Short-eared owl (Asio flammeus) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

Habitat conversion/degradation has been correlated with population declines of Short-eared Owls in the Fraser River delta of British Columbia (Campbell et al. 1990) and has been implicated in declines across the Prairie Provinces (e.g., Smith 1996) and southern Ontario (Hunt 2004). Habitat conversion has probably been negligible in central and northern Canada. Telfer (1992) reported a 39% decline in native pastureland between 1946 and 1986 in the Prairie Provinces. Samson and Knopf (1994) reported dramatic losses of native grasslands in Alberta (61% of mixed grass prairie), Saskatchewan (81% of mixed grass prairie and 85% of shortgrass prairie), and Manitoba (99% of both tallgrass and mixed grass prairie), as well as further south along the western and central Great Plains. Thus, the extensive loss of native grasslands throughout the central portions of the range have likely had a significant negative impact on Short-eared Owl abundance and population viability.

Widespread and intensive livestock grazing occurs over much of the remaining grasslands on the Canadian prairies and the U.S. Great Plains (Samson and Knopf 1994). Excessive livestock grazing is a direct threat to Short-eared Owl habitat, as relatively tall grasslands are typically preferred nesting areas (e.g., Fondell and Ball 2004). While programs (e.g., Permanent Cover Program in Canada, Conservation Reserve Program in the U.S.A.) aimed at reverting grasslands and agricultural fields to wildlife habitat have no doubt acted to increase the amount of foraging and roosting habitat available to Short-eared Owls, it is unclear how such programs may have affected breeding success.

In areas where Short-eared Owls breed amid crop fields, mowing and harvesting of hay and grains can be a significant source of egg and nestling mortality (e.g., Arroyo and Bretagnolle 1999). Although probably not a significant long-term factor in the species’ decline, collisions with vehicles, barbed-wire fences, and utility lines are known to contribute to (largely winter) mortality of adults (Fitzer 1975).

Although elevated levels of DDE and heptachlor epoxide have been found in Short-eared Owl eggs (Peakall and Kemp 1980, Henny et al. 1984), no negative effects on owl reproduction have been detected (Cadman and Page 1994). The effects of toxic chemicals on Short-eared Owl prey populations has not been studied. As noted by Cadman and Page (1994), more data on pesticide effects are needed.

Although the extent to which West Nile virus is currently affecting Short-eared Owls is unknown, they are known to contract the virus (Fitzgerald et al. 2003).