Description
Owls have large forward-facing eyes and ear-holes; a hawk-like beak; a flat face; and usually a conspicuous circle of feathers, a facial disc, around each eye. The feathers making up this disc can be adjusted in order to sharply focus sounds that come from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey sport eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting. Although owls have binocular vision, their large eyes are fixed in their sockets — as are those of other birds — so they must turn their entire head to change views. Owls can rotate their heads and necks as much as 270 degrees in either direction.As owls are farsighted, they are unable to see clearly anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes — like feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.
The smallest owl — weighing as little as 31 g (1.1 oz) and measuring some 13.5 cm (5.3 inches) — is the Elf Owl (Micrathene whitneyi Some of the pygmy owls are scarcely larger. The largest is the Great Grey Owl with 72 centimeters average, the second and third largest are two of the eagle owls; the Eurasian Eagle-Owl (Bubo bubo) and Blakiston's Fish Owl (Bubo blakistoni) — which may reach a size of 60 – 71 cm (28.4 in) long, and have a wingspan of almost 2 m (6.6 ft), and an average weight of nearly 4.5 kg (10 lb).).
Different species of owls make different sounds; this wide range of calls aids owls in finding mates or announcing their presence to potential competitors, and also aids ornithologists and birders in locating these birds and recognizing species. As noted above, the facial disc helps owls to funnel the sound of prey to their ears. In many species, these discs are placed asymmetrically, for better directional location.The plumage of owls is generally cryptic, but many species have facial and head markings, including face masks, ear tufts and brightly coloured irises. These markings are generally more common in species inhabiting open habitats, and are thought to be used in signaling with other owls in low light conditions.Owl eggs usually have a white color and an almost spherical shape, and range in number from a few to a dozen, depending on species. Eggs are laid at intervals of 1 to 3 days and do not hatch at the same time. This fact accounts for the wide variation in the size of sibling nestlings. Owls do not construct nests, but rather look for a sheltered nesting site or an abandoned nest in trees, underground burrows, or in buildings, barns and caves.[citation needed]
Behavior
Most owls are nocturnal, actively hunting their prey only in water and darkness. Several types of owl, however, are crepuscular — active during the twilight hours of dawn and dusk; one example is the Pygmy owl (Glaucidium). A few owls are active during the day also; examples are the Burrowing Owl (Speotyto cunicularia) and the Short-eared Owl (Asio flammeus).
Much of the owls' hunting strategy depends on stealth and surprise. Owls have at least two adaptations that aid them in achieving stealth. First, the dull coloration of their' feathers can render them almost invisible under certain conditions. Secondly, serrated edges on the leading edge of owls' remiges muffle an owl's wing beats, allowing an owl's flight to be practically silent. Some fish-eating owls, for which silence has no evolutionary advantage, lack this adaptation.
An owl's sharp beak and powerful talons allow it to kill its prey before swallowing it whole (if it is not too big). Scientists studying the diets of owls are helped by their habit of regurgitating the indigestible parts of their prey (such as bones, scales and fur) in the form of pellets. These "owl pellets" are plentiful and easy to interpret, and are often sold by companies to schools for dissection by students as a lesson in biology and ecology
Adaptations for hunting
All owls are carnivorous birds of prey and live mainly on a diet of insects and small rodents such as mice, rats and hares. Some owls are also specifically adapted to hunt fish. They are very adept in hunting in their respective environments. Since owls can be found in nearly all parts of the world and across a multitude of ecosystems, their hunting skills and characteristics vary slightly from species to species, though most characteristics are shared among all species.
Flight and feathers
Most owls share an innate ability to fly almost silently and also more slowly in comparison to other birds of prey. Most owls live a mainly nocturnal lifestyle and being able to fly without making any noise gives them a strong advantage over their prey that are listening for any sign of noise in the dark night. A silent, slow flight is not as necessary for diurnal and crepuscular owls given that prey can usually see an owl approaching. While the morphological and biological mechanisms of this silent flight are more or less unknown, the structure of the feather has been heavily studied and accredited to a large portion of why they have this ability. Owls’ feathers are generally larger than the average birds’ feathers, have fewer radiates, longer pennulum, and achieve smooth edges with different rachis structures. Serrated edges along the owl’s remiges bring the flapping of the wing down to a nearly silent mechanism. Research has shown that the serrations are more likely reducing aerodynamic disturbances, rather than simply reducing noise. The surface of the flight feathers are covered with a velvety structure which absorbs the sound of the wing moving. These unique structures reduce noise frequencies above 2 kHz,making the sound level emitted drop below the typical hearing spectrum of the owl’s usual prey. and also within the owl’s own best hearing range . This optimizes the owl’s ability to silently fly in order to capture prey without the prey hearing the owl first as it flies in. It also allows the owl to monitor the sound output from its flight pattern.
Vision
Another characteristic of the owl which aids in their nocturnal prey capture is their eyesight. Owls are part of a small group of birds that live nocturnally, but do not use echolocation to guide them in flight in low-light situations. Owls are known for their disproportionally large eyes in comparison to their skull. An apparent consequence of the evolution of an absolutely large eye in a relatively small skull is that the eye of the owl has become tubular in shape.This shape is found in other so-called nocturnal eyes, such as the eyes of prosimians and bathypelagic fishes. Since the eyes are fixed into these sclerotic tubes, they are unable to move the eyes in any direction.Instead of moving their eyes, owls swivel their head to visualize their surroundings. The swiveling radius of the owl’s head is around 270˚, easily enabling them to see behind them without relocating the torso. This ability keeps bodily movement at a minimum and thus reduces the amount of sound the owl makes as it waits for its prey. Owls are regarded as having the most frontally placed eyes amongst all avian groups, which gives them some of the largest binocular fields of vision. But owls are farsighted and cannot focus on objects within a few centimeters of their eyes. While it is commonly believed that owls have such great nocturnal vision due to their large (and thus very light-gathering) eyes and pupils and/or extremely sensitive rod receptors, the true cause for their ability to see in the night is due to neural mechanisms which mediate the extraction of spatial information gathered from the retinal image throughout the nocturnal luminance range. These mechanisms are only able to function due to the large sized retinal image. Thus, the primary nocturnal function in the vision of the owl is due to its large posterior nodal distance; retinal image brightness is only maximized to the owl within secondary neural functions.These attributes of the owl cause the nocturnal eyesight to be far superior to that of its average prey.
Hearing
Owls exhibit specialized hearing functions and ear shapes that also aid in hunting. They are often noted for the asymmetrical ear placements on the skull in some genera. Owls can have either internal or external ears, but the genera exhibiting asymmetrical ear geometry only have external ear placements. Asymmetry has not been reported to extend to the middle or internal ear of the owl. The asymmetrical ear placement on the skull allows the owl to pinpoint the location of its prey. This is especially true with the strictly nocturnal species such as the barn owls Tyto or Tengmalm’s Owl With the ears set at different places on the skull, the owl is able to determine the direction in which the sound is coming from by the minute difference in time that it takes for the sound waves to penetrate the left and right ear.[citation needed] The owl turns its head until the sound reaches both ears simultaneously, at which point it is directly facing the source of the sound. This time difference between ears is a matter of about 0.00003 seconds, or 30 millionths of a second. In coordination with this left to right calibration, some owls (like the barn owl) have slightly asymmetrical ears up and down that allow the owl to recognize whether the sound being received is higher or lower in vertical space by the volume of the sound in each ear. If the sound is louder in the ear facing more upwards, then the prey is higher up than the owl’s focus. Like the eyes, that utilize feather movements to focus light, the ears are surrounded by feathers to maximize hearing capabilities. Behind the ear openings there are modified, dense feathers which are densely packed to form facial ruff which creates an anteriorly-facing concave wall which cups the sound into the ear structure. This facial ruff is poorly defined in some species and yet prominent and nearly encircling the face in other species. The facial disk also acts to direct sound into the ears and a downward-facing, sharply triangular beak allows for little sound reflection away from the face. The shape of the facial disk is able to be adjusted at will to focus sounds more effectively. Owls have an audible range similar to that of humans, but are far more acute to certain frequencies which allow it to detect even the slightest movements of its prey. Once the owl has determined the location of its prey, it flies towards it according to the last sound perceived. If the prey moves, the owl is able to adjust its flight pattern mid-flight.
Talons
While the auditory and visual capabilities of the owl allow it to locate and pursue its prey, the talon and beak of the owl does the final work. The owl’s prey is killed by using these talons to crush the skull and knead the body. The crushing power of an owl’s talon varies according to prey size and type as well the size of the owl itself. The Burrowing Owl (Athene cunicularia), a small partly insectivorous owl, has a release pressure of only 500 grams. The larger Barn Owl (Tyto alba) needs pressure of 3000 grams to release its prey, and one of the largest owls, the Great Horned Owl (Bubo virginianus) needs pressure of over 13,000 grams to release prey in its talons. An owl’s talons, like most birds of prey, can seem massive in comparison to the body size outside of flight. The Masked owl has some of the proportionally longest talons of any bird of prey and appears enormous in comparison to the body when fully extended to grasp prey The foot design of owls differs from that of diurnal birds of prey because owls have a zygodactyl toe arrangement, with two toes directed forwards and two toes directed backwards during perching. During flight, the outer front toe swivels forwards due to a flexible joint so that three of the four toes are forward facing. Some owls’ talons have been adapted to include filoplume feathers that aid in sensing the prey once it is close or has in fact made contact with the talon, which is useful during the capturing of small prey at night. The underside of the talon foot is covered in a rough, knobby surface which allows the owl to grasp its prey and hold it without having to keep the muscles tightly contracted An owl’s claws are sharp and curved. The family Tytonidae have inner and central toes of about equal length while the family Strigidae have an inner toe which is distinctly shorter than the central one. These different morphologies allow for more effective capturing to prey specific to the different environments in which they inhabit.
Beak
The beak of the owl is short, curved and downward-facing. The beak is typically hooked at the tip for gripping and tearing its prey. Once prey is captured, the scissor motion of the top and lower bill is used to tear the tissue and kill. The sharp lower edge of the upper bill works in coordination with the sharp upper edge of the lower bill to deliver this motion. The downward-facing beak allows to the owl’s field of vision to be clear, as well as direct sound into the ears without deflecting sound waves away from the face.
Camouflage
The coloration of the owl’s plumage plays a key role in its ability to sit dormant and blend into the environment, making themselves nearly invisible to prey. Owls tend to mimic the colorations and sometimes even the texture patterns of their surroundings, with the common barn owl being an exception. Nyctea scandiaca, or the White Owl, appears nearly bleach-white in color with a few flecks of black which mimic their snowy surroundings perfectly. Likewise, the Muted Wood-Owl (Strix ocellata) displays shades of brown, tan and black which make the owl nearly invisible in the surrounding trees, especially from behind. Usually, the only tell-tale sign of a perched owl will be vocalization outside of a hunt or their vividly colored eyes.