RESEARCH

SHORT-EARED OWL (Asio flammeus)

SPECIES PROFILE


Description:
The Short-eared Owl (Asio flammeus) is a light-brown bird, heavily streaked below, and with a prominent facial disk.  The upper breast is darker than the lower underparts.  It is a medium-sized owl, averaging 38 cm long.  As the name implies, the ear feathers are typically inconspicuous.  

Males average 315 - 350 grams, while females are approximately 20% heavier, at 378 - 411 grams.  However, there is a lot of variation in weight, with some females weighing as much as 550 grams; perhaps this is related to geographic and/or subspecific variation.  Males tend to be paler than females, but identifying sex by appearance can be problematic.  Measurements of body dimensions, in combination with weights, can yield more consistent results.

Juveniles are a dark sooty brown on the back, and plain buff below.

Similar species:
Among North American species, the Long-eared Owl is most often confused with the Short-eared Owl.  The Short-eared is paler overall, especially below, and feature prominent black wrists and wing tips contrasting with the overall light-coloured wings.


Short-eared Owl in winter.
Photo 2002 www.arttoday.com


Habitat:
The Short-eared Owl is generally found in open country, often grasslands, tundra, or wetlands.  For nesting, areas with tall vegetation are favoured to provide concealment from predators.  Wintering grounds are typically selected for their combination of abundant prey and shelter from the elements - often groves of conifers bordering fields.

Range:
In North America, the Short-eared Owl ranges from Alaska to Newfoundland, and as far south as California in the west, and Virginia in the east.  In most provinces it is primarily a summer resident, while in many central and eastern states it is usually found only in winter.  In some western states, as well as southern Ontario, Short-eared Owls can be found throughout the year, but it is not clear whether these are in fact non-migratory populations.

The Short-eared Owl also is resident on Cuba and Hispaniola, as well as parts of South America.  It breeds across much of Eurasia, with northerly populations generally thought to be migratory.

Behaviour:
One of the most notable characteristics of a Short-eared Owl is its somewhat erratic mothlike flight.  On breeding grounds, they often land right on the ground, but on migration and in the winter, they can be found roosting in trees, sometimes communally.  Unlike most owls, the Short-eared adopts a relative horizontal posture when perched.

Vocalizations:
The Short-eared Owl is relatively quiet, vocalizing much less frequently than many woodland owls.  The regular call is a low hoot, repeated in sets of up to 20 notes at a time.  It is heard most often during courtship, and often given by the male while giving its display flight.  The species also produces a variety of barking, squawking, and hissing notes, especially at or near the nest.  Short-eared Owls also engage in wing-clapping, especially during courtship displays.

Diet:
Mammals are the primary prey of the Short-eared Owl, typically accounting for 80-99% of their diet.  Voles in particular are highly favoured.  Of the remaining prey, most are birds, mostly marsh or grassland species, including a variety of shorebirds and songbirds.

The typical hunting method is to fly low over the ground, often less than 2 metres up, and usually heading into the wind, breaking up their flight with occasional bouts of hovering, often just before pouncing down on their quarry.  Hunting activity often peaks within the last few hours of daylight, but can occur at any time of day or night.

Life history:
The Short-eared Owl becomes sexually mature within its first year of life, and one-year-old females have been recorded breeding on a number of occasions.  They are generally thought to be monogamous within a single breeding season, though some suspect that polygyny occurs.  There appears to be relatively little nest site fidelity from year to year, and it may be that pair bonds also do not last beyond the breeding season.

Courtship displays begin in late winter - February or March depending on latitude.  As they reclaim their nesting territories, they commence flight displays.  These involve frequent loud wing-clapping, as well as elaborate aerobatic display flights by the males.  These usually see the males rising in circles, hovering for periods, and ultimately diving erratically toward the ground.  Later in courtship, males provide females with food prior to mating.

Information on nesting is limited, but it appears that the Short-eared Owl makes some effort to build a nest by digging a small scrape in the ground and lining it with available vegetation.  

In the north part of the Short-eared Owl's range, eggs are laid beginning in early June; further south they may start as early as late March, though May is more typical.  As many as 16 eggs are laid, but a range of 5 - 7 is most common.  Eggs are an average of 39 mm long and 31 mm wide, weighing just under 20 grams.  Incubation commences with the first egg, and lasts 24 - 29 days.  It is thought that the female is largely or entirely responsible for incubation.

The young weigh approximately 15 grams at hatching, but increase their weight tenfold within just a week and a half.  By just over two weeks of age, they are mobile, and sometimes wandering a fair distance from the nest.  The actual age of fledging ranges from 24 - 36 days.

Conservation issues:
Reproductive success tends to be low, often due to predation.  Agricultural practices can pose a serious threat, both through direct disturbance / destruction of nests, as well as by reducing cover around the nest and making it more conspicuous to predators.  Inclement weather and lack of prey can also severely limit reproduction.

Analysis of the annual Breeding Bird Surveys in North America from 1966 to 1993 revealed a population decline of over 3% per year.  The northeast population appears to be particularly troubled.  As the Short-eared Owl has a short lifespan (longevity record of under 4.5 years), such steady declines indicate a serious threat to the survival of the population.

Recommended references:
Clark, R.J.  1975.  A field study of the Short-eared Owl, Asio flammeus (Pontopiddan) in North America.  Wildlife Monographs 47: 1-67.

COSEWIC.  2008.  COSEWIC assessment and update status report on the Short-eared Owl Asio flammeus in Canada.  Committee on the Status of Endangered Wildlife in Canada.  Ottawa.  vi _ 24 pp.  www.sararegistry.gc.ca/status/status_e.cfm

Dechant, J.A., M.L. Sondreal, D.H. Johnson, L.D. Igl, C.M. Goldade, M.P. Nenneman, and B.R. Euliss.  2003.  Effects of management practices on grassland birds: Short-eared Owl.  (Version 12DEC2003).  Northern Prairie Wildlife Research Center, Jamestown, ND.  http://www.npwrc.usgs.gov/resource/literatr/grasbird/seow/seow.htm.

Herkert, J.R., S.A. Simpson, R.L. Westemeier, T.L. Esker and J.W. Walk. 1999. Response of Northern Harriers and Short-eared Owls to grassland management in Illinois. Journal of Wildlife Management 63: 517-523.

Holt, D.W. 1992. Notes on Short-eared Owl (Asio flammeus) nest sites, reproduction and territory sizes in coastal Massachusetts. Canadian Field-Naturalist 106: 352-356.

Holt, D.W. 1993. Breeding season diet of Short-eared Owls from Massachusetts. Wilson Bulletin 105: 490-496.

Johnsgard, P.A.  2002.  North American Owls.  Second edition.  Smithsonian Institution Press, Washington DC, USA, pp 213-220.

USFWS.  2001.  Short-eared Owl habitat model.  United States Fish and Wildlife Service.  Available at http://www.fws.gov/r5gomp/gom/habitatstudy/metadata/short-eared_owl_model.htm.

Wiggins, D. 2004. Short-eared owl (Asio flammeus): A technical conservation assessment.  USDA Forest Service, Rocky Mountain Region. Available at: http://www.fs.fed.us/r2/projects/scp/assessments/shortearedowl.pdf.

Wiggins, D.A., D.W. Holt and S.M. Leasure.  2006. Short-eared Owl (Asio flammeus). The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Laboratory of Ornithology; The Birds of North American Online database: http://bna.birds.cornell.edu/BNA/account/Short-eared_Owl/.

See also the Links section for a variety of additional online references.

2002- The Migration Research Foundation Inc.