Food Habits

Main Foods Taken

Small mammals; less frequently birds. Strong positive correlations between vole abundance and owl abundance in Scotland (Village 1987) and Finland (Korpimaki and Norrdahl 1991).

Food Capture And Consumption

Hunts day and night, probably dictated by hunger, prey density, and nestlings’ hunger. Winter birds, however, may be essentially crepuscular (Clark 1975). Erkinaro (1973) reported that diel activity of Short-eared Owls coincided with activity periods of prey. Extent of diurnal foraging may be over emphasized, facultative, or due to ease in which this owl is observed in open habitats.

Hunts primarily on the wing, quartering area approximately 0.3 to 3.0 m above ground (Clark 1975, Village 1987); also by hovering 2–30 m above ground (Clark 1975) or, less frequently, perched on poles or hills. Courses area like Northern Harriers (Circus cyaeneus) do and quickly adjusts flight to drop down on prey (Fig. 2). Often hovers during descent to make position adjustments. If windy, may hover or hang in wind; hunts less often from perch. Foraging technique probably dictated by weather conditions. Of 628 foraging attempts, 70.1% unsuccessful, 20.7% successful, and 8.1% unknown; coursing and hovering hunting techniques about equally effective for a single male owl (Clark 1975). Marr and McWhirter (1982) reported differential hunting success for (what they believed) was a family group of 2 adults and 1 immature: adults 70.6% (12/17), 43.8% (7/16) successful foraging attempts; immature 15.4% (2/13).

Consumes small mammals by swallowing whole or by clipping off head and eviscerating by pulling out trachea and esophagus region. Clips wings off most small birds, then swallows body whole. Clips wings off all medium to large avian prey. Techniques for eating terns as follows: nestlings 1–3–d–old swallowed whole; 4–10–d–old sometimes decapitated then swallowed; 11 d to fledging (also adults) found on backs with head clipped off (less frequently wings, tail and legs; Holt 1994). Primary feeding area is breast, pieces of keel often torn out. Does not pluck avian prey (DWH).

Adaptations Which May Aid In Hunting

Uses hearing, vision, feather, and flight adaptations for foraging. Ear openings vertically asymmetrical (Kuroda 1967, Feduccia and Ferree 1978), but size and shape equal or nearly so (Voous 1988). Left ear opening higher than right (Martin 1990), but opposite also reported Voous (1988). Slight asymmetry of squamosal wing on skull (Kuroda 1967). Prey detected primarily by acoustical cues (Rice 1982), with highest sensitivity at about 6 kHz (Voous 1988). Asymmetrical ear openings allow owls to localize sound (prey) in horizontal and vertical planes simultaneously and with same accuracy (Norberg 1987).

Laboratory experiments report better success with increasing illumination, suggesting vision also aids in prey detection (Clarke 1983). Apparently sees well by day, but no red droplets or other histological characteristics of diurnal birds found in photoreceptor cells of retina (Gallego et al. 1975). Not known to use olfactory cues in hunting, but area of brain served by olfactory nerve is well-developed and comparable to avian species that have shown olfactory discrimination (Cobb 1960, Martin 1990). Wing-loading relatively low: 0.33 g/cm2 (Clark 1975), 0.43 g/cm2 (Voous 1988). Low wing-loading allows for slow flight speed, aerial agility, reduced aerodynamic flight noise, ability to hear while in flight, and reduced detection by prey (Norberg 1987).

Major Food Items

Determined by pellet analysis and remains of prey. Diet varies little throughout range, with apparently little difference in food eaten seasonally or by sex or age of individuals. Small mammals, particularly Microtus, are dominant prey throughout most of North America (Craighead and Craighead 1956, Clark 1975, Colvin and Spaulding 1983, Holt 1993a, b). Other species include: shrews (Sorex, Blarina), moles (Scapanus), rabbits (Lepus, Sylvilagus), and pocket gophers (Thomomys). Rodent families include: Heteromyidae (pocket mice, kangaroo rats), Cricetidae (harvest mice, deer mice, voles, lemmings), Muridae (rats, house mice), Zapodidae (jumping mice). Uncommon and rarely eaten mammals: big brown bat (Eptesicus fuscus; Holt 1993a), norway rat (Rattus norvegicus; Johnston 1956), and short-tailed weasel (Mustela erminea; Walley 1982); also “blond” color morph of meadow vole (Microtus pennsylvanicus; Holt 1990), and adult Muskrat (Ondatra zibethicus; Wiebe 1991).

Many birds eaten, but in low numbers; more in coastal areas than at inland sites (Tomkins 1936, Johnston 1956, Fisler 1960, Holt 1993a, b). Page and Whitacre (1975) report 51.7% and 88.0% birds in diet from two nonbreeding seasons in coastal California. In Massachusetts, the owls kill and eat adult and nestling Common Terns (Sterna hirundo), Laughing Gulls (Larus atricilla), shorebirds, and passerines (Holt 1993a). In Wisconsin, Kumlien (1899) collected over 600 bird feathers, representing at least 32 prey species from one Short-eared Owl nest; he found no mammalian prey remains. In the Galapagos Is., seabirds form the major part of Short-eared Owl diet (Abs et al. 1965, Harris 1969, Grant et al. 1975, De Groot 1983). In Kauai (Hawaiian Is.), this owl a key predator of the endangered Hawaiian Thrush (Puaiohi; Myadestes palmeri), mostly fledglings (Snetsinger et al. 2005).

Other avian prey include families of Hydrobatidae (storm-petrels), Rallidae (rails), Charadriidae (plovers), Scolopacidae (sandpipers and allies), and many species within the order Passeriformes. Uncommon and rarely eaten avian prey include Wilson’s Storm-Petrel (Oceanites oceanicus), Leach’s Storm-Petrel (Oceanodroma leucorhoa; Holt 1987), White Tern (Gygis alba; Schulmeister 1980), and Clapper Rail (Rallus longirostris; Johnston 1956).

For dietary studies in Palearctic region see Glutz von Blotzheim and Bauer (1980) and Cramp (1985). For dietary review throughout much of Northern Hemisphere see Mikkola (1983), Voous (1988). Rau et al. (1992) is the only South American study, but see also Fulk (1976).

Quantitative Analysis

Pellet analysis reliable reflection of actual diet and best method for quantifying small mammalian prey. In a review of Short-eared Owl feeding ecology in the Nearctic, Holt (1993b) reported > 79.0% mammals in the diet from 9 studies with > 500 prey items each (20,416 prey total). Of these mammals, meadow voles dominated in all but two sites, with > 78.0% of total diet (see Table 1). Trophic diversity (number of prey) and dietary evenness (distribution of prey in the diet) of small mammalian prey for Short-eared Owls is narrow, although many species have been recorded (Holt 1993b). Regression analysis of certain cranial measurements give more accurate estimate of prey biomass and age class than standard mean or maximum prey biomass estimates that may introduce inaccuracies (Blem et al. 1993; see nutrition and energetics, below).

From Graber (1962), the standard metabolic rate of one captive female Short-eared Owl kept in a winter aviary yielded a food value equivalent of 35 kcal/bird/d, or 87 kcal/kg/d. This owl needed 41 g of food to maintain its weight for 24 h—the equivalent of 164 kcal/d. When accounting for caloric loss due to excretion and pellet deposition, the metabolized energy yielded a value of 87.0% to 99.0%, or 142 kcal/d (350 kcal/kg) of the total caloric intake. Four other wild Short-eared Owls consumed 229 kcal/d (about 53 g), but lost 13 kcal in pellet ejection, equaling 216 kcal/d as gross energy intake. These values equal about 1–2 adult-sized voles/d.

In Europe, Mikkola (1982) reported 105 g average daily food intake for captive Short-eared Owls—28% of the owls’ body mass, but in Finland Korpimaki and Norrdahl (1991) estimated 50 and 40 g of food/d for adult and young, respectively. Blem et al. (1993) used cranial measurements from prey contents in Short-eared Owl pellets and a snap trap sample of local prey to determine body mass of prey captured by these owls during a vole high in w. Montana. They reported mean mass of Meadow Voles and Montane Voles (M. montanus) eaten by the owls to be 30.2 ± 0.6 g, and 28.9 ± 0.6 g, respectively (about 3/4 adult vole weight). These data show that the owls were capturing sub-adult voles and standard biomass estimates of adult weights may be misleading. Short-eared Owls probably eat 1 or 2 voles/d; adult voles of the above species weigh about 40–50 g.

A captive female Short-eared Owl yielded a standard metabolic rate in terms of oxygen consumption of 7,521 ml/d. Respiration rate ranged from 24 to 28 beats/min, with peak oxygen consumption occuring between 22:00 and 02:00 h (Graber 1962). Graber predicted that metabolized energy of wild Short-eared Owls from Feb to Mar, with ambient temperature 2° to 6°C, would yield a value of 188 kcal/bird/d. Ability of this owl to withstand cold not known.

Defecation data not known. Uric acid described as buffy with stringy black centers (Clark 1975, Holt et al. 1987).

Approximately one pellet per meal; interval between pellets directly related to meal weight (Duke et al. 1976). Individuals cast pellets 8.5 h (range 1.5 to 13) after eating 30 to 40 g prey (Chitty 1938). A captive Short-eared Owl studied by Duke et al. (1976) cast pellets 10.2 h after feeding. It also ate more food than expected, based on its body mass. Its MPI (meal pellet interval)/g and MPI/g/kg were low, given its body mass. These data indicate a more rapid meal digestion and pellet ejection. Duke concluded that Short-eared Owls may have a higher metabolic rate than other owls.

Techniques for distinguishing Short-eared Owl pellets from those of sympatric Northern Harriers described by Clark (1972) and Holt et al. (1987). Short-eared pellets range in length (27.8–119.0, mean 49.4 mm) and width (17.4–30.8, mean 22.3 mm) n =180; elongated, with bone content generally near the surface and almost 50% of pellet mass (mean 44.0 g); about 1.2 and 1.7 prey/pellet. Northern Harrier pellets shorter (range = 22.6–82.2, mean 36.9 mm), round in diameter (range = 18.2–43.2, mean 23.5); bone content often wrapped within pellet; about 18 g of bone weight per pellet; generally < 1 prey/pellet (Holt et al. 1987).

Not experimentally measured, although dietary information suggests selection of small mammals, usually 1 or 2 species of vole. Habitat is usually occupied by few prey species, so little selection may actually occur.

Food storage or caching infrequently reported (Young et al. 1988, Ingram 1962), but this behavior may be common. Seen during all seasons and adults feeding flightless (dispersed) young known to stock-pile up to five prey items (DWH). Caching may reduce kleptoparasitism from sympatric harrier species; provide a buffer during food shortages due to weather; or increase foraging time during prey activity periods (DWH). Lack of a crop in owls may also contribute to selection for this behavior.