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Erie, Colo., homeowners inherited this owl box and the female owl who raised her babies there. (Photo by Sarah Hill)

Winter is the perfect time to mount an owl box

Boulder County’s owl population ranges from the tiny 6-inch flammulated owl (birds are measured from the tip of the beak to the tail) to the 2-foot-tall great horned owl. In between are owls of all sizes: screech, burrowing, northern pygmy, short eared, long eared, barn, boreal, northern saw-whet and possibly snowy owls.

Great horned owl in a tree hollow. (Photo courtesy Steve Frye)

Although our local owls vary in size and habitat, they all have one thing in common: homelessness. “An owl’s talons and beak aren’t built for digging out trees,” says Steve Frye, owner of Boulder’s Wild Bird Company. “Owls don’t have the equipment to make nests.”

Instead, these crafty raptors take over others’ nests, or nest in tree crevices or burrows. Great horned owls nest in hollows and ledges, and are adept at stealing raven and osprey nests. “They’re so fierce and large, they just take over what they want, so they have an advantage over other owls,” Frye says.

So much so, in fact, that one owl could use some help from humans. “The barn owl has suffered because of the presence of great horned owls, who prey on them,” Frye explains. “Putting up an owl box would help that population.”

Barn owl boxes are easy to build, and instructions can be found online at www.scvas.org/pdf/cbrp/BuildingBarnOwlBoxes.pdf. Other owls also nest in boxes, particularly screech owls, which need nearby access to water. Barn owls get all the water they need from their prey, and like to hunt in open fields and meadows typical in the eastern county. “Lots of folks there have inactive silos that would be the perfect place to mount a barn owl box,” Frye notes. True to their name, barn owls also nest in barns, and mounting a box inside would keep the rodent population to a minimum, as barn owls eat mice, voles and gophers. A male must catch about 25 mice a night to feed himself, his mate and four owlets.


Barn owlets ready for a mice meal. (Photo by Scott Rashid/Colorado Avian Research and Rehabilitation Institute)

When it comes to a box, barn owls prefer two chambers inside so they can stash owlets in the back chamber, away from any predators that may come to the entrance, and a front chamber “that’s more like a porch where the adults can just kind of hang,” Frye says.

He recommends using wood that isn’t pressure treated, and because barn owl boxes aren’t small, plywood would be easier and less costly to work with than dimensional lumber. Mount the box out of prevailing winds and hot afternoon sun if you place it in a tree or on a pole. The entrance hole should be perch-less and large enough to allow the adults to enter, but keep out raccoons and other predators. Holes for drainage and ventilation are needed, and a nice layer of wood shavings makes for a cozy nest, since owls don’t gather nesting materials.

A bark-covered owl box made from a hollowed-out log. (Photo courtesy Steve Frye)

Winter is the perfect time to mount an owl box, as courtship starts in January and parents scout for nest sites beforehand. They use the nest for about four months, through egg laying, incubation, rearing and fledging. And if she raises a successful brood, the female often returns to the same nest, which is why you should clean it out after the owls leave.

“Owls are very popular with folks just because they’re so fascinating,” Frye says. “Having one in your yard would be wonderful.”

—by Carol Brock