Northern Hawk Owl

Surnia ulula

Order:
Strigiformes
Family:
Strigidae
Sections

Diet and Foraging

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Feeding

Main Foods Taken

Small mammals, especially rodents in summer; shifts in winter to birds such as ptarmigan and grouse (extent unknown). Its specialized small-mammal diet, and hence hunting habitat, is remarkably similar to that of other circumpolar boreal forest owl species, such as Boreal, Long-eared (Asio otus), Short-eared (A. flammeus), and Great Gray owls. Yet there appears to be little aggression among or between these species when they converge to breed in areas of high prey abundance (see Behavior: spacing, and Behavior: social and interspecific behavior, below).

Microhabitat For Foraging

Generally prefers open areas with suitable perches, including open spruce forests (Rohner et al. 1995), burned areas (Mindell 1983b), cutovers, and marshland with shrubs (PAD, JRD). Sometimes hunts along roadsides with large rights-of-way, and in farmland, especially in winter (Lane and Duncan 1987, Nero 1995). In Norway, changes foraging habitat from mature forest to clear-cuts after snow has melted, in response to prey availability; in winter, prey more active above snow cover in mature forests than in open areas (Nybo and Sonerud 1990). In ne. Ontario in summer, selected mixed-wood cutovers aged 11–15 yr where herbicides were used to control deciduous shrub layer (R. Russell pers. comm.); understory variable, including blueberry (Vaccinium), dogwood (Cornus), willow (Salix), sphagnum moss (Sphagnum), alder (Alnus), sedges, and grasses (PAD, JRD). In Alaska, hunted in white spruce (Picea glauca) bog characterized by widely spaced, stunted trees <4 m high; ground cover of scattered shrubs and thick sphagnum (Kertell 1986). In Manitoba, observed hunting in all seasons in moist cutovers and older burns with spruce and/or tamarack trees 2–3 m high; in winter, treed watercourses and adjacent fields and marshlands (PAD, JRD); grassy and/or weedy fields near trees (Nero 1995).

Food Capture And Consumption

No less diurnal than Snowy Owl or Short-eared Owl. Will hunt in bright sunlight; also at night. At northern latitudes with continuous daylight in summer and darkness in winter, hunts during all hours (Mikkola 1983, Huhtala et al. 1987, Voous 1988b). In s. Manitoba in winter, 1 owl observed to hunt from sunrise until 09:00, then from 15:00 to sunset (Nero 1995). In Minnesota in winter, ceased hunting at dusk, possibly to avoid Great Horned Owls (Bubo virginianus; Lane and Duncan 1987); see Behavior: predation, below.

Habit of selecting high hunting perches (see below) in open habitat, and behavior, suggests that this species depends more on eyesight than on hearing for hunting. Can detect prey visually up to 800 m away (JRD, PAD). Lacks the asymmetrical ear openings found in Boreal and Great Gray owls, species that regularly use hearing to catch voles under thick vegetation or snow (Norberg 1987). Rather, hawk owl has an elliptical ear opening 13 mm high, without enlarged margin or transverse ligament (Voous 1988b). Its ear cavity is not covered by an operculum (Eckert 1974), and its ear openings are symmetrical in position and size (Voous 1988b). Nonetheless, its hearing is likely keen; it readily responds to prey-like sounds by immediately turning its face directly toward source, and it can capture prey concealed by snow cover (JRD, PAD; see below).

When scanning for prey, leans forward so that body and tail are almost horizontal; tail-pumping observed; when it locates prey, head is “snapped” into position and body becomes rigid. Sometimes opens wings and appears ready to pounce, almost falling off perch before regaining balance. Swoops down on prey fast and low (Mikkola 1983). Leaves perch into gliding dive, and may flap wings several times if strike distance (from hunting perch to prey) is great.

In Alaska, mean hunting perch height is 5.41 m ± 2.61 SD (range 0.61–10.61, n = 25), and mean strike distance for hunting male is 8.1 m ± 5.47 SD (range 0.91–21.2, n = 18; Kertell 1986). In Norway, observed to hunt for longer periods from higher perches (Sonerud Sonerud 1992, Sonerud 1997). Portion of prey capture attempts successful 68% for males (n = 28), 25% for females undergoing tail molt (n = 5; Kertell 1986). In Norway, individuals were most successful at capturing prey close to hunting perches when ground was free of snow (Sonerud 1992).

Snow-plunging (capturing prey concealed by snow) observed, but infrequently and in winter (Nero 1980, Jones 1987a, Lane and Duncan 1987, Radloff 1992). Hovering, sometimes persistent, observed (Dement'ev and Gladkov 1966, JRD, PAD). Occasionally seizes prey in flight (Dement'ev and Gladkov 1966). Recorded hunting using searching flight at height of 2–4 m (Cramp 1985a). Captures young birds in nests on the ground (Kertell 1986). Observed running through snow chasing a lemming (Synaptomys spp.; D. Russell pers. comm.), briefly stalking prey on the ground in a hay field (S. Crocoll pers. comm.), and catching a red squirrel (Tamiasciurus hudsonicus) off a limb near hawk owl fledglings (C. Garber pers. comm.). Rarely carries prey in bill (Eckert 1974).

Small rodents are swallowed whole, or the head is removed and swallowed and then the remainder eaten, or the head is eaten and the body eviscerated. If not swallowed whole, head is always removed first and eaten, including whole heads of red squirrels. Eviscerates mammalian prey by tearing into the side, and discards intestines and stomach; eats remaining organs. Numerous discarded rodent stomachs observed under feeding perches in Alaska (T. Osborne pers. comm.). Often partially plucks large prey items before eating them (Kertell 1986, JRD, PAD).

Will retrieve, and eat or cache, dead prey placed by humans on ground or on tree limbs (James 1994, Collister 1995, JRD, PAD). Eckert (Eckert 1974) reports it to “streak in and speed away with snipe or woodcock that have been downed by hunters.” Observed eating carrion: pile of deer (Odocoileus virginianus) guts and carcass of moose (Alces alces) in Ontario (S. Walshe pers. comm.), and “meat” placed out for other birds (James 1994). Observed following farmers and forest workers, pouncing on prey exposed by their equipment (Bent 1938b, B. Geisler pers. comm.).

Diet

Major Food Items

Few data for North America. Voles (Microtus spp.) are primary prey in Eurasia (Mikkola 1983, Cramp 1985a, Voous 1988b) and much of North America (Johnsgard 1988a, Duncan 1993; Appendix 1). In North America, may be less a vole specialist than S. u. ulula in Europe and Asia (Rohner et al. 1995). Microtus species taken in North America include Alaska vole (M. miurus), meadow vole (M. pennsylvanicus), tundra vole (M. oeconomus), long-tailed vole (M. longicaudus), and yellow-cheeked vole (M. xanthognathus).

Other Documented Prey

Prey less frequently taken in North America include shrews (e.g., masked shrew [Sorex cinereus]), moles, rabbits, hares (e.g., snowshoe hare [Lepus americanus]), squirrels (e.g., red squirrel), mice, rats, voles, lemmings, weasels (e.g., short-tail weasel [Mustela erminea]), partridge, grouse (e.g., Spruce Grouse [Falcipennis canadensis]), doves, woodpeckers (e.g., Pileated Woodpecker [Dryocopus pileatus]), jays, robins, starlings, sparrows (e.g., White-crowned Sparrow [Zonotrichia leucophrys]), buntings, blackbirds, grackles, and finches (e.g., House Sparrow [Passer domesticus]; Fisher 1893c, Smith 1922, Sprot 1927a, Roberts 1932c, Bent 1938b, Nero 1963a, Smith 1970a, Savaloja 1973, Eckert 1974, Snyder and Wiley 1976, Axelrod 1980b, Johnson 1981b, Kertell 1986, Lane and Duncan 1987, Johnsgard 1988a, Kiliaan 1989, Torres 1990, Larivée 1991, James 1994, Nero 1995, Rohner et al. 1995, T. Cameron, I. Carruthers, C. Garber, R. Gehlert, J. Jehl, S. Loch, T. Osborne, and U. Waterman pers. comm., PAD, JRD).

Also preys on Tengmalm's Owl (Eurasian Aegolius funereus funereus) in Finland in winter (Mikkola 1983). Adults known to eat their young (Mikkola 1972).

Quantitative Analysis

Appendix 1. Information primarily from pellet analysis; remains of prey in pellets can be reliable indicators of diet, but caution is needed in analysis of pellets collected at nest sites, because smaller prey (e.g., shrews) may be consumed by hunting adults away from the nest (Marti 1987, Rohner et al. 1995). Most pellets collected at nest sites in Alaska and in Manitoba did not contain skull or tooth material (J. Jehl and T. Osborne pers. comm.). In 20 pellets collected in Iowa in winter: 23 meadow voles and 2 white-footed mice (Peromyscus spp; Conrads 1993).

During winter in central Norway, bank voles (Clethrionomys glareolus) were 97% of diet Jan–Apr (Hogstad 1986). Of 868 prey items from Norway: microtines 97.2%, shrews 2.7%, least weasel (Mustela nivalis) 0.1%, birds 0.6%, and insects 0.2% (Nybo and Sonerud 1990). For more on diet of S. u. ulula, see Cramp 1985a.

Food Selection and Storage

During the breeding season, depends on microtines (especially Microtus spp.) for successful nesting (Cramp 1985a, Mikkola 1983, but see Rohner et al. 1995 for importance of other prey). In winter, more birds taken, but extent in North America unknown. In Finland, Mikkola (Mikkola 1972) found birds taken 30 times more often in winter; up to 90% birds in winter diet in Fennoscandia (Mikkola 1972, Cramp 1985a, Hogstad 1986, Kiliaan 1989). In Alaska, nesting hawk owls exploited prey depending on availability and ease of capture; juvenile snowshoe hares taken 31 May–24 Jun, red squirrels (predominantly young) captured 17 May–2 Jul, juvenile Gray Jays (Perisoreus canadensis) taken 25 May–19 Jun. In Norway, hawk owl diet changed during the breeding season from primarily bank voles to Microtus voles as the season progressed (Nybo and Sonerud 1990).

Breeders in Alaska cache food more often after young leave the nest. While female incubates and broods, male conducts all caching and food retrieval. All sizes of prey cached (Kertell 1986). At a nest In Norway, a male cached an equal amount of prey before and after egg-laying, incubation, and hatching (Sonerud et al. 1987). Ritchie (Ritchie 1980a) suggested that caching helps define duties by the pair, and reduces potential conflicts at the nest site; storing prey at locations other than the nest may reduce conspicuousness and facilitate sanitation.

Hawk owls in Finland store prey for intensive feeding of young; prey stores act as buffer during food shortages (Huhtala et al. 1987). During winter, individuals observed capturing and caching up to 20 prey items in < 3 h (D. Fast pers. comm.). Caches prey in crevices and old woodpecker excavations in deciduous trees, dead stumps, and spruce boughs (McNair 1994, Collister 1995, Nero 1995, Schaefer et al. 2007, R. Gehlert pers. comm.), and at base of trees in snow (observed using bill to push snow over cached prey until it was covered) and at base of fence posts in grass (JRD, PAD). During nesting, usually caches prey at least 3 m above ground on spruce boughs (Kertell 1986). Caching also observed in captivity (Collins 1976b, K. McKeever pers. comm.).

Nutrition and Energetics

Needs study; in captivity in winter, consumes 30–45 g/d (K. McKeever pers. comm.).

Metabolism and Temperature Regulation

Of 109 cloacal body temperatures (73 in daytime, 29 at night, 7 in transition hours), overall mean temperature 39.4°C ± 0.59 SD (range 38.4–40.8), mean daytime temperature 39.6°C ± 0.42 (range 38.4–40.8), and mean nighttime temperature 38.8°C ± 0.24 (range 38.4–39.5). The 0.78°C difference in temperatures was statistically significant (t = 8.67, p < 0.001). A sample of 64 core (2.5 cm into large intestine) temperatures averaged 0.91°C ± 0.32 higher (Collins 1991). Higher daytime temperature relates to species' primarily diurnal activity, e.g., Northern Hawk Owls hunt mostly by day.

Metabolic rate of adult female hawk owl: Two determinations, at 24°C, were both 0.93 ml O2/g/h (35.0 kcal/day); bird mass 333 g. Owl could fluctuate between 293 and 375 g with no apparent ill effects. Resting cloacal body temperature ranged from 38.2 to 41.8°C, slightly higher during day than at night. Metabolic rate same as that of other non-passerine species of comparative body mass, but lower than that of other owls (Johnson and Collins 1975).

Drinking, Pellet-Casting, and Defecation

Observed consuming snow after eating prey (T. Osborne pers. comm.). Pellets relatively small, grayish, mucus-coated, regurgitated easily; up to 7.5 cm long (Nero 1995): Twenty winter pellets averaged 4.0 cm ± 1.0 SD x 2.3 cm ± 0.4 SD (JRD, PAD). Nero (Nero 1995) noted that, in winter, smaller pellets were found under perches in the open; largest beneath night roosts; occasionally, undigested prey parts were found in pellets. Eurasian pellet measurements are in Mikkola 1983. For number of prey per pellet, see Appendix 1. Fecal wastes are more liquid than viscous; dark chalky color; usually expelled as bird leaves perch (Eckert 1974).

Recommended Citation

Duncan, J. R. and P. A. Duncan (2014). Northern Hawk Owl (Surnia ulula), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.356