Effects of Human Activity
Shooting And Trapping
Difficult to quantify. Historically, some native groups killed hawk owls for consumption (Duncan 1993). Vulnerable to shooting because of diurnal habits, bold nature, and use of exposed perches; called "practice owl" by some northern residents (Austen et al. 1994). In Newfoundland, gunshot wounds, leghold traps, and collisions with transmission lines have been dangers for this species (M. Pitcher pers. comm.). In Yukon and British Columbia, found in traps set for furbearers (Siddle 1984, N. Barichello pers. comm.). Of 23 hawk owls received at a rehabilitation center in Vineland, Ontario, from 1976 to 1991, 8 had been shot, 3 were from vehicle or train collisions, 1 had been accidentally trapped, 7 had died of unknown causes, and 4 were juveniles from destroyed nests (K. McKeever pers. comm.). Sometimes used for falconry in Scandinavia (Duncan 1993).
Pesticides And Other Contaminants/Toxics
No data. Other owl species killed by consuming vertebrates, including voles, poisoned with Dieldrin or organophosphate Azodrin (Marks et al. 1994).
Degradation Of Habitat
More than 50% of hawk owl's North American breeding range occurs in non-commercial boreal forests. From 1975 to 1995, modern forestry started transforming semi-pristine and continuous commercial northern forests into a mosaic of patches interspersed with clear-cuts and plantations. This practice has likely reduced nest-site and hunting-perch availability, yet has increased Microtus prey abundance (Duncan and Harris 1997, Sonerud 1997; see also Distribution, and Food habits, above). Policy of fire suppression has also resulted in habitat degradation (N. Barichello pers. comm.). Modified forestry practices can mitigate these impacts (see Management, below). Nothing short of widespread elimination of northern conifer forests is likely to seriously threaten continued existence of this species (Errington 1933, Voous 1988b).
Disturbance At Nest And Roost Sites
In Manitoba and n. Minnesota, has nested to within 30 m of busy highways (JRD, PAD). In Europe, bred twice within 100 m of houses (Mikkola 1983). Tourism-related bird-watcher activity in Churchill, Manitoba, led to destruction of 1 nest site (JRD). Can be intolerant and dangerously aggressive to humans close to nests (Lane and Duncan 1987).
Exceptionally tolerant of humans, especially in winter while hunting; frequently permits approach to base of hunting or roost perch (JRD, PAD). In Alaska, found in housing developments with average lot size about 2 ha (Meehan and Ritchie 1982). One established and maintained a winter territory in suitable habitat within a city of >700,000 people (Nero 1995).
Considered "Not at Risk" in Canada (Duncan 1993) and "Globally Secure" (Nature Conservancy 1996a). Alberta, Saskatchewan, and Quebec list Northern Hawk Owl as a "Sensitive or Vulnerable Species," but currently "Not at Risk" or "Apparently Secure" (J. Keith, L. Takats, and G. Jolicoeur pers. comm.). Ranked of "Medium" concern (eighty-fifth of 297 birds considered) among Canadian birds evaluated for setting conservation, research, and monitoring priorities (Dunn 1997).
Modified clear-cut logging can benefit the hawk owl by creating habitats that are more profitable for hunting than the original or selectively cut forest. Can use open clear-cuts for long-range visual scanning and capture of ground-dwelling small mammals; from perches at least 9 m above ground, this species can effectively search an estimated 70-m radius in winter (Sonerud 1997). By hunting from trees along edges, it can access prey in square-shaped clear-cuts up to about 2 ha, or in rectangular cuts with the short side no wider than 140 m. A higher portion of larger clear-cuts would be available to hunting hawk owls if cut edges were convoluted and/or if residual trees were left after logging. Other clear-cut variables important to consider include type and extent of ground cover, and composition and abundance of prey species (Sonerud 1986, Duncan and Harris 1997, Sonerud 1997).
Residual trees in clear-cuts also used for feeding, loafing, and calling. Cavities in residual trees left standing in clear-cuts have been used for nesting (Duncan 1993, Wilson 1993b). A mix of old forest and variably sized clear-cuts <100 ha and staggered over time would be an optimal management goal to meet the year-round lifetime habitat requirements of this species (Duncan and Harris 1997).
Measures Proposed And Taken
In Fennoscandia, species has used nest boxes (Sonerud 1997). One successful design measured 50 cm high, 30 cm wide, and 35 cm deep; entrance 13 cm in diameter and 20 cm from bottom of box (G. Sonerud pers. comm.). No published records of nest box use in North America (Pearman 1992, Duncan 1993). Nest boxes placed in muskegs or old burned areas would be most likely to attract hawk owls. Nest boxes could mitigate local loss of nest sites in clear-cuts, but this would not be practical over large areas and timescales. Nest boxes can facilitate research on breeding biology, but can also bias results (Gehlbach 1994c).
No reported loss of mass for 5 hawk owls radio-marked in Norway (Baekken et al. 1987). No other published studies on hawk owls involving radiotelemetry. Two hawk owls were radio-marked in n. Minnesota (S. Loch pers. comm.) and 1 in se. Manitoba (JRD), by either backpack harnesses or tail-mounted transmitters.
Visual searches in suitable habitat on foot or by vehicle. Call playback not tested; needs study. Hawk owl vocalizations detected only rarely and incidentally during nocturnal owl censuses in Manitoba, by playback of Boreal Owl and Great Gray Owl male territorial call (Duncan and Duncan 1997).
Caution required when handling hawk owls. Short tarsus is hard to hold and restrain, and struggling bird can easily free its feet and injure itself or the handler. Placing soft nylon mesh over bird's head and folded wings helps restrain it for banding, etc. Often resumes prior activity after capture and release. On breeding territory, female observed copulating within 2 min of her release after being weighed, measured, and banded (Lane and Duncan 1987).
Some concern that U.S. Fish and Wildlife Service (USFWS) bands cause damage to legs (T. Swem, S. Ambrose, and B. Ratcliff pers. comm.). Needs study. No such evidence when USFWS size 7B, "special short" butt end bands are used (PAD, JRD).