Owl Movements in Fall and Winter

In general, owl movements in fall and winter are caused by lack of food such as Meadow Voles.  Recently, some owls have found less to eat as Least and Eastern Chipmunks and Jumping Mice have gone into hibernation. Owl populations increase when vole and other small mammal numbers are high.  When prey numbers crash (high and low cycles), the owls move or starve to death or both happen.  The above comments apply to the northern forest owls such as Great Gray, Boreal and Northern Hawk Owls.  Not all these species eat exactly the same prey, for example, Boreal Owls eat voles, but may be able to survive better in poor vole years because they also prey on Northern Flying Squirrels.  Interestingly, Boreal Owls are sometimes seen around bird feeders in central Ontario.  These Boreals catch Short-tailed Shrews that are eating the spilled seeds buried in the snow under the feeder.  Southward movements of northern Great Horned Owl are caused by crashes in Snowshoe Hare populations.  Snowy Owls move south when lemming populations crash on the tundra.  The Northern Saw-whet Owl is a regular migrant, but in years of high Deer Mouse and Red-backed Vole numbers, many Saw-whet Owls apparently stay on territory all winter in Algonquin Park where the snow cover is deep.

Cold and Deep Snow: It is a myth that northern owls are forced south  because of cold weather and deep snow cover, but you'll hear this repeated all the time.  Some owls such as the Eastern Screech Owl and Long-eared Owl are greatly affected by extremely cold weather and deep snow cover, but these conditions generally do not affect the true northern forest owls.  The life cycles of northern forest owls are adapted to cold weather and deep snow.  Most years we see the boreal forest deep in snow and if prey populations are high, the owls stay on territory.

Snow Crusts: Sometimes crusts form on top of deep snow following a thaw or rain in mid-winter.  Do crusts prevent northern forest owls from catching prey?  Crusts may actually help owls find prey!  A metre or yard below the snow surface at ground level, the temperature is above freezing; bacterial decomposition of dead leaves and plants goes on all winter.  This decay releases carbon dioxide gas which is trapped under the crust.  The buildup of carbon dioxide gas agitates the voles and causes other behavioural changes, forcing voles to the surface and out through openings into the talons of waiting owls.

Northern forest owls, cold temperatures, deep snow, crusts and voles are  part of the boreal forest ecosystem in winter. Great Gray, Boreal and Northern Hawk Owls are adapted to cold and snow.  Extreme cold, deep snow and crusts do not force northern owls south, but lack of food does.

By permission of Ron Pittaway,  Ontario Field Ornithologists  November 18, 2000

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