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 virginianus
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 (Phodilus
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 arenamedfor 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, longeared,
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-andpounce
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
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 species-
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
fromabove; 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
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 provide
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.
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
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