Night descends and a motionless bump on a tree limb comes suddenly to life. Moving silently across the landscape on large, soft wings, a great horned owl begins its nighttime hunt.
Owls. Something about these birds, with their haunting calls and knowing eyes, fascinates us. Maybe it’s the upright posture, round face and large, forward-looking eyes, so like our own, that intrigue us. Or perhaps the owl’s familiarity with the night, a world we retreat from, makes owls seem mysterious and otherworldly.
We tend to think of all owls as large-eyed night birds sitting about by the treefull asking the question, Who? But there is great variety among owls. Of the 18 species of owls in North America, 13 have been recorded in Colorado. The large, widespread resident owl, the great horned, lives throughout the state in nearly every habitat, including our densely populated cities. The larger snowy owl visits Colorado occasionally. Our smallest owl is the rare and reclusive flammulated owl, only six to seven inches in length. The long-eared owl looks like a slender version of the great horned. When threatened it raises its ear tufts and draws its body up to be very long and narrow. 
The sparrow-sized pygmy owl is tiny but fierce, hunting by day for birds and swooping on them in flight like a falcon. With it’s ear tufts, the screech-owl looks like a small great horned. This common owl often lives in city parks and neighborhoods, where its call is heard at night – less a screech than a tremulous hoo-oo-oo-o-o-o. The familiar barn owl has a unique, heart-shaped face; biologists don’t know why its face is so different from other owls. The Mexican spotted owl, a subspecies related to the northern spotted owl, nests in coniferous trees in the bottoms of deep canyons in southwestern and south central Colorado.
Most owls are stay-at-home birds, but a few migrate. Burrowing owls nest in Colorado and spend the winter in the southwestern U.S. and northern Mexico. Snowy owls nest on the artic tundra and seem to migrate cyclically, possibly in response to fluctuating populations of artic lemmings, a primary food. Short-eared owls also move around in response to prey availability.
Some owls have “horns,” some don’t. These ear tufts, found on owls like the great horned, long-eared and screech, aren’t ears at all, but tufts of feathers with no function in hearing. They may alert other owls or communicate mood – horns laid back while relaxed and roosting, up when alert or sensing danger.
Great horned owls are probably the most easily seen owl for wildlife watchers, by virtue of their size, activity, wide distribution, and tolerance for humans. They become active at dusk, materializing suddenly on a tree limb or other perch. An exceptionally early nester, the great horned owl moves into the old nest of a hawk or some other large bird in late January or early February. Fed by her mate, the female faithfully incubates her eggs even as the snow falls on her back. Great horned owls are attentive parents, feeding their downy, wide-eyed owlets for 60 to 70 days before the babies are ready to leave the nest. (If you are fortunate enough to discover and watch an owl nest, be very careful not to approach too closely or frighten the adults. Owlets are very subject to predation by mammals and birds, including other owls.)
Many owls nest in tree cavities, but not all owls nest and roost in trees. The burrowing owl is a grassland bird, nesting in prairie dog burrows, and hanging out during the day atop burrow mounds or nearby fenceposts. Snowy owls lay their eggs in grass-lined depressions on the artic tundra. Barn owls often take up residence in barns, attics and abandoned buildings.
All owls are hunters, but their diets differ. Great horned owls have been nicknamed “winged tigers” for their fierceness and the sheer numbers of prey animals they kill. Though rabbits are their preferred dish, these three-and-a-half pound birds will kill ducks, geese, house cats, even porcupines and skunks (with relatively little sense of smell, they are apparently impervious to skunk perfume). A study of barn owl pellets estimated the birds each consume an average of 2000 rats and mice a year. That’s five to six rodents a night, no mean feat of rodent control.
Smaller owls, like the pygmy owl, hunt birds. Burrowing owls consume a great many grasshoppers and ground beetles. Screech-owls are known for their indiscriminate palates, eating anything and everything—all kinds of insects, spiders lizards, shrews, bats, mice, and birds. The tiny flammulated owl feeds largely on insects.
Owls fill niches occupied during the day by hawks. Sometimes direct parallels can be seen between species using the same hunting methods, habitat, and prey. Screech-owls replace kestrels; short-eared owls take over for harriers; the great horned replaces the red-tailed hawk and the barn owl the rough-legged hawk. Though these two groups feed on the same food source, they avoid direct competition by working different “shifts.”