by Larry Powell
| The Great Gray Owl (Strix nebulosa). Photo by Drsarahgrace, public domain.|
A new study in Manitoba shows how the “Great Gray Owl,” a common site, either soaring over the plains and perching and nesting in the forests of the eastern Canadian prairies, overcomes many obstacles to find its prey.
|The prairie vole (Microtus ochrogaster) - Photo by Soebe, public domain|
But the snow presents the owl with other problems way before the “moment of capture,” too. Not only does it hide its prey from site, forcing the bird to rely on its hearing only, it deadens, or attenuates any sound the vole is making, and even "bends" or refracts it, creating an “acoustic mirage,” or false impression of its location. (See above.) The denser the snow, the more pronounced is both the attenuation and refraction.
The owl soars towards its prey from its perch (above), then hovers as directly over it as it can until it reaches a “listening position” of least refraction and attenuation - defeating that "acoustic mirage" in the process. Then it plummets straight down on its target, forming a “plunge-hole” in the snow.The owl is superbly adapted for this. While it has no ear tufts, it has the largest “facial disc” of any owl. That's where its ring of feathers filters and amplifies sound at its ears. (See above) This also allows it to pick up low-frequency sound, the kind that transmits best through snow. And its wing feathers are formed in such a way as to allow it to fly and hover more quietly than just about any bird, anywhere.
There are other features that make the “Great Gray” unique, too.
It is the largest owl in North America, with a wing span of well over a metre. It can be found across the province, year-round. And, since Manitobans “adopted” it in 1987, it’s been our official, provincial bird, too!
A three-member team, two from the US, along with James Duncan from “Discover Owls” in Balmoral, Manitoba, used loudspeakers and special cameras in their research.
The above images were extracted, with thanks, from the team's official study, just published in the proceedings of The Royal Society.
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