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Short-eared Owl

Limiting Factors

Factors that limit the population of Short-eared Owls include those that reduce either reproductive successor survival. Unfortunately, controlled studies have not been conducted to delineate critical conservation needs of Short-eared Owls. Some of the limiting factors for the Short-eared Owl include Habitat Loss and Degradation, Food Abundance, and Pesticides.

Habitat Loss and Degradation:   Loss and degradation of habitat through agriculture, grazing, recreation, or development has been implicated in Short-eared Owl declines in some areas. In central California, where numbers are said to be greatly down, Short-eared Owls nest primarily in marshland that is being lost to agriculture and urbanization. Similarly, the urbanization of coastal grasslands in Massachusetts has contributed to considerable declines. As with any ground nesting bird, the removal of dense ground cover renders nesting Short-eared Owls vulnerable to mammalian predators. 

Food Abundance:  More than any single factor, reproduction and survival of Short-eared Owls is clearly tied to small mammal abundance. In the Canadian prairies, the Meadow Vole appears to be the predictive resource. Meadow Vole populations are characterized by two- to five-year cyclic fluctuations in density. The amplitude of such cycles is influenced by many factors such as climate, food quality, predators, and physiological stress.

The mechanisms and geographic extent of vole peaks are not well documented. Scientists noted high vole densities and large numbers of Short-eared Owl nests in Saskatchewan, when winter had come early and snow had covered the unharvested swathes of grain in this largely agricultural landscape. Presumably, this condition provided good forage and subnivean shelter, allowing voles to reproduce throughout the winter. 

Pesticides:  Raptors that eat herbivorous mammals usually accumulate low levels of organochlorine residues. Although Short-eared Owls have not been extensively studied for pesticide contamination, detrimental concentrations have not been documented in North America.

Reprinted from Alberta Wildlife Status Report No. 28 (2000), with permission from Alberta Sustainable Resource Development.

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