Demography and Populations

Age At First Breeding

Few data, but appears to breed at an early age, e.g., ≥1 yr old. A female banded as nestling in Massachusetts trapped the following year brooding young in nest (Tate 1992; see also natal philopatry, below). In Europe, breeding at 1 yr or less (Glutz von Blotzheim and Bauer 1980).

Clutch

For North America; mean 5.6, range 1–11 (n = 186; Murray 1976). Clutch size increases significantly with latitude—increase also to the west, but not significantly (Murray 1976).

Specific regions: New Jersey, mean 6.8 (n = 4; Urner 1925); Alaska, mean 6.3, range 4–8 (n = 22; Pitelka 1955a); Alberta, mean 6.2, range 4–10 (n = 25; Trann 1974); Manitoba, mean 8.8, range 8–10 (n = 4; Clark 1975); N. Dakota and S. Dakota, mean 7.0, range 4–9 (n = 13; Duebbert and Lokemoen 1977); Massachusetts, mean 5.7, range 3–8 (n = 7; Holt and Melvin 1986); Massachusetts, mean 6.2, range 4–10 (n = 9; Holt 1992). In Montana, mean 8.1 ± 1.4 (n = 29), range 6–11 (DWH and SML). In Europe, mean 7.3, range 2–13 (n = 121; Mikkola 1983); Galapagos, mean 3.3 (n = 7; De Groot 1983).

May lay replacement clutches, but marked individuals needed to conclusively determine that same individual laid replacement clutch.

Annual And Lifetime Reproductive Success

Hatching success (number of young hatched per nest); Washington, mean 3.5 (n = 2; Kitchin 1919); New Jersey, mean 4.8 (n = 4; Urner 1925); Manitoba, mean 7.5 (n = 4; Clark 1975); Massachusetts mean 3.4 (n = 7; Holt and Melvin 1986); N. Dakota and S. Dakota 100% hatching success (n = 14; Duebbert and Lokemoen 1977).

In Montana, of 235 eggs laid, 174 (74.0%) hatched, yielding about 5.8 hatched/nest (n = 29; DWH and SML). In W. Germany, 44 (36.4%) of 121 eggs hatched from 17 nests (Holzinger et al. in Cramp 1985). In Scotland, only 5 of 24 broods hatched because of predation (Lockie 1955).

After hatching, pre-fledging dispersal from nests (Montana) was 159 (91.4%) nestlings, or about 5.5 per nest (n = 29; DWH and SML). Fledging success data per nest limited and difficult to assess; Manitoba, mean 4.0 (n = 4; Clark 1975); Massachusetts, mean 2.1 (n = 7; Holt and Melvin 1986); Massachusetts, mean 3.2 (n = 22; Holt 1992). Pitelka (1955a) reported 56.0% (14/25) survival for at least one nestling or fledging surviving the incubation period. In Montana, Fondell and Ball (2004) found 60% and 10% nest success (at least one young successfully fledged) in ungrazed and grazed grassland, respectively.

In Scotland, two broods fledged from 24 nests (Lockie 1955). Besides predation, annual fluctations in prey density, habitat changes, and catastrophic events (poor weather) may all contribute to the wide variation in reproductive success reported here.

North American Bird Banding Laboratory recovery records limited (n = 47), but longevity record for a wild Short-eared Owl is 4 yr, 2 mo. In Europe, longevity record 12 yr, 9 mo for wild bird (Cramp 1985).

Causes Of Death

Data limited for North America. Occasionally hit by cars and airplanes, shot, trapped (Clark 1975, Holt 1992). May also suffer from collisions with barbed-wire fences (e.g., see Figure 8), but extent of problem is unknown.

Disease

A road killed bird in England found infected with Tuberculosis (Mycobacterium tuberculosis avium; Harrison 1943). In California, secondary poisoning from fowl cholera (Pasteurella bacteria) passed through waterfowl, rodents, and raptors, killing 44 Short-eared Owls (Rosen and Morse 1959). Single cases of mortality due to West Nile virus in midwestern USA (Fitzgerald et al. 2003) and avian cholera (Pasteucella multocida) in the Texas panhandle (Taylor and Pence 1981).

Initial Dispersal From Natal Site

Few data. One female banded as nestling retrapped the following year brooding young 98 m from her natal nest (Tate 1992). Another female banded as a nestling found dead 740 d later on the same island, 4.8 km from natal nest (DWH).

Recent “invasion” of s. Florida by individuals (mostly juveniles) from Antillean sub-populations (most likely Cuba) suggests an ability to colonize new areas, even over water, at a distance ≥ 145 km (Hoffman et al. 1999). Presence of this species on oceanic islands, distant from mainland (e.g., Hawaii), suggests this ability is well developed.

Fidelity To Breeding Site

Apparently limited, perhaps not surprising for a species that depends on a fluctuating prey base. In Scotland, only 4 of 28 wing-tagged adults kept same site the following year (Village 1987). In Montana, of 28 females captured between 1987 and 1993, no recaptures, suggesting little fidelity there.

Natal Philopatry

Few data. One female banded as nestling retrapped the following year brooding young 98 m from her natal nest (Tate 1992). Another female banded as a nestling found dead 740 d later on the same island, 4.8 km from natal nest (DWH).

Fidelity To Winter Home Range

No data; needs study. Same areas and roost sites often occupied in consecutive years, but not known if by same individuals (Clark 1975).

Despite widespread concern over declines in their abundance in many areas of North America, this owl classified as G5 (secure) by the Nature Conservancy. Population status of this species difficult to assess because individuals are nomadic and prone to annual fluctuations in numbers; also contributing to difficulties are nomadic nature, crepuscular habits, and overall low abundance. Consequently, fixed-area census projects such as the Breeding Bird Survey do not adequately sample for Short-eared Owls. Most recent regional ornithological summaries (including state and provincial bird atlas projects) suggest that declines have occurred across the species’ range, including many areas in Region 2 (Wiggins 2004).

Abundance map generated from Breeding Bird Surveys (Fig. 9) shows low abundance (< 1 per route) over most of breeding range. Numbers fluctuate on an annual basis in most areas. Over 6 yr period at Barrow, AK, breeding population fluctuated between 0 (in 3 yr) and one pair (in 2 yr), with peak of 28 pairs during a high lemming year (Pitelka et al. 1955a).

1985 estimates showed total of 20-25 pairs bred annually on islands off the coast of se. Massachusetts (Holt 1986a). Christmas Bird Count totals for wintering populations in New Jersey, 1976-1997, show numbers fluctuating on 4 year cycles (likely reflecting prey cycles farther north), with highs statewide about 80 individuals, lows < 20 (Fig. 10; Walsh et al. 1999). Likewise in Lowland Mainland, British Columbia, Christmas Bird Count data from 1972–1987 varied from 170 to 5 individuals, often in the space of just a year or two (Campbell et al. 1990).

On San Clemente I. CA, this owl the least numerous of 3 others wintering there (Dec-Apr), with only 0.1 individuals detected/hr in nighttime surveys (Condon et al. 2005).

Historically (1960s-early 1980s; Tate 1986), of 7 North American regions, 3 (Central Southern, Prairie Provinces, Middle Pacific Coast) species reported “greatly down in numbers” and 4 (Hudson-Delaware, Ontario, Middle-Western Prairie, Southern Great Plains) reported “down in numbers;” declines also in midwest, California, and s. Ontario (Tate 1992). Melvin et al. (1989) considered this owl to be the “rarest and most threatened species of owl” in ne. United States (see also Distribution: historical changes). Erskine (1987), however, doubted the value of these regional reports.

Recent (1980-2003) BBS data (Sauer et al. 2005; http://www.mbr-pwrc.usgs.gov/bbs/) suggest declines in western United States and Canadian prairie provinces, with populations stable or increasing in the Dakotas, ne. Minnesota, and e. Montana and Wyoming; e.g., see Figure 9 .

Working on the arctic tundra near Barrow, Alaska, Pitelka et al. (1955a, b) showed that large, annual fluctuations in the number of breeding Short-eared Owls was tightly correlated with the abundance of small mammals.

Few data on mortality; also needed is better information on dispersal and nomadism in this species – when vole outbreaks occur, where do the Short-eared Owls that exploit them come from? And how quickly?

Predation clearly an important factor for this ground nesting species, and probably influences numbers in some areas (see Behavior: predation). Probably also helped speed breeding declines in ne. United States (see Population Status); e.g., this species now generally restricted to nesting on offshore islands with no (or few) mammalian predators.