993; Harris et al. 1994; see references in Block et al. 1994; see references in Duncan and Kirk 1995; Iverson et al. 1996); and can lead to local extirpation when the extent of logging is great (Petty 1989; Kenward et al. 1991 in Crocker-Bedford 1994; Crocker-Bedford 1998).

Because large-volume stands have high economic value and are also preferred as nesting habitat by Queen Charlotte Goshawks, logging is usually concentrated in forests with the highest quality goshawk habitat. Typical forestry practices such as partial cutting, understorey brushing, patch cutting and clear-cutting, result in a reduction in stem density and canopy volume, which reduces current habitat quality for nesting or foraging (Crocker-Bedford 1990b; Iverson et al. 1996). Therefore, logging may reduce the ability of a landscape to provide a suitable mixture of structural habitat attributes needed by goshawks, although individual logged areas may recover over time as suitable habitat. It is hypothesized that the cumulative effect of logging may result in fewer pairs, less opportunity to locate a new mate, higher proportions of habitat unoccupancy (Crocker-Bedford 1994), and larger home ranges (Crocker-Bedford 1998).

In coastal British Columbia, second-growth stands on good growing sites can become suitable for nesting Queen Charlotte Goshawks after 50 years, as several nests have been found in such stands on Vancouver Island (D. Doyle pers. comm.). If logging rotations of 100 years were standard, then provision of extensive amounts of suitable breeding habitat could be ensured. However, it is becoming increasingly common for second-growth stands that are in this age class and which have size and structure suitable for goshawks, to be logged because of the timber values represented by trees of that size. The result is that, once a stand is logged, it may be continually logged just as it is becoming suitable for goshawks. In such a scenario, these stands would never recover sufficiently to provide habitat suitable for breeding goshawks.