Demography and Populations
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Measures of Breeding Activity
Age At First Breeding; Intervals Between Breeding
Age at first breeding unknown; many suggest that it probably breeds in first year based on comparisons with other species (Hagen 1956, Mikkola 1983, Cramp 1985a, Johnsgard 1988a). Does not breed in years of prey scarcity; e.g., in Lapland, nests only twice every 4 yr, in accordance with fluctuating vole and lemming populations (Voous 1988b).
Range 3–13, commonly 7 (see medialink). Varies with prey availability (Mikkola 1983). In Lapland/Finland, clutch size decreased southward (Mikkola 1983).
Annual And Lifetime Reproductive Success
Annual number of young fledged/nest ranges from 1 to 7 (see Appendix 2). No relationship between laying date and brood size (Rohner et al. 1995). One of 10 nests in sw. Yukon was abandoned with 2 eggs still in it (Rohner et al. 1995). Radio-marked female abandoned nest 1 d after 5 young (14–22 d old) were killed by a predator (JRD). No data on lifetime reproductive success.
Number Of Broods Normally Reared Per Season
Life Span and Survivorship
Few data; needs study. Life span 10 yr in captivity and wild (Eckert 1974). Juvenile banded at nest in n. Minnesota in 1987 (JRD, PAD) was recaptured 8 yr later in se. Manitoba (H. Copland and R. Nero pers. comm.).
Disease and Body Parasites
Fewer fleas and feather lice than most other owls have (Eckert 1974). Lice frequently observed on winter-caught owls in se. Manitoba and n. Minnesota (PAD, JRD). Lice (Stigiphilus spp.), fleas (Orchopeas spp.), feather mites (Order Acarina), and ticks (Haemaphysalis spp.) found on 6 specimens examined in Manitoba in 1997 (T. Galloway pers. comm.). Syngamus spp., a nematode parasite, caused a captive hawk owl to die in captivity (K. McKeever pers. comm.).
Few quantitative data on natural (not caused by humans) rates of mortality. Unknown what proportion of individuals that disperse south of breeding range in winter irruption years (see Migration, above) ever return to breeding grounds. Most hawk owls captured and banded during irruptions in Manitoba appear to be healthy (JRD, PAD). Of an estimated 100 hawk owls recorded during winter 1996–1997 influx in Minnesota, only 1 known to die (Svingen 1997b). In contrast, 202 of 263 Boreal Owl records were birds found dead during same winter in Minnesota (Wilson 1997). See also Behavior: predation, above.
Initial Dispersal From Natal Site
No data; needs study.
Fidelity To Breeding Site And Winter Home Range
No data; fidelity is unlikely, because of nomadic habits (Cramp 1985a). Needs study.
Dispersal From Breeding Site Or Colony
During invasion years, many juveniles are out of breeding range by Oct–Nov (Hagen 1956, Bernard and Klugow 1963, Forsman 1980a, Eckert 1992).
Averaged 372 ha (range 140–848 ± 281 SD, n = 5 owls: 3 males, 2 females) in Norway (Baekken et al. 1987); immature male on 50-ha winter home range in s. Manitoba (Nero 1995). Needs study in North America.
Poorly known; estimates only. Breeding densities low (Fyfe 1976, Errington 1933); 0–6 pairs/100 km2 in the Yukon (Rohner et al. 1995). In Europe, estimates of density from 0.2 to 20 pairs/100 km2 (Hagen 1956, Cramp 1985a).
Numbers reported to fluctuate up to 100% with cycles of small-mammal prey populations. On the basis of its North American breeding range (see Distribution, above), population estimated to be 10,000–50,000 pairs (Duncan and Harris 1997). Estimates from Fennoscandia include < 10,000 pairs in Sweden and 3,600 pairs in Finland (Cramp 1985a).
Anecdotal evidence suggests that populations have declined since the late 1800s and early 1900s, but there are few data to confirm this. Numbers fluctuate locally (Fyfe 1976), but the North American population has likely been stable over the last 100 yr (Duncan and Harris 1997). Breeding Bird Survey (BBS) data for Canada are sparse, therefore not conclusive; decreasing trend noted for Labrador and central Prairie Provinces between periods 1966–1977 and 1978–1983 (Collins and Wendt 1989).
During the 1800s, the European range is thought to have retreated northward and numbers to have decreased (Mikkola 1983, Voous 1988b). No population trends for Ontario, since it is too poorly represented on BBS. Rare status in Ontario, owing to lack of breeding records, threats to the species, and need to monitor populations for evidence of declines (Austen et al. 1994).
Unknown; variable brood size, dispersal movements of banded birds of > 1,000 km, and occasionally intense predation are likely important factors and related to small-mammal prey abundance. Competition between boreal avian predators is perhaps less important than influence of distribution and abundance of prey (Mikkola 1983). An apparent lack of adaptations for locating and catching voles under thick snow cover led Rohner et al. (Rohner et al. 1995) to suggest that the most severe selection takes place during winter. However, hawk owls do snow-plunge (infrequently; see Food habits: feeding, above), and successfully capture prey under considerable snow crusts (PAD, JRD). More study needed.