Northern Hawk Owl

Surnia ulula

Order:
Strigiformes
Family:
Strigidae
Sections

Breeding

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Figure 3. Annual cycle of breeding, molt, and migration of N. Hawk Owl

Annual cycle of breeding, molt, and migration of the Northern Hawk Owl in North America. Molt is based in part on European studies. Thick lines show peak activity; thin lines, off-peak.

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Figure 4. Female Northern Hawk Owl delivering prey to young at the nest.

Drawing by N. John Scmitt.

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Northern Hawk Owl nestlings, Churchill, Manitoba, June.

Typical nest site for this species in a broken snag.; photographer Rick and Nora Bowers

Phenology

Pair Formation

In n. Minnesota, males start singing as early as 10 Feb (Lane and Duncan 1987). Pair observed copulating 19 Feb in Alberta (Henderson 1919). On 1 Feb in Alberta, male observed courtship-feeding (R. Gehlert pers. comm.). In Labrador, female sitting on nest site (hollow top of snag) on 13 Mar, evidence of nesting on visit of 13 Jun; after 3 nest checks by observer, site was still used (Todd 1963a). In North America, initiation of breeding can be as late as Jun (Appendix 2).

First/Only Brood Per Season

Figure 3; Appendix 2 . In North America, extreme dates for egg-laying 30 Mar–30 Jun (Bent 1938b, Gabrielson and Lincoln 1959, Eckert 1974, Johnsgard 1988a). In Finland, 30 Mar–23 Jun (Mikkola 1972). May be no breeding in years when prey is scarce (Hagen 1956).

Second/Later Brood Per Season

No second brood reported in North America. In Europe, replacements laid after egg loss (Cramp 1985a).

Nest Site

Selection Process

In Europe, male sits below or near a prospective site and attracts female with Advertising Call (see Sounds: vocalizations, above). Pair inspect site together, or female inspects site while male continues to give Advertising Call while staring at site. Both perform scraping. Female may utter a Lure Call (see Sounds: vocalizations, above), taking the initiative in attracting the male to a potential site; thus male and female share initiative in nest-showing. Choice of alternative nest sites by female is thought critical to successful breeding in captivity (K. McKeever pers. comm.).

Microhabitat

In Scandinavia, more than half of 16 nests were in open forests with scattered trees, clear-cut areas or bogs; 30% in open spruce forests; remainder in closed spruce forests or ecotone areas between open and closed forests (Sonerud 1985c). Similar variation found in North America (Duncan 1993, Patrikeev 2006 ); nests in Alaska in closed black spruce (Picea mariana) forest with > 60% canopy cover 45 m from river; another nest in open stand with < 60% canopy cover of aspen trees 35 m from river (Mindell 1983b). Proximity to water (within 100 m from nest) thought to be important because of increased availability of prey attracted to water's edge (Bombardier and Gauthier 1996).

Site Characteristics

Table 1.  Nest sites located in cavities in decayed trees, open decayed hollows where treetops have broken off, burnt-out stumps, and vacant woodpecker nests. Occasionally uses stick nests. Use of cliff sites reported in Bent 1938b. Uses nest boxes in Norway (Sonerud et al. 1987) and Sweden (Voous 1988b). One nest found in metal cavity of structure of Trans-Alaska Pipeline (Reakoff et al. 2003). Shows preference for dead hollow stub at 45° angle, with top of stub 2.6 m above ground, sometimes < 2 m (Eckert 1974); 2 nests at 7 m and 8.5 m above ground (Patrikeev 2006). Favors nest holes that are shallow (Huhtala et al. 1987).

Nest

Construction Process

Uses existing structures. Does not add fresh material to nest.

Structure And Composition Matter

Floor of nest cavity often composed of decomposed sapwood, feathers, prey fur, and matted pellets up to 5 cm thick (Smith 1970a, Kertell 1986, PAD, JRD).

Dimensions

See Table 1.

Microclimate

No data.

Maintenance Or Reuse Of Nests, Alternate Nests

No information on nest-site fidelity, but doubtful, considering nomadic tendencies (Cramp 1985a). Year-to-year reuse of nest structures reported, but individuals not marked.

Eggs

Shape

Blunt elliptical, oval to elongate oval. Resemble eggs of Short-eared Owl (Bent 1938b).

Size

Mean measurements in North America: 31.47 mm (range 30.63–32.15) x 40.09 mm (range 38.64–42.48, n = 51 eggs from 9 clutches; Western Foundation of Vertebrate Zoology [WFVZ]). Relatively small compared to eggs of other owl species (K. McKeever pers. comm.).

Mass

In North America, 21.6 g; in Europe, 21.8 g (Mueller 1986). Mean empty shell mass 1.656 g (range 1.465–1.790, n = 51 eggs from 9 clutches; WFVZ).

Color

White, infrequently with vague yellowish tinge (Eckert 1974).

Surface Texture

Smooth and slightly glossy.

Eggshell Thickness

Average thickness 0.251 mm (range 0.231–0.271, n = 51 eggs from 9 clutches; WFVZ).

Clutch Size

Three to 13, usually 7 (see medialink).

Egg-Laying

Mean laying interval 1.6 d, on basis of 6 nests at Kluane, Yukon (1988–1993; C. Rohner pers. comm.). In Fennoscandia, 1–2 d (Mikkola 1983, Cramp 1985a).

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

Incubation begins after first egg is laid (Cramp 1985a).

Incubation Patch

Female only. Measures 7.6 x 12.7 cm (n = 1; Smith 1922).

Incubation Period

Lasts 25–29 d (Godfrey 1986) or 25–30 d (Cramp 1985a). Minimum 25 d, not > 30 d (Mikkola 1983).

Parental Behavior

Generally only female incubates (Bent 1938b, Mikkola 1983, Kertell 1986, Voous 1988b); remains on nest except for short periods to receive food, preen, cast, or defecate. Male perches atop trees about 30 m from nest when not foraging (Kertell 1986). Male feeds incubating female. Usually exchange food away from nest, at nearby perch. Female returns to nest within 1–2 min. Male does not incubate when female is off nest. Incubating female most frequently faced east on snag-top nest in Alaska (T. Osborne pers. comm.). Incubating or broody female is hard to flush off nest; male exhibits intense interest in nest with eggs or young, tries to peer under female; upset when female off nest, male rarely enters nest to “fuss” about; adults will remove cracked eggs before they hatch, puncture eggshell with bill, and carry away from nest (K. McKeever pers. comm.).

Vigorously and repeatedly attacks intruders at the nest, including large avian and mammalian predators, also humans (Bent 1938b, Voous 1988b). Capable of inflicting painful scalp wounds on humans. Both sexes attack; more defensive at nest site, but will give Alarm Calls (see Sounds: vocalizations, above) and attack in nest vicinity (Lane and Duncan 1987). Within a few hours of midnight, less aggressive at nest (Voous 1988b). Males seem to be more aggressive, especially when female is incubating or brooding (Eckert 1974, Mikkola 1983). During nesting, adults away from the nest or young allow close approach by humans (Lang et al. 1991).

Hatching

Preliminary Events And Vocalizations

Young start calling from within egg (Cramp 1985a).

Shell-Breaking And Emergence

No information.

Parental Assistance And Disposal Of Eggshells

Eggshells either removed or eaten after chicks hatch (K. McKeever pers. comm.).

Young Birds

Condition At Hatching

Mean mass 18 g; mean wing length 13 mm (n = 6 young from 1 nest; Huhtala et al. 1987). No difference in hatching asynchrony between nests and years (Rohner et al. 1995). At hatching, blind and helpless (altricial) and naked (psilopaedic); remain in nest for 3–5 wk (nidicolous); hatch with eyes closed and no means of temperature regulation.

Growth And Development

Young gain mass rapidly, about 9 g/d (13%/d) in first 2.5 wk (A. Gilyason pers. comm.). This is 3 times the rate of the Boreal Owl (Aegolius funereus).  Mean nestling wing growth 5.6 mm/d (n = 6 young from 1 nest; Huhtala et al. 1987). Of 6 young in a nest in Finland, mass went from 59 to 180, 72 to 186, 90 to 206, 114 to 214, 130 to 220, and 135 to 228 g, respectively, from 28 May to 6 Jun (Pulliainen 1978a). Mass gain of nestlings ceases at 2.5 wk (Huhtala et al. 1987). Adults take advantage of long daylight hours and cache surplus prey near the nest site; these stores are used for intensive feeding of young and help account for rapid growth (Huhtala et al. 1987). After natal down, young acquire mesoptile feathers (soft semi- or second down and definitive remiges and rectrices); leave nest in this “mesoptile attire” (Dement'ev and Gladkov 1966).

Parental Care

Brooding

By female only; begins immediately after hatching and lasts 10–14 d. In Europe, female broods young until oldest is 13–18 d old (Leinonen 1978).

Feeding

While the female broods, male brings food to her and the female feeds the young. At the end of the brooding period, the male flies directly to the nest with prey (Jones 1987a); male perches at least 100 m from female when not foraging. At this time, the female receives food at nearby perches (Kertell 1986, Lane and Duncan 1987). Begging by young seems to regulate rate at which they are fed; only those young that respond vocally to an incoming adult are fed (Simon and Simon 1980, Cramp 1985a, Lang et al. 1991). Indirect evidence suggests that very small young are not fed gastrointestinal organs of prey (Simon and Simon 1980, T. Osborne and K. McKeever pers. comm.).

In Sweden, pair fed young once per hour during daylight; after 17:00, feeding decreased (Mikkola 1972). In Finland, fed usually between 04:00 and 19:00; 0.8–2.5 feedings/h, with no obvious peaks (Leinonen 1978). Brood of 1 fed every 30–45 min at night, every 95 min by day in Finland (Simon and Simon 1980).

Foraging activity from 4 nests in central and n. Finland showed owls feeding young at the nest around the clock, except for a 2- to 3-h pause around midnight. The frequency of nest visits was greatest in early morning (03:00–04:00), around noon (11:00–13:00) and in late evening (20:00–23:00). During the hatching period, 16.6 visits/d were made to the nest, 41.4 visits/d during the nestling period. The hawk owl visits its nest 3–4 times more often than any other northern owl (Huhtala et al. 1987).

In Manitoba, average of 2.1 food deliveries/h to nest area (after owls had left the nest; 27.5 h of observation at 1 nest). Adults delivered food at intervals averaging 48.5 min ± 34.9 SD (16–19 Jun, n = 12 intervals [ranging from 10 to 130 min]). Longest intervals recorded just after midnight, in early afternoon, and in mid-afternoon; adults fed up to 2 young/trip (Lang et al. 1991). In Ontario, food delivery intervals range from 49 - 188 min (n = 8) during 14.5 h observation at 1 nest (Patrikeev 2006). Males occasionally brought food directly to fledglings, but were observed offering only small intact prey (Figure 4), while females fed the young larger pieces of prey (Kertell 1986).

Nest Sanitation

Females eat pellets of young and defecate outside of the nest (Leinonen 1978, T. Swem pers. comm.). Caching prey away from the nest may facilitate sanitation (see Food habits: food selection and storage, above). Indirect evidence suggests that the female eats the feces of young until within a few days of fledging (K. McKeever pers. comm.).

Cooperative Breeding

Not reported.

Brood Parasitism

Not reported.

Fledgling Stage

Departure From Nest

Young leave nest at 3–5 wk of age (Mikkola 1983); 25–35 d (Torres 1990); 23 d (Huhtala et al. 1987); 31 d (T. Osborne pers. comm.). In Alaska, young left the nest at 20–22 d (Kertell 1986); in n. Minnesota, at 28–30 d (Lane and Duncan 1987); in Europe, at 21–35 d. In n. USSR, youngest chick left 1 wk after the oldest (Cramp 1985a). Young are unable to fly when they leave the nest; walk, jump, and climb out to limbs of the nest tree (Mikkola 1972, Leinonen 1978, Lane and Duncan 1987). Occasionally fall to the ground, but can jump, climb, and flutter to low branches (Kertell 1986, JRD, PAD). Young are able to fly up to 30 m horizontally (usually landing on ground) when about 30–33 d old (Leinonen 1978, Cramp 1985a, Kertell 1986, Lane and Duncan 1987, C. Garber pers. comm.). In Finland, mass of young is > 200 g when leaving the nest (Pulliainen 1978a); mean body mass of fledging young: 230 g (78% of adult mass); mean wing length 141 mm (60% of adult wing length; n = 6 young from 1 nest; Huhtala et al. 1987).

Association With Parents Or Other Young

In Alaska, when young 37–39 d old, female absent for up to 5 h; seldom brings food to the young. Male feeds and guards young while female is absent; fledglings are left alone while the male forages (Kertell 1986). After 6–8 wk, young are still usually found in the nest area (Mikkola 1983). Feeding rates (see Parental Care, above) and behavior of adults and young in the postfledging period need study.

Immature Stage

Few data. Young are independent at about 2.5–3.0 mo of age; at this time, may begin to disperse (Hagen 1956, Mikkola 1972, Cramp 1985a, Torres 1990).

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