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Arb Notes

February 20, 2008 at 11:44 am
By Arb Naturalists

Last Friday afternoon, the student naturalists set off with Carleton staff member Gene Bauer in pursuit of some of the arb’s most elusive predators: owls. Birds of prey that are typically nocturnal and solitary hunters, owls are notoriously hard to find. Although we were stymied in our quest for the real thing, we did find plenty of signs that owls are indeed frequenting the arb. Owls advertise their roosting trees by leaving plenty of white “splash” on the branches and the trunk, as well as regurgitated pellets at the base of the tree. These roosting sites can be found in the mixed forests and denser pine plantations of the lower arb. And if you are lucky, you may even spot owl tracks: the feather swoop marks left on the snow where a careless mouse met its demise.


Owls are impressive hunters that are masterfully adapted for night hunting. Their eyes are extremely large and elongated, and are fixed in their head, which means owls must turn their head to change their view. Although they are relatively nearsighted at close range, owls have exceptionally acute vision at longer ranges. Their eyes are estimated to be three to five times better than our own. It addition to impressive vision, owls also possess extremely good ears. The feathers of the facial disc channel sound waves to the ear drums, which are asymmetrically placed on either side of the eyes. This uneven placement, along with the fact that each ear is tuned to hear noise at a slightly different frequency, allows owls to precisely locate their prey through triangulation. The leading edge of owl flight feathers are finely serrated, an adaptation that allows them to fly all but silently. Finally, having used eyes, ear and silent flight to locate and sneak up on their prey, owls finish the job with their talons and strong beak.
The arb may be home to up to nine species of owls, depending on the time of year. Of these nine, there are three species which are most commonly seen. These include the Barred Owl, Strix varia (look for black eyes, a yellow beak, and vertical barring on the chest), the Great Horned Owl, Bubo virginianus (a very large owl with ear tufts and yellow eyes) and the Eastern Screech Owl, Otus asio (a smallish owl with vertical streaks on its breast, ear tufts and yellow eyes, that can often be observed peering out from a tree cavity).

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