Asio flammeus

A small to medium-sized owl. Published lengths range from 37-39 cm (Cramp 1985) to 34-42 cm (Mikkola 1983), with females slightly larger than males and considerably heavier, averaging 411 g compared to 350 g for males (Mikkola 1983). They are excellent flyers with long wings (95-110 cm) (Cramp 1985), and light wing-loading (0.333 g/cm squared) (Clark 1975). There is little difference in wing length between the sexes (Clark and Ward 1974).

The back and upper wing surfaces are tawny brown to buff-colored with heavy but indistinct streaking. The ventral surfaces are much lighter, with bold, vertical brown streaking on the breast, and a pair of barely visible "ear" tufts close together at the top of the facial disk; belly is pale, lightly streaked; wings are long and have a buffy patch beyond the wrist above and a dark patch at the base of the primaries below; dark facial disk contrasts with yellow eyes; legs and feet are feathered (NGS 1983). Mature males are bright white on the underwing, while mature females show somewhat more buff coloration (Bent 1938, Village 1987). It is, nonetheless, difficult to sex or age these birds in the field. Females are generally darker than males but young birds are also darker than older ones (Mikkola 1983), thus a young male may be darker than an old female. Both sexes have a distinct, black carpal bar and dark wingtips. Juveniles possess full adult plumage by October of the first year (Bent 1938, Cramp 1985).

The facial disc is circular and whitish with dark areas around the bright yellow eyes. Recently fledged and juvenile owls show much darker coloration overall and a much darker facial disc which whitens with age. The owl gets its common name from the small ear-tufts over the eyes. These tufts are part of the facial disc and are erected when the bird is annoyed or alert. They may possibly aid in making birds more cryptic when in vegetation by breaking the line of the circular facial disc.

VOCALIZATIONS: The bird is generally silent but does vocalize in courtship (a low, repeated, hooting: "voo, hoo, hoo, hoo," or in conjunction with defensive behavior or annoyance: "yaps" or "barks"). Young owls give a food-begging call ("pssssip") that apparently aids adults in locating them from the time they leave the nest until after fledging. Adult owls may squeal while feigning injury during broken-wing acts to distract intruders from nests or young. Both young and adults will clack their bills when annoyed or in defense. Apparently, no data exist on the use of broadcasting tape-recorded vocalizations for detection or monitoring purposes.

These are probably the most diurnal of owls (Lockie 1955, Clark 1975) and may often be observed from late afternoon until nightfall, or at dawn. A crow-sized owl abroad during daylight in open country will most likely be a short-eared owl. However, they also hunt at night. Easily recognized by its blunt-headed profile and the fact that it glides with its wings held horizontally. This contrasts with the shallow V-shape of the northern harrier (Circus cyaneus) with which the owl often shares habitat and may be confused. Harriers may also be distinguished by their white rump patch. Habitat is useful in separating short-eared owls from long-eared owls (Asio otis), the latter being predominantly a woodland dweller. The long-eared owl is also more slender with much longer ear tufts. Burrowing owl also inhabits open country but is smaller (24 cm vs. 38 cm), has relatively longer legs, and (in adults) has at least some horizontal barring on the breast. The short-eared owl's style of flight is unique and has at times been called mechanical, moth-like, or even "slovenly" (Peterson 1934).

BREEDING: Broad expanses of open land with low vegetation for nesting and foraging are required. Habitat types frequently mentioned as suitable include fresh and saltwater marshes, bogs, dunes, prairies, grassy plains, old fields, tundra, moorlands, river valleys, meadows, savanna, open woodland, and heathland (Dement'ev et al. 1951, Clark 1975, Mikkola 1983, Holt and Melvin 1986). In general, any area that is large enough, has low vegetation with some dry upland for nesting, and that supports suitable prey may be considered potential breeding habitat, although many will not have breeding short-eared owls. Dement'ev and Gladkov (1951) assert that "nearby water" is a requirement for nesting habitat. Roosts by day on ground, on low open perch, under low shrub, or in conifer. Reported from "forest" habitats in Hawaii.

Nests on ground, generally in slight depression (Terres 1980), often beside or beneath a bush or clump of grass. Many nests are near water but generally are on dry sites. In coastal Massachusetts, nested in secondary herbaceous grass/sand dune vegetation dominated by Ammophila (Holt 1992). Same nest site may be used in successive years. Moves into and breeds in areas with high rodent densities.

Generally nest on high ground or upland sites (Pitelka et al. 1955; Clark 1975; Holt and Melvin 1986; Tate and Melvin 1987, 1988; Combs and Melvin 1989). Urner (1925) reported nests in a saltmarsh, one of which was subsequently flooded by a high tide, but in general, drier sites are preferred. During five years of study on Nantucket and Tuckernuck islands, all 41 nests found were in dry upland areas, though wetter sites were available (Holt and Melvin 1986; Tate and Melvin 1987, 1988; Combs and Melvin 1989; Combs and Griffin 1990). Eight nest sites at Monomoy National Wildlife Refuge, east of Nantucket, found between 1982 and 1985, were also all on dry upland sites (Holt and Melvin 1986).

Using a line-intercept technique (Brower and Zar 1977, Holt and Melvin 1986), vegetation characteristics of 15 nest sites on Nantucket were evaluated in 1986 and 1987. This analysis showed that low dense shrubs, mainly black huckleberry (Gaylussacia baccata) and bayberry (Myrica pennsylvanica), that were less than 0.5 m comprised 40.4%, and high dense shrubs (same species, >= 0.5 m) comprised 37.14% of the cover within five meters of the nest (Tate and Melvin 1987, 1988). Other vegetation included low sparse shrubs (11.1%), low dense grass (8.1%), and high dense grass (3.0%, mostly Andropogon scoparius and Ammophila breviligulata). These data demonstrate that in choosing nest sites on Nantucket, dense shrub cover is usually sought.

NON-BREEDING: Suitable breeding habitat may also be occupied by wintering birds. Conversely, Clark (1975) noted two occasions when winter territories became breeding territories. Short-eared owls tend to congregate and roost communally in the winter (Banfield 1947, Craighead and Craighead 1956, Clark 1975), often in sheltered sites near hunting areas. Winter roosts have been reported in abandoned dumps, quarries, gravel pits, storage yards, stump piles, old fields, small evergreen groves, bayberry thickets, dunes, and open, abandoned cellars (Clark 1975, Bosakowski 1986). May also roost directly on the ground in tall grasses, possibly choosing vegetation of a coloration that blends with their plumage (Craighead and Craighead 1956).

In winter the ground roosting habit may be abandoned for trees, possibly in response to deep snow (Banfield 1947, Bosakowski 1986). Smith (1989) noted drastic decreases in numbers of short- eared owls at known winter roosts on Point Peninsula in Jefferson County, New York, after a heavy snowfall created deep cover.

Somewhat gregarious in winter; groups may gather where prey is abundant (NGS 1983, Tate 1992). Breeding density in different areas 0.6-6 pairs per sq km. May defend feeding territory in winter (where prey is sufficient). Reported average home range size: 15-200 ha. In coastal Massachusetts, 10 territories averaged 64 ha (48-126 ha) (Holt 1992). In Manitoba, mean size of five territories was 73.9 hectares (Clark 1975).

Local abundance varies with vole abundance. In the winter, short-eared owls congregate at sites that provide good foraging (Craighead and Craighead 1956). Congregations of up to 200 birds have been reported (Bent 1938). Assemblage sites usually provide shelter and are within, or adjacent to, hunting areas (Clark 1975). In wintering areas in New York where vole densities were high, Clark (1975) saw owls establish and defend hunting territories. Territories were less distinct when vole numbers were low.

DISEASES, PARASITES, AND PREDATION: Disease is presently not known to limit populations. Harrison (1943) reported an owl infected with Mycobacterium tuberculosis avium In 1987, four young owls from two widely separated nests on Nantucket Island were discovered to be suffering from a feather disorder of unknown cause (Tate 1991), in which the juvenal plumage was not developing properly and the emerging feathers were twisted or malformed. All of these young had developed open sores, apparently from picking at the skin around these feather shafts with their beaks. This problem is not known to have been reported previously in the literature and no cause was determined. Avian predation is known from: great horned owl (Bubo virginianus), snowy owl (Nyctea scandiaca), peregrine falcon (Falco peregrinus), and marsh harrier (Circus aeruginosus) (Clark 1975). Northern harrier, American crow (Corvus brachyrhynchos), and European kestrel (Falco tinnunculus) have been known to steal prey from short-eared owls (Village 1987, Tate 1991). As a ground-nesting species, eggs and young are vulnerable to various mammalian ground predators such as foxes, raccoons, and mustelids, populations of which have been augmented because of human-caused increases in food resources. Predation by the striped skunk may be one of the reasons for the extirpation of the short-eared owl as a breeding bird on Martha's Vineyard (Melvin et al. 1989). Domestic cats and dogs have been known to disturb owl nests on Nantucket Island (Tate 1991). The potential for an increase in the threat of predation or disturbance by domestic or feral cats and dogs may be high.

See Johnsgard (1988) for egg dates (timing of nesting varies with latitude and prey abundance). Often only the oldest chicks survive.

COURTSHIP AND TERRITORIAL DEFENSE: Wing-clapping, exaggerated or deep wing-beats, and skirmishing are three displays seen predominantly during the breeding season (Lockie 1955, Clark 1975). Short-eared owls wing-clap along territorial boundaries or during flights within a territory, in aggressive displays to other birds or to human observers, and in courtship flight. When wing-clapping, the owl's wings are brought below the body and clapped together in short, rapid bursts. Both males and females may wing-clap. The courtship flight is unique and involves song, a spiraling flight, and wing-clapping by the male (DuBois 1924, Mikkola 1983, Holt 1985). Exaggerated wing-beats also occur in the same contexts as wing-claps. In this behavior the owl brings its wings high over its body, prominently displaying the underwing. Owls often patrol territorial boundaries using exaggerated wing-beats. Skirmishing involves other neighboring owls, usually along territorial boundaries, and is aggressive and territorial in nature. Exposure of talons, hovering, and sometimes actual striking of the other bird is involved. Any of these displays observed during the breeding season may signify a territorial bird. Observation and mapping of these behaviors over a nesting season is the best way to delineate an owl's breeding territory (Lockie 1955, Village 1987, Tate 1991).

Generally begin courtship in mid- to late March on Nantucket Island along the coast of Massachusetts (Holt and Melvin 1986; Tate and Melvin 1987, 1988). Courtship has been reported as occurring in mid-March in Montana (Dubois 1924) and as early as late February in Jefferson County, New York (G. Smith, pers. comm.). Pitelka et al. (1955) reported initial courtship activity in the first week of June at Barrow, Alaska. Unpaired males may engage in courtship flights well into the breeding season (Clark 1975; G. Tate, pers. obs.). The breeding season is often reported to commence in direct relation to vole abundance with a larger prey population yielding an earlier start to breeding activities (Randall 1925, Snyder and Hope 1938, Lockie 1955, Mikkola 1983).

NESTING: Depending on latitude, nesting activities generally begin in late winter to early spring across the owl's distribution. Timing of nesting may be correlated with latitude and prey abundance (Mikkola 1983, Cramp 1985). The nesting cycle from nest initiation to fledging of young takes approximately seven to nine weeks in temperate zones lasting from mid-March to mid-September in the Northeast Region. During a four-year study of breeding ecology on Nantucket Island, egg-laying began in April each year (as early as the first week) and all young were fledged by the first week of September (Holt and Melvin 1986; Tate and Melvin 1987, 1988; Combs and Melvin 1989). Late nests or renests accounted for young fledging in late August and September (Tate and Melvin 1987, 1988). Polygyny may result in two nests within one short-eared owl territory. On Nantucket, two broods from different females that overlapped temporally were raised within a territory defended by a single male (Tate 1991).

Unlike most owls that nest in holes or take over the abandoned nests of crows or other birds, the short-eared owl is unique within its family (Strigidae) in building a nest, albeit a crude one, on the ground. The female makes a small scrape in the ground with her body and lines it with nearby material. Nests may be lined with grass, leaves, twigs or feathers (Bent 1938, Clark 1975). These small nest depressions do not last long after the young have dispersed from the site (G. Tate, pers. obs.).

Generally between four and nine eggs are laid, and sometimes more (Bent 1938), although Mikkola (1983) reported a range of two to 13 from 121 European records. Murray (1976) reported a mean clutch size of 5.61 from 186 nests in North America. A trend for mean clutch size to increase from south to north was also noted in this sample. The largest clutch ever reported in the literature is 16 from Finland (Mikkola 1983). Large clutches of 14 in Scotland (Adair 1982) and 13 from Finland (Mikkola and Sulkara 1969) have also been reported. All exceptionally large clutches were laid in years of peak vole abundance in these areas.

Clark (1975) reported a mean clutch of 8.6 from five clutches in 1969 in Manitoba, Canada. Pitelka et al. (1955) reported a range in clutch size of four to eight with a mean of 6.3 from 22 nests in Alaska. A four-year study of nesting owls on Nantucket reported clutch sizes of 5.8 (n = 6), 7.7 (n = 9), 6.8 (n = 8), and 5.2 (n = 8) in 1985-88 respectively, with an inclusive range of four to nine (Holt and Melvin 1986; Tate and Melvin 1987, 1988; Combs and Melvin 1989).

Two broods are sometimes raised and, if the nest is destroyed or depredated, the female may renest (Lockie 1955, Mikkola 1983). Pitelka et al. (1955) saw no evidence of renesting by short-eared owls in Alaska; this was apparently tied to the shorter season. In 1986 and 1987, single late nests with eggs were found on Nantucket in mid-July (G. Tate, unpubl. data). These were suspected to be either second broods or renests.

Witherby et al. (1938) reported an incubation period of 24-28 days in temperate zones. With data from six eggs in four nests, Pitelka et al. (1955) reported an incubation period for Barrow, Alaska, that ranges from 26-37 days (mean = 30). He saw no evidence that incubation takes longer there than at lower latitudes. From a Finnish study of four nests, Gronlund and Mikkola (1969) reported an incubation period of 24-29 days (mean = 25.7). In 1986, three eggs, each from separate nests on Nantucket, were documented as having 29-, 30-, and 31-day incubation periods (mean = 30) (Tate 1991).

Normally the female does all of the incubation (Witherby et al. 1938, Dement'ev et al. 1951, Pitelka et al. 1955, Clark 1975) and lays at approximately 24-hour intervals (Mikkola 1983). She begins incubation with the first egg laid and hatching is therefore asynchronous. According to Mikkola (1983), the first and last eggs laid take the same length of time to incubate. Young owls leave the nest before fledging and wander into the surrounding area at about two weeks of age (Lockie 1955, Clark 1975). The young owlets stay concealed but continue to wander and are found and fed by both parents by means of the food-begging call given by the young. Fledging has been reported variously at 24-27 days (Witherby et al. 1938) and 31-36 days (Urner 1923). On Nantucket, young owls dispersed from the nest at 14-17 days and fledged when about 30 days old (Holt and Melvin 1986).

Age of first breeding is reported as one year or less (Mebs 1966, cited by Mikkola 1983; Glutz von Blotzheim and Bauer 1980). Field evidence of breeding at one year has been obtained on Nantucket in 1990, when a sitting female that had been banded as a nestling in 1989 was trapped. This female was brooding on a nest only 98 m from her natal nest site (K.P. Combs unpubl. data). These owls have been known to live as long as 12.5 years (Mebs 1966, cited by Mikkola 1983).

Short-eared owls usually offer little defense of the nest from human intruders. Wing-clapping, circling overhead with deep wing beats, "barks" or "yaps," and broken-wing acts are employed when any defense is attempted. Adults perform a distraction display that is a dramatic broken-wing act accompanied by vocalization. It is most often used by the male when an observer is at, or near, a nest or dispersed young (Clark 1975; G. Tate, pers. obs.). However, often both owls vacate the vicinity of the nest site while an intruder is present. At times the female may desert the area, retreating to another part of the breeding territory, while the male remains nearby. Females may return to the nest by flying low and remaining inconspicuous (G. Tate, pers. obs.).

While short-eared owls have been observed diving at house cats (G. Tate, pers. obs.), the best defense is their cryptic coloration and the fact that the female sits tightly on the nest. On Nantucket, some females remained on the nest while observers passed within two meters, and on Tuckernuck Island, a female on a nest would not budge in spite of repeated attempts from as close as one meter to flush her (G. Tate, pers. obs.). These behaviors make it extremely difficult to find nests.