Feathers and Flight
Birds have up to five feather types:
1. Contour feathers cover the body, wing (remiges) and tail (rectrices).
2. Down feathers - these soft and fluffy feathers trap air and create a layer of
insulation
next to the bird's body.
3. Semiplumes function to fill in between contour and down feathers.
4. Bristles are small feathers with a stiff shaft and barbs only on the base, or
often not at all. Bristles occur most commonly around the base of the bill,
around the eyes, and as eyelashes.
5. Filoplumes are hairlike feathers that consist of a very fine shaft with a few
short barbs at the end. They are typically covered by other feathers, and may
function as pressure and vibration receptors - they sense the location of other
feathers so they can be adjusted properly.
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An Owl has very few down feathers, but has downy barbules on the parts of the
contour feathers closest to the skin.
Many of the Owl's feathers are specially designed - around the face there are the stiff
facial disc feathers or ruff, crown feathers, ear-flap feathers and also bristles around
the bill.
The feet have feathery tendrils that work somewhat like feelers, to help the Owl react
to things they touch, such as prey.
The most unique adaptation of Owl feathers is the comb-like leading edge of the
primary wing feathers. With a normal bird in flight, air rushes over the surface of the
wing, creating turbulence, which makes a gushing noise. With an Owl's wing, the
comb-like feather edge breaks down the turbulence into little groups called microturbulences. This effectively muffles the sound of the air rushing over the wing
surface and allows the Owl to fly silently.
Silent flight gives Owls the ability to capture prey by stealth, and also allows the Owl
to use its hearing to locate potential prey. This adaptation is not present on some
Owl species that hunt in the daytime.
Pattern and Color
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In general, an Owl's cryptic colors and pattern allow it to blend in with its
surroundings, hiding it from potential danger. This is especially important for the
nocturnal owls, as they need to remain hidden when roosting in the daytime.
When threatened, an owl will often take up a concealing posture, with closed eyes,
raised ear tufts, and compacted feathers.
The ear tufts are actually nothing to do with hearing - they are display feathers, used
to indicate moods, such as fear, anger and excitement. They also help with
camouflage. It is also interesting to note that although owls of the same species look
alike, each individual owl within a species has slightly different markings.
Preening
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All birds frequently clean and groom their feathers in order to remove dust, dirt and
parasites. Owls, like most other birds, use their beak and talons to do this.
Flight feather barbs have tiny barbules that lock the barbs together, making the
feather into a single continuous surface. These barbules often become unhooked
during harsh flying conditions or collisions. A bird will use its beak to realign the
unhooked barbs and restore the feather to peak condition.
There is small gland called the uropygial, located at the base of the tail, that
produces a thin oily liquid. This gland is stimulated by the beak, which is then used to
transfer the liquid to the feathers to provide them with a protective coating.
Molting
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When an Owl hatches, it has no flight feathers, but is covered with downy feathers
that keep it warm. This down is gradually replaced with feathers as the Owl grows.
Juvenile plumage is similar to adult's, but often paler, and sometimes with different
markings.
An adult bird's color is derived from mature feathers. During the normal course of the
bird's life, these feathers suffer from damage caused by abrasion, flexing and even
collisions.
Like other birds, Owls regularly replace their feathers in a process known as molting.
This usually happens once a year, beginning after the parent birds have raised a
brood that has fledged and can care for themselves.
The process takes up to 3 months, during which feathers are shed and re-grown
over the entire body in a regular pattern. In order to minimize the impact of the molt
on the Owl's flight and hunting skills, this molting pattern only allows a few of the
primary or secondary flight feathers to be shed at time.
When birds molt, new feathers grow to replace the ones that have fallen out. The
new feathers immerge from the skin tightly bound in a thin shaft of tissue. These are
called pin feathers. The shaft splits shortly after, allowing the new feather to unfurl
and grow to its full size.
Flight
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Most owls have relatively large, rounded wings. The wings are broad, with a large
surface area relative to the weight of the bird i.e. a low wing loading. This allows
them to fly buoyantly and effortlessly, without too much flapping and loss of energy.
They can glide easily and fly slowly for long periods of time. Many species use this
slow flight to hunt ground-dwelling prey from the air.
Food and Hunting
Food:
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Owls are Birds of Prey, which means that they must kill other animals to survive.
Their diet includes invertebrates (such as insects, spiders, earthworms, snails and
crabs), fish, reptiles, amphibians, birds and small mammals.
The main food largely depends on the species of Owl. For example, Screech Owls
feed on insects mostly, while Barn Owls eat mainly mice, shrews and voles. Larger
Owls such as the Eagle Owl will prey upon hares, young foxes and birds up to the
size of ducks and game birds. Some species have specialized in fishing, such as the
Asian Fish Owls (Ketupa) and African Fishing Owls (Scotopelia). Although certain
species have these preferences for food type, most owls are opportunistic, and will
take whatever prey is available in the area.
Hunting:
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Owls generally have a hunting territory away from their daytime roost.
All Owls are equipped with special adaptations that make them efficient predators.
Keen eyesight allows them locate quarry even on dim nights. Sensitive, directional
hearing helps locate concealed prey. Some species can even hunt in complete
darkness using sound alone to guide them to a successful kill. An Owl's flight is
silenced by special wing feathers, that muffle the sound of the air rushing over the
surface of the wing. This allows an Owl to hunt by stealth, taking their victims by
surprise. It also allows the Owl to listen for prey movements while still flying.
Most species hunt from a perch, such as a low branch, stump or fence post. They
will wait for prey to appear, and swoop down with open wings, and their talons
stretched forward. Some species will fly or glide a little way from the perch before
dropping on the victim. In some cases, the Owl may simply drop on the target,
opening their wings at the last moment.
Other species prefer to soar, or make quartering flights, scanning the ground below
for a suitable meal. When a target is located, the Owl will fly towards it, keeping its
head in line with it until the last moment. This is when the Owl pulls its head back,
and thrusts its feet forward with the talons spread wide - two pointing backwards and
two forwards.The force of the impact is usually enough to stun the prey, which is
then dispatched with a snap of the beak.
Owls can adapt their hunting techniques depending on the type of prey. Insects and
small birds may be snatched in mid air, sometimes after being flushed from the cover
of trees or bushes by the Owl.
Once caught, smaller prey is carried away in the bill, or eaten immediately. Larger
prey is carried off in the talons.
In times of plenty, Owls may store surplus food in a cache. This may be in the nest,
or tree hole, or a fork in a branch.
Digestion
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Like other birds, Owls cannot chew their food - small prey items are swallowed
whole, while larger prey are torn into smaller pieces before being swallowed. Some
Owl species will partially pluck bird and larger mammal prey.
Unlike other birds, Owls have no Crop. A crop is a loose sac in the throat that serves
as storage for food for later consumption. Since an Owl lacks this, food is passed
directly into their digestive system.
Now, a bird's stomach has two parts:
(1) The first part is the glandular stomach or proventriculus, which produces
enzymes, acids, and mucus that begin the process of digestion.
(2)The second part is the muscular stomach, or gizzard. There are no digestive
glands in the gizzard, and in birds of prey, it serves as a filter, holding back
insoluble items such as bones, fur, teeth and feathers (more about this below).
The soluble, or soft parts of the food are ground by muscular contractions, and
allowed to pass through to the rest of the digestive system, which includes the small
and large intestine. The liver and pancreas secrete digestive enzymes into the small
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intestine where the food is absorbed into body. At the end of the digestive tract (after
the large intestine) is the cloaca, a holding area for wastes and products from the
digestive and urinary systems. The cloaca opens to the outside by means of the
vent. It is interesting to note that birds (apart from the Ostrich) do not have a bladder.
The excretion from the vent is largely made up of an acid which is the white part of a
healthy dropping.
Several hours after eating, the indigestible parts (fur, bones, teeth & feathers that are
still in the gizzard) are compressed into a pellet the same shape as the gizzard. This
pellet travels up from the gizzard back to the proventriculus. It will remain there for up
to 10 hours before being regurgitated. Because the stored pellet partially blocks the
Owl's digestive system, new prey cannot be swallowed until the pellet is ejected.
Regurgitation often signifies that an Owl is ready to eat again. When the Owl eats
more than one prey item within several hours, the various remains are consolidated
into one pellet.
The pellet cycle is regular, regurgitating the remains when the digestive system has
finished extracting the nutrition from the food. This is often done at a favorite roost.
When an Owl is about to produce a pellet, it will take on a pained expression - the
eyes are closed, the facial disc narrow, and the bird will be reluctant to fly. At the
moment of expulsion, the neck is stretched up and forward, the beak is opened, and
the pellet simply drops out without any retching or spitting movements.
Owl pellets differ from other birds of prey in that they contain a greater proportion of
food residue. This is because an Owl's digestive juices are less acidic than in other
birds of prey. Also, other raptors tend to pluck their prey to a much larger extent than
Owls.
Ears and Hearing
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Because Owls are generally active at night, they have a highly developed auditory
(hearing) system. The ears are located at the sides of the head, behind the eyes,
and are covered by the feathers of the facial disc. The "Ear Tufts" visible on
some species are not ears at all, but simply display feathers.
The shape of the ear opening (known as the aperture) depends on the species of
Owl - in some species, the opening has a valve, called an operculum covering it .
The opening varies from a small, round aperture to an oblong slit with a large
operculum. All owls of the family Tytonidae have rounded openings with large
opercula, while in Strigidae, the shape of the outer ear is more varied.
An Owl's range of audible sounds is not unlike that of humans, but an Owl's hearing
is much more acute at certain frequencies enabling it to hear even the slightest
movement of their prey in leaves or undergrowth.
Some Owl species have asymmetrically set ear openings (i.e. one ear is higher than
the other) - in particular the strictly nocturnal species, such as the Barn Owl or the
Tengmalm's (Boreal) Owl. These species have a very pronounced facial disc, which
acts like a "radar dish", guiding sounds into the ear openings. The shape of the disc
can be altered at will, using special facial muscles. Also, an Owl's bill is pointed
downward, increasing the surface area over which the sound waves are collected by
the facial disc.
An Owl uses these unique, sensitive ears to locate prey by listening for prey
movements through ground cover such as leaves, foliage, or even snow. When a
noise is heard, the Owl is able to tell its direction because of the minute time
difference in which the sound is perceived in the left and right ear - for example, if the
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sound was to the left of the Owl, the left ear would hear it before the right ear. The
Owl then turns it's head so the sound arrives at both ears simultaneously - then it
knows the prey is right in front of it. Owls can detect a left/right time difference of
about 0.00003 seconds (30 millionths of a second!)
An Owl can also tell if the sound is higher or lower by using the asymmetrical or
uneven Ear openings. In a Barn Owl, the left ear left opening is higher than the right so a sound coming from below the Owl's line of site will reach the right ear first.
The translation of left, right, up and down signals are combined instantly in the Owl's
brain, and create a mental image of the space where the sound source is located.
Studies of Owl brains have revealed that the medulla (the area in the brain
associated with hearing) is much more complex than in other birds. A Barn Owl's
medulla is estimated to have at least 95,000 neurons - three times as many as a
Crow.
Once the Owl has determined the direction of its next victim, it will fly toward it,
keeping its head in line with the direction of the last sound the prey made. If the prey
moves, the Owl is able to make corrections mid flight. When about 60 cm (24") from
the prey, the Owl will bring its feet forward and spread its talons in an oval pattern,
and, just before striking, will thrust it's legs out in front of it's face and often close it's
eyes before the kill.
Talons and Feet
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Many Owl species have feathered feet to protect them from cold weather. The
feathers may also serve to sense contact with prey, and to protect against prey that
might bite when seized.
An Owl's foot has four toes. When in flight, 3 of these toes face forward, and one
backwards. When perched, or clutching prey, the outer front toe on each foot swivels
to face the rear. It is able to do this because of a unique flexible joint.
Owl's talons are very powerful, as they are used to capture prey. The bony structures
in an Owl's feet are shorter and stronger than the equivalent bones in other birds.
This is in order to withstand the force of an impact with prey.
When attacking prey, the talons are spread out wide to increase the chance of a
successful strike.
The actual length, thickness and color of the talons varies greatly with Owl species,
but all have very sharp claws. Color may vary from near-black to pale grey or ivory.
The underside of an Owl's foot is covered with a rough, knobby surface that helps
grip prey or a perch. Barn Owls have a serration on the underside of their middle toe
which may help grip prey and also aids grooming.
As with other birds of prey, Owls have the locking, ratchet-like mechanism in their
foot which keeps the toes locked around a perch or prey without the need for the
muscles to remain contracted.
In some Owl species, it is thought that the feet help to regulate body temperature.
Excess body heat is radiated through the soles of the feet, which are supplied with
extra blood vessels.
Behavior
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Most Owls are active at dusk and dawn, spending the daytime at a quiet,
inconspicuous roost. They generally roost singly or in pairs, but may form flocks
outside of the breeding season. (A group of Owls is called a parliament)
An Owl's daily activity begins with preening, stretching, yawning and combing its
head with its claws. The plumage is often ruffled up, and claws and toes are cleaned
by nibbling with the beak. The Owl will then leave its roost, sometimes giving a call
(especially in breeding season).
Owls have a very expressive body language. Many species will bob and weave their
head, as if curious about something - this is in fact to further improve their threedimensional concept of what they are viewing.
When relaxed, the plumage is loose and fluffy. If an owl becomes alarmed, it will
become slim, its feathers pulled in tightly to the body, and ear-tufts, if any, will stand
straight up. A pygmy Owl will cock its tail and flick it from side to side when excited or
alarmed. Little owls bob their body up and down when alert.
When protecting young or defending itself, an Owl may assume a "threat" or
defensive posture, with feathers ruffled to increase apparent size. The head may be
lowered, and wings spread out and pointing down. Some species become quite
aggressive when nesting, and have been known to attack humans.
Owls will bathe in shallow water, and also in rain.
Calls: Owls have a very wide range of vocalisations, ranging from the hoots so often
associated with Owls, to whistles, screeches, screams, purrs, snorts, chitters and
hisses. Hooting is often territorial, and is also associated with courting, the male
usually having the lower pitched Hoot. It should be noted that not all Owl species
Hoot.
Owls can also make clicking noises with their tongues, often as part of a threat
display. They may also clap their wings in flight as part of a mating display.
Mobbing: Because Owls are predators, they are feared by many birds. For this
reason, they are often attacked or harassed by groups of smaller birds. This is not
limited to one species, as once the attack begins, many different birds will join in.
Interestingly, the Owl rarely responds to the harassment, and it is just as rare for the
Owl to be injured in any way! The mobbing may succeed in forcing the Owl to move
on to a different area. The retreating Owl is often pursued by the mob.
Migration: Owls are generally resident birds. Some Northern populations of certain
species may escape harsh winters by moving south.
Eyes and Vision
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Of all the Owl's features, perhaps the most striking is its eyes. Large and forward
facing, they may account for one to five percent of the Owl's body weight, depending
on species. The forward facing aspect of the eyes that give an Owl its "wise"
appearance, also give it a wide range of "binocular" vision (seeing an object with
both eyes at the same time). This means the owl can see objects in 3 dimensions
(height, width, and depth), and can judge distances in a similar way to humans. The
field of view for an owl is about 110 degrees, with about 70 degrees being binocular
vision.
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By comparison, humans have a field of view that covers 180 degrees, with 140
degrees being binocular. A woodcock has an amazing 360 degree field of view,
because its eyes are on the side of its head. However, less than 10 degrees of this is
binocular.
An Owl's eyes are large in order to improve their efficiency, especially under low light
conditions. In fact, the eyes are so well developed, that they are not eyeballs as
such, but elongated tubes. They are held in place by bony structures in the skull
called Sclerotic rings. For this reason, an Owl cannot "roll" or move its eyes - that
is, it can only look straight ahead!
The Owl more than makes up for this by being able to turn its head around, and
almost upside-down. It is able to achieve this by having a long and very flexible neck,
which is not always apparent, as it is hidden by feathers and the Owl's posture. An
owl's neck has 14 vertebrae, which is twice as many as humans. This allows the owl
to turn its head through a range of 270 degrees - not, as some rumours state, a full
circle.
Cross-section of an Owl's Eye
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As most owls are active at night, their eyes must be very efficient at collecting and
processing light. This starts with a large cornea (the transparent outer coating of the
eye) and pupil (the opening at the centre of the eye). The pupil's size is controlled by
the iris (the colored membrane suspended between the cornea and lens). When the
pupil is larger, more light passes through the lens and onto the large retina (light
sensitive tissue on which the image is formed).
The retina of an owl's eye has an abundance of light-sensitive, rod-shaped cells
appropriately called "rod" cells. Although these cells are very sensitive to light and
movement, they do not react well to color. Cells that do react to color are called
"cone" cells (shaped like a cone), and an Owl's eye possesses few of these, so
most Owls see in limited color or in monochrome.
Since Owls have extraordinary night vision, it is often thought that they are blind in
strong light. This is not true, because their pupils have a wide range of adjustment,
allowing the right amount of light to strike the retina. Some species of Owls can
actually see better than humans in bright light.
To protect their eyes, Owls are equipped with 3 eyelids. They have a normal upper
and lower eyelid, the upper closing when the owl blinks, and the lower closing up
when the Owl is asleep. The third eyelid is called a nictitating membrane, and is
a thin layer of tissue that closes diagonally across the eye, from the inside to the
outside. This cleans and protects the surface of the eye.
Owls - Nesting and Rearing of Young
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For Owls, as with other birds, the period during which nesting and rearing young
occurs is the most important part of the annual cycle. It is the time during which genes are
passed on to the next generation, and considerable effort is expended by the parents in this
respect. In the case of most Owl species, especially those found in temperate or sub-Arctic
regions, breeding occurs during the spring.
However, all the upbringing of their young, and the period immediately following their
fledging, is invariably timed to coincide with the maximum abundance of prey animals. As a
general rule Owls are monogamous, that is to say pairs are comprised of one male and
one female, neither one of which has any involvement with other nesting birds. This is in
contrast to some species of passerine birds where a single male may mate with more than
one female, and take partial responsibility for the upbringing of several broods.
The reverse case, wherein one female mates with several males, is less commonly
observed among birds. With some Owl species the pair bonds last only for the duration of
the breeding season, especially if the species involved is dispersive or migratory. In others,
particularly sedentary species such as the Little Owl, pairs may remain together throughout
the year. Tawny Owl pairs are similarly faithful to one another, their bonds remaining for life.
Owls are territorial, a fact that is particularly evident during the breeding season. They
vigorously defend the nest and a well-defined surrounding feeding territory against members
of the same species and other birds that might conceivably compete for the same
resources.
If the Owls are dispersive, this territoriality lasts only for the duration of the breeding season.
Year-round residents such as Tawny Owls and Eagle Owls will defend their territories
throughout the year, their efforts extending to offspring of the previous year once they are
more than a few months past fledging. Attacks on intruders are invariably uninhibited and
vicious if the intruder stands its ground. Smaller Owl species (potential competitors for food
and nesting sites) are also attacked, and Tawny Owls, for example, will readily kill Long-
eared Owls in their territory. This effectively results in the fact that two species do not
overlap in terms of their precise distributions.
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Diurnal species of Owls will sometimes advertise their territories in a visual manner.
Thus Short-eared Owls perform buoyant-flight and wing-clapping displays to announce their
presence to other birds, both potential mates and intruders. Nocturnal species of Owls
invariably use sound as a means of advertisement. The calls of sedentary species often
involve duets between established pairs, rather than individual birds.
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In the strict sense of the word, Owls do not construct nests in the same way as songbirds
do. Instead they are opportunistic nesters, using ready-made sites or taking over the
abandoned nests of other birds. Owl species that breed in open terrain are often ground
nesters. The Snowy Owl, which favors the Arctic tundra, will use a hollow in the ground
which the female may attempt to scrape out and line with plant material.
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Short-eared Owls often nest in or beside tussocks of grass; similar sites are sometimes
chosen on rare occasions by Long-eared Owls and Tawny Owls, both usually tree-nesting
species. Holes in trees are another preferred site for a wide variety of Owls, and a few
species, notably the Barn Owl, have adopted the man-made equivalent of these sites namely, holes in barns and other outbuildings. The abandoned nests of crows and birds of
prey are also favored by many Owl species, with sometimes little or no attempt to embellish
the previous owner's construction. Lastly, natural rock crevices or ledges are used by a few
species, including that most impressive of Owls, the Eagle Owl.
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Owls lay between one and thirteen eggs, depending on the species and also on the
particular season; for most, however, three or four is the more common number.
The eggs are rounded and white; there is little need for cryptic markings given the
concealed nature of most nest sites, and the vigor with which they are defended.
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Incubation of the eggs usually begins when the first one is laid, and lasts, in most species,
for around thirty days. Because eggs are laid over a period of several days, the hatching is
also staggered. This means that there is always a gradation in the size of the chicks in the
nest, the larger and more active individuals invariably getting more food from the parents
than their smaller, weaker siblings. As a result, it is rare for all the chicks that hatch from a
clutch to survive, except of course when food is plentiful. In most seasons the youngest
chicks starve, or are sometimes even killed by their brothers or sisters.
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This seemingly brutal approach to the rearing of young has in fact positive survival
advantages for the family as a whole: it ensures that, whatever the food availability, some
offspring will always survive and produce further offspring. If all the young were fed equally
there would be a chance that all might starve in years of poor food supply.
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However callous the parent Owls' approach to feeding their offspring may appear, they
cannot be criticized for their courage in defending the nest. Many of the medium-sized and
large species will unhesitatingly attack even a human that strays too close, often directing
blows with the feet and talons at the intruder's face and eyes. There are well-documented
cases of people losing an eye due to Tawny Owl attacks, and the ferocity of a Great Grey
Owl at its nest is nearly legendary.