Tuesday, September 24, 2013

The Night Shift

Great Horned Owl with rabbit (c) John Ashley
Adult Great Horned Owl with a rabbit for a midnight snack (click to enlarge)
I read somewhere that owls are 80% feathers and 100% attitude. I think that's still an underestimate of the attitude displayed by Great Horned Owls (Bubo virginianus).

As of midnight we now have one less rabbit in the yard, compliments of a hungry Great Horned Owl. One evening several summers back, our company here at the end of the road watched a Great Horned Owl pluck a sleeping duck off a floating log at dusk. American Coots and striped skunks are both favored dinner fare for these, our largest owls, who will take on most anything their size and smaller.

Many years ago, we hiked in to find out why a Bald Eagle nest near Waterton Lake, on the Glacier Park side, had failed after the lone chick was almost one month old. We found the young but large eagle carcass on the ground below its nest, decapitated, apparently by a Great Horned Owl. They've been known to take other large raptors as well, like Osprey, Peregrine Falcons, and even other owls.

(On a side note, we used a cheap, white Styrofoam cooler to smuggle the Bald Eagle carcass across the border into Canada, and then from Canada back into the U.S., without any of the necessary permits. If any one of the Border Patrol agents had bothered to look in the cooler, I would have had another interesting story to reminisce about.)

Great Horned Owl primary feather adaptations (c) John Ashley
Folded primary feathers showing leading
edge (left) and trailing edge adaptations
Another long-ago spring and summer, I conducted owl surveys one night each week along the Camas Road on the west side of Glacier Park. The protocol at the time was to play a cassette tape of recorded owl calls, one species at a time, and record the locations of any owl responses. But you had to start with the smallest owl species and finish with the largest, otherwise all of the other owls would turn silent after hearing a Great Horned call.

Most birds make a fair amount of noise in flight, including day-hunting owls. But nocturnal owls like the Great Horned have several feather adaptations to soften and disperse the noisy turbulence that occurs during flight.

If you look closely at the folded flight feathers on this owl's wings (left), you'll notice two adaptations. First, the leading edge has a row stiff hairs sticking up that look like a short comb. These mostly help with stability, but they also offer some sound dampening effect. Second, the trailing edges of these feathers are flexible and tattered, and this is the area most responsible for the owl's noise reduction.

The third adaptation is mostly noticeable if you get the chance to stand up close, like at a rehab center. The small, body and leg feathers are mostly velvety-soft down and contour feathers. This fuzziness further reduces the amount of noise created when air passes over the owl in flight.

Great Horned Owl talons (c) John AshleyGreat Horned Owls also have a talon adaptation for feeding that I once felt on the feet of a deceased owl. The inner side of the middle talon has a sharp, knife-like edge. Holding its prey with one foot, the owl uses its hooked, upped mandible to pull the prey against this sharp edge, helping it tear the meal into bite-sized chunks.

No doubt last night's rabbit snack was eaten this way. And soon, somewhere in the nearby woods, coughed-up pellets of rabbit fur will fall around the base of another tree. The pellets might eventually add to the duff layer under the tree, and help grow a little bit of pine grass to be nibbled on by a hidden rabbit.