Short-eared owl (Asio flammeus) COSEWIC assessment and status report: chapter 6

Biology

Reproduction

Short-eared Owls are primarily summer residents in Canada, particularly in arctic regions. South of arctic regions, Short-eared Owls settle on breeding areas from March to May, with egg laying occurring from late April through early June. In the Churchill region, Jehl (2004) notes early June as the period when nests are initiated. There is no information available on which sex chooses the site, but only the female builds the nest (Mikkola 1983), which typically is a simple scrape in the ground, lined by grasses and a few feathers. In wet areas, the nest is often on a small rise or knoll. One egg is laid every 1-2 days, and clutch size averages 7 eggs (range 3-11: Wiggins 2004). Clutch size is positively correlated with local food abundance (Clark 1975). Only a single brood is raised, but a replacement clutch is laid if the first clutch is lost.

Incubation starts once the first egg is laid, which leads to strong hatching asynchrony. Females perform all of the incubation and are fed extensively by their mates during the laying and incubation periods (Lockie 1955, Clark 1975). The incubation period is approximately 27 days, but has been reported to last up to 37 days, possibly depending on local food abundance and the amount of time females spend off the nest hunting (Lockie 1955, Clark 1975). Females brood the nestlings during the early nestling stage and do all the actual feeding of the young. During this period, males provide the majority of the food consumed by the brooding female and nestlings. Once they reach the relatively early age of 14-17 days, and before they can fly, nestling Short-eared Owls typically begin dispersing short distances from the nest, hiding in nearby vegetation.

Survival

Pre-fledging survival of nestlings is relatively high for a ground-nesting species, with most studies (4 of 7) reporting over 50% fledging success (Wiggins 2004). In southern Manitoba, Clark (1974) found 86% hatching success and 46% fledging success. There is no information available on juvenile mortality post-fledging, or on adult survival rates.

Movements/dispersal

In most areas of their North American range, Short-eared Owls are nomadic, settling in areas with plentiful prey (e.g., Poulin et al. 2001). Exceptions occur in temperate coastal areas and on islands, where the strong dependence on small mammal populations is tempered by a broadened prey base that includes birds.

In most other areas of the Canadian range, some migration occurs during spring and fall, but it isn’t clear to what extent individuals may overwinter at coastal breeding sites. In the Yukon, Short-eared Owls are summer residents, with migration typically peaking in late April and from late August to October (Sinclair et al. 2003, C. Eckert, pers. comm.). Recovery of owls banded in British Columbia suggests that many individuals move south to winter along the Pacific coast, although some individuals remained over the winter (Clark 1975). Owls breeding in the Prairie Provinces are thought to move southward in winter, wintering primarily on the U.S. Great Plains (Clark 1975).

Although the same breeding areas may be used from year to year, it isn’t known whether the same individuals are involved. There is some degree of natal philopatry on islands, as Holt (1992) found an owl breeding within 5 km of her natal site on an island off coastal Massachusetts.

Interspecific interactions

In many areas of North America, Short-eared Owls are found in habitats used by Northern Harriers (Circus cyaneus) and on a few occasions harriers have been observed chasing owls and causing them to drop prey (Clark and Ward 1974). Nonetheless, Clark and Ward (1974) concluded that there was little competition between the two species in Pennsylvania, presumably due to minimal overlap in the timing of foraging. The extent to which competition may be more important at more northerly latitudes (e.g., Canada), where there is greater overlap in foraging schedules, remains unknown.

Predation on eggs and nestlings is likely the most significant source of reproductive failure in Short-eared Owls (Lockie 1955, Pitelka et al. 1955). Mammalian predators include foxes, skunks, and feral cats and dogs, while avian predators include Great Horned Owls (Bubo virginianus), Snowy Owls (B. scandiaca), Red-tailed Hawks (Buteo jamaicensis), Rough-legged Hawks (B. lagopus), Northern Harriers, Northern Goshawks (Accipiter gentilis), Peregrine Falcons (Falco peregrinus), Herring Gulls (Larus argentatus), Stercorarius jaegers (I. McDonald, pers. comm.), and Common Ravens (Corvus corax). As nest predation may be more common in fragmented habitats (Johnson and Temple 1986), habitat degradation and fragmentation is often cited as a significant factor in the decline of Short-eared Owls in Canada (e.g., Campbell et al. 1990, Cadman and Page 1994).

Behaviour/adaptability

Disturbance

In general, Short-eared Owls prefer to nest in habitats (e.g., arctic tundra, wetland fringes, extensive grasslands) that are not prone to extensive human disturbance. They are apparently sensitive to human disturbance during the laying or incubation stages, as females typically desert the nest if disturbed during this period (Leasure and Holt 1991). However, in some areas they will nest in a mosaic of grassland, abandoned fields, and row crops in close proximity to agricultural activities such as haying, mowing, and livestock grazing.

Food/foraging

Although their diet is often comprised largely of Microtus voles, Short-eared Owls feed on a variety of small mammals including shrews (Blarina and Sorex spp.), pocket gophers (Thomomys), mice (Peromyscus), kangaroo rats (Dipodomys), and lemmings (Holt 1993). Holt (1993) summarized diet studies from across North America and found that small mammals typically make up over 75% of the diet. In coastal areas, their diet is more varied and more often includes small birds. Foraging is particularly intensive at dusk and dawn, presumably when prey species are most active.

The species’ ability to cue in on local food sources makes them relatively flexible in breeding and wintering habitat choice, with birds (often temporarily) settling into previously unused areas that support large small mammal populations.

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