Natural Selection In Snowy Owls

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Any of the Strigiformes, a group of birds with highly specialized characteristics for nocturnal activity, including soft feathers and enhanced hearing and eyesight.

Principal Terms asymmetrical ears: in some species of owls, the ears are of unequal size and located unequally on the sides of the head asynchronous: an uneven event; in hawks and owls, the staggered hatching of eggs that results in a nest of different-aged young facial disk: the distinctive concentric circles of feathers that encircle the eyes of owls, helping direct sound toward the ears owl pellets: compacted packets of undigested prey that is regurgitated. Owl pellets may be used to determine food habits owls: any of the strigiformes, a group of birds with highly specialized characteristics for nocturnal activity, including soft feathers and enhanced hearing and eyesight nest box programs: construction and placement of nest boxes in suitable habitat to provide nesting platforms for specific birds of prey pair bond: close relationship between a male and female for breeding purposes

With their rounded facial disk encircling their large, forward-looking eyes, owls are the most recognizable of birds. Their unique traits also include superb auditory abilities and soft feathers for silent flight. Sharp talons for catching and killing prey and powerful bills for tearing flesh complete their basic characteristics. Most owls are colored in drab shades of brown, buff, and gray, either spotted or streaked, which helps conceal them during daylight hours. Woodland owls tend to be darker, while those of open country are lighter and paler. Thus, the eastern North American race of great horned owl (Bubo virgin-ianus virginianus) is much darker than the pale northern race (B. v. lagophonus) of interior Alaska and the Yukon. Afew smaller owls have rounded, eyelike disks on the back of their head to deter predators. Although once thought to be the nocturnal kin of hawks and eagles, owls are actually most closely related to the frogmouths and nightjars. The similarities between hawks and owls result from the evolutionary convergence of morphological features that facilitate their roles as avian hunters of live animals.

The 205 species of owls are a widespread and successful group that occupies virtually all habitats on all continents, from tundra to tropics, and are even found on most oceanic islands. They range in size from the forty-gram sparrow-sized elf owl (Micrathene whitneyi) of the Southwest desert to the formidable eagle owl (Bubo bubo) of Eurasia, which may reach 0.6 meter in length and weigh forty kilograms.

All the owls are placed in a single order, the Strigiformes, in which two owl families are recognized. The family Tytonidae includes sixteen species of barn (Tyto sp.), grass, and bay owls (Pho-dilus sp.), defined by small, dark eyes set in a narrow skull with a heart-shaped facial disk. The other 189 species are loosely grouped in the family

Strigidae, all of which have rounded skulls and large, wide-set eyes in a concentric facial disk. Owls of both families are named for their plumage colors or patterns (the tawny owl, Strix aluco, the black-and-white owl, Strix nigrolineata, the spectacled owl, Pulsatrix perspicillata, and the spotted owl, Strix occidentalis), habitats (the barn owl, Tyto alba, and jungle owllet, Glaucidium radiatum), size (great gray owl, Strix nebulosa, and little owl, Athene noctua), power and strength (eagle owls, Bubo sp.), presence of ear tufts (great horned, long-eared, Asio otus, and short-eared owls, Asio flammeus), or for their distinctive songs (screech owls, Otus spp., saw-whet owls, Aegolius sp., and barking owl, Ninox connivens).

Throughout history, owls have been alternately revered and feared. To the ancient Greeks, the solemn owl was the bird of wisdom and a companion of their warrior goddess, Athena. The Romans attached more ominous signs and portents to the ghostly cries of owls in the night. During the Middle Ages, owls were thought to be the companions of witches and the harbingers of evil and death. Many Native American tribes placed owls on a higher footing. The Arikara Plains Indians had secret owl societies, in which initiates were adorned with facial masks of owl feathers, while the Pimi Indians believed spirits of departed warriors assumed the shape of owls. Thanks to enlightened conservation efforts, owls are at long last recognized as important, interesting, and beneficial birds and all are protected by law.

Hunting and Food

All owls are predators, hunting a variety of animals commensurate with their size and strength. Woodland owls mostly hunt by the perch-and-pounce method, but hawk owls (Surnia ulula), short-eared owls, and other open country species may forage, harrier-like, over fields and meadows in search of prey. Burrowing owls (Speotyto cunicularia) are more terrestrial than most, spending a good deal of time running across the ground pursuing insects and small mammals. Most owls have broad wings—shorter in woodland species that maneuver in vertically complex habitats, longer and more hawklike in species that hunt open

Pygmy Owl Catching Prey
The owl's soft, fluffy feathers allow it to fly in complete silence through the night. (Corbis)

country or are migratory. Bird-chasing owls, such as pygmy owls (Glaucidium sp.) and brown owls (Ninox sp.) have longer wings and tails for agile flight.

Owls use their combination of large eyes, superb hearing, and silent flight to hunt and catch prey. Their relatively large wings and small bodies give owls a low wing loading which, combined with soft, fluffy feathers, enables the quiet flight that makes owls such efficient nocturnal hunters. The large eyes of owls are densely packed with light-gathering rods for seeing in very low light, while the overlapping fields of their binocular vision enable precise parallax judgment of distance to prey. Head bobbing movements seen in many owls also help estimate distance and angle to prey.

If light is absent or nearly so, owls can continue to hunt, substituting ears for eyes. Studies by ornithologist Roger Payne have shown that barn owls, for example, can locate prey in total darkness entirely by sound. When an owl hears rustlings of prey, it turns its head toward the sound, using the facial disk of flattened feathers to direct and amplify faint sounds toward the ears set on either side of the wide, flat face to pinpoint the location of the source. The asymmetrical ears of some spe-cies—the right ear is larger and higher on the skull than the left—permits determination of vertical and horizontal direction to the sound. If the sound reaches the higher right ear first, then the source is from above; if it reaches both ears at the same time, the source lies straight ahead. By turning its head, the owl can determine the precise distance and angle of the flight path to the prey to within 1.5 degrees. When flying toward prey, the head is forward of the body to detect prey movements and make minor adjustments. Once within striking range, the owl extends its legs and spreads the talons in a wide oval to snare prey. Most prey are killed by the powerful, slashing talons, but larger animals may be dispatched by a bite to the back of the neck.

Owls tend to be opportunistic in their food habits, hunting a wide variety of mammals, birds, and other vertebrates. Rabbits, rats, and mice are staples of many of the large and medium-sized

Owl Facts

Classification:

Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Aves Order: Strigiformes

Families: Tytonidae (barn owls); Strigidae (owls), with subfamilies Buboninae (true owls, eighteen genera, eighty-three species), Striginae (long-eared and dish-eyed owls, six genera, twenty-six species) Geographical location: All continents except

Antarctica

Habitat: Virtually all habitats Gestational period: Around thirty days, depending on species Life span: Depending on species, three to ten years

Special anatomy: Rounded facial disk that directs sounds toward the ears; soft feathers for soundless flight; sharp talons and beak; large eyes with densely packed rods to enhance night vision; ability to turn the head up to 270 degrees; some species have asymmetrically spaced ears to assist in locating prey by sound owls, but birds, lizards, snakes, insects, fish, and occasionally carrion are also consumed when available. The smallest owls, such as the elf owl, tropical screech owls, and boobook owls (Ninox boobook) are mostly insectivorous, while the larger and more powerful owls take hares, rabbits, and other medium-sized mammals and birds. The Eurasian eagle owl is a champion hunter, fully capable of killing chamois and foxes. With their long, bare shanks, the fishing owls (Ketupa sp.) of Africa and Southeast Asia spear fish from woodland streams or wade in the shallows to search for frogs, crabs, and crayfish.

Although some owls have a reputation for taking chickens and other poultry, most owls help keep injurious rodent populations in check. This awareness has led to the establishment of barn owl nest box programs in some inner cities to pro vide nest sites for an urban owl population to control rodents.

Small prey is swallowed whole but larger prey is broken into pieces. Prey is digested to a semiliquid consistency and then passed on to the intestinal tract. The undigested remains, mostly fur, feathers, bones, teeth, and other indigestible substances, are compacted into a small ball which is regurgitated in a reflexive, choking motion that casts the pellet out. The process takes several hours, so that today's pellet contents represent yesterday's meal. Owl biologists collect and analyze the pellets to determine food habits and impact of owl hunting on different species.

Owl Seasons

Tropical owls may breed at any time of year, but temperate owls commence breeding activity in late winter or early spring. Males claim a territory with territorial and courtship songs and postures. Songs of almost all owls consist of a series of hoots or wailing cries that echo ghostly through the night skies. Many owl species pair for life. Courtship may involve alternate duetting, billing and cooing, and mutual preening. Males of many species present food offerings, both as a courtship gift and to display their hunting ability.

Following courtship the female selects a suitable nest site. Although snowy owls (Nyctea scandiaca) of the Arctic select a spot beneath a clump of tundra sedge, most owls choose secluded tree hollows, cavities in cliffs, rock outcrops, or caves. Some species may also appropriate hawk or squirrel nests. Nest improvement usually consists of scraping a shallow hole or lining the nest with a few breast feathers. The oval or nearly oval eggs are usually laid at two-day intervals, but incubation begins with the first egg, resulting in a nest of different-aged young. The female incubates the eggs and the male brings food to the female or to a delivery site near the nest. The male may replace the female on the nest for short periods while she hunts, but his role in incubation is unclear.

Recently hatched young are typically fed pieces of food by the female. Later the adults simply deposit food at the nest and allow the young to pick and tear at it. Nest defense is weak or nonexistent during the early stages of nesting, but intensifies when young are in the nest. Defense varies from alarm calls and vigorous bill clacking in smaller species to aggressive and determined attacks by great horned owls and other large species. The female is usually most active in nest defense.

The fledged young typically remain in company of the adults for a few weeks before dispersing in search of new territories, usually in late summer or early fall. This is the most dangerous period of their lives, as they must perfect their hunting skills while avoiding enemies.

Longevity varies greatly; great horned owls and other large species may live ten or more years but the life span of smaller owls is usually only a few years. Other than humans, owls have few enemies. Larger owls prey on smaller owls while ravens and crows steal an occasional egg. Humans continue to be the main threat to owl populations. About thirty-five species are currently listed as threatened or endangered. Pollutants, disturbance, and collisions with vehicles and structures all take their toll of owls, but habitat loss factors most heavily. Several island races have disappeared following habitat alteration or introduction of exotics, and island populations continue to be at risk.

—Dwight G. Smith See also: Beaks and bills; Birds; Claws, nails, and hooves; Domestication; Feathers; Flight; Hearing; Molting and shedding; Nesting; Predation; Respiration in birds; Vision; Wildlife management; Wings.

Bibliography

Bent, Arthur C. Ow/s.Part2in Life Histories of North American Birds of Prey. 1938. Reprint. New York: Dover, 1961. Bent exhaustively summarized all of the available reports of ecology, behavior, and food of North American owls. Still an important reference.

Burton, James A., ed. Owls of the World: Their Evolution, Structure, and Ecology. Rev. ed. London: Peter Lowe, 1992. An entertaining but still very informative book written for a more general audience. Contains excellent summaries of the natural history of the world's owls.

Clark, Richard J., Dwight G. Smith, and Leon Kelso. Working Bibliography of Owls of the World. Washington, D.C.: National Wildlife Federation, 1978. All the books, papers, articles, and news about owls ever published (to 1978) is presented in this bibliography, which also includes summaries of owl taxonomy and distributional status across the globe.

Del Hoyo, Josep, Andrew Elliott, and Jordi Sargatal, eds. Barn-Owls to Hummingbirds. Vol. 5 in Handbook of the Birds of the World. Barcelona, Spain: Lynx Edicions, 1999. A stunningly illustrated tour de force of all of the world's owls. Photos and plates make this massive, oversized volume a must-see for anyone even remotely interested in owls. Lengthy and learned introductory chapters precede the section on barn and bay owls of the world and another equally impressive chapter introduces the typical owls. What makes this volume exceptional is that each species account is by noted field researchers, most of whom have actually conducted research on the particular species.

Johnsgard, Paul A. North American Owls: Biology and Natural History. Washington, D.C.: Smithsonian Institution Press, 1988. Detailed accounts of all species of North American owls written in an authoritative and entertaining manner. Natural history and ecology is emphasized throughout.

Konig, Claus, Friedhelm Weick, and Jan-Hendrik Becking. Owls: A Guide to the Owls of the World. New Haven, Conn.: Yale University Press, 1999. An authoritative treatment of all of the world's owls. The book includes an overview describing why owls are really owls and detailed accounts of every recognized owl species. Each account succinctly describes identification, vocalizations, distribution, habitat, measurements and weight, habits, food, and breeding, status and conservation, and lists recent references. There are also chapters on evolution of owls, how to study owls, and owl conservation. Excellent color plates of owls are a highlight of the book.

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