Sunday, January 27, 2008

Life After Dark

Photograph by David Powell

“Whoo-hoo-o-o, whoo-hoo-o-o, who” reverberated from the snow-topped tree, only to be answered a few moments later by a higher-pitched, more elaborate response from the nearby forest. One of the birds flew to the top of a utility pole, where he continued his elaborate duet with the female of his choice. Apparently she returned his affection, as the hooting continued throughout much of the night. Among the earliest of nesters, Great Horned Owls may already be incubating this year’s clutch of eggs. Widespread throughout North America, and occurring locally south to Tierra del Fuego, South America, this highly adaptable species might be considered the supreme generalist among birds. The word variable personifies the species as a whole; as a result the population is reported to be robust without a need for special management in scientific literature.

Great Horns have many advantages that contribute to their overall success. As one of the largest owls in North America (the other is the Snowy Owl of the Arctic tundra), adults have few predators and are long-lived. Some studies have reported non-captive owls 21-28 years old. Monogamous pairs remain on territory year-round, and aggressively defend it from other owls. They do; however, often nest in close proximity to other diurnal (daytime hunting) raptors such as hawks and eagles. In 2003, we documented a Great Horned Owl nest and a Red-tailed Hawk nest on opposite sides of the same promontory in northwestern New Mexico, a total distance between nests of about 25'. Our raptor biologist speculated that only one nest would succeed, and wondered which it would be. In the end, only the owl nest was successful, fledging two young, while the hawk nest failed. There was no evidence of predation by the owls on the other nest, and we speculated the more visible hawk nest might have failed due to human disturbance while the more camouflaged, cryptically colored owls may have escaped detection. The pair successfully nested at the same spot in 2004 and 2005, but the hawk paired move on to another location. Other studies report owl nests in close proximity to other species such as Golden Eagle, Harris Hawk, and in one instance a Great Horned Owl and a Turkey Vulture nested in different cavities in the same tree. There is an apparent benefit to the nesting birds due to additional protection afforded by two separate species, one active during the day and the other at night. At the same time, there is little competition for food.

The prey taken by the Great Horned Owl varies according the occupied habitat. For instance, desert dwelling owls consume a greater number of reptiles than birds living near ponds and marshes that take waterfowl and muskrats. Documented prey includes everything from earthworms, crickets, and scorpions to mice, rabbits, and even skunks. Free-ranging house cats often fall victim to the talons of this owl. Other species of birds are routinely hunted by some owls. Indeed, I once located an owl nest in the cavity of a cottonwood tree after finding the headless body of an adult Red-tailed Hawk at the base of the tree.

Several remarkable biological adaptations make all owls supremely adapted to their inconspicuous nightlife. Their feathers are very soft, with comblike fringes along the leading edge of each feather, and a fuzzy surface that muffles sounds, enabling them to fly almost silently. They have large, forward facing eyes with binocular vision like us. But, unlike humans, they are unable to move them in their eye-sockets. They compensate for this limitation by bobbing their heads up and down and from side to side. Possessing more vertebrae in their necks than humans, they also can rotate their heads about 270 degrees!

The feathered tufts atop their heads, often called ears, are used for communication between owls and to break up the owl’s silhouette when it is roosting during the day. Their true ears are asymmetrically located on their heads within the feathered facial disk that helps direct sound into the ear canal. Some owls, such as the Barn Owl, can hunt in nearly total darkness. Owls use a combination of hearing and vision to locate prey. Cryptic coloration serves this species particularly well, and the birds are often indistinguishable from the tree in which they are sleeping, unless a mobbing actions of a flock of crows, ravens, or jays draws attention to them. Even within New Mexico, the plumages of Great Horned Owls vary by habitat with soft browns and tans characterizing the lowland desert dwellers setting them apart from the charcoal gray and dark brown of those that live in the coniferous forest.

Truly a remarkable species, survivors in a tough world, these owls adapt well to human activities, and often nest close to our dwellings. One might then wonder then, why it is that the bird chooses to nest in the dead of winter when little food is available and keeping eggs warm might present a greater problem. Owls do not build their own nests, but instead occupy nests constructed by other birds or cavities in cliffs or trees. Because they remain on territory year-round, they have ample opportunity to select the best nest site within the area without interference from the original nest builder who may be spending the winter further south. In the Middle Rio Grande bosque, they often choose the largest Cooper’s Hawk nest of the previous year! Their offspring leave the nest about May 1, the time when most of the migrant songbirds are returning to their nesting territories. Fed by their owl parents for another 30-45 days, the young owlets begin learning to hunt and catch their first live food at about the time that young, vulnerable songbirds are just hatching and leaving their nests. It makes perfect sense, from an owl’s point of view!

No comments: