The Owl

Owl Anatomy

Owls are a group of birds that belong to the order Strigiformes, constituting 200 extantbird of prey species. Most are solitary and nocturnal, with some exceptions (e.g., the Northern Hawk Owl). Owls hunt mostly small mammals, insects, and other birds, although a few species specialize in hunting fish. They are found in all regions of the Earth except Antarctica, most of Greenland and some remote islands. Though owls are typically solitary, the literary collective noun for a group of owls is a parliament. Owls are characterized by their small beaks and wide faces, and are divided into two families: the typical owls, Strigidae; and the barn-owls, Tytonidae.

 

General Information on Owls

The smallest owl—weighing as little as 31 grams and measuring some 13.5 centimetres—is the Elf Owl. The largest owl by length is the Great Grey Owl which measures around 70 cm on average and can attain a length of 84 cm. However, the heaviest (and largest winged) owls are two similarly-sized eagle owls; the Eurasian Eagle-Owl and Blakiston's Fish Owl. These two species, which are on average about 2.53 cm shorter in length than the Great Grey, can both attain a wingspan of 2 m and a weight of 4.5 kg in the largest females.

Sub-Species of Owl

There are over 200 sub-species of owl. Here we have summarised them into genera in order to simplify the list.

Tytonidae: Barn-owls

• Genus Tyto – barn-owls, 15 species

• Genus Phodilus – bay-owls, 2 species

Strigidae: Typical owls

• Aegolius – saw-whet owls, four species

• Asio – eared owls, 67 species

• Athene – 2–4 species

• Bubo – horned owls, eagle-owls and fish-owls, some 25 species

• Ciccaba – 4 species

• Glaucidium – pygmy-owls, about 30–35 species

• Gymnoglaux – Bare-legged Owl or Cuban Screech-owl

• Jubula – Maned Owl

• Lophostrix – Crested Owl

• Megascops – screech-owls, some 20 species

• Micrathene – Elf Owl

• Mimizuku – Giant Scops-owl or Mindanao Eagle-owl

• Ninox – Australasian hawk-owls, some 20 species

• Nesasio – Fearful Owl

• Otus – scops-owls, about 45 species

• Pseudoscops – Jamaican Owl and possibly Striped Owl

• Ptilopsis – white-faced owls, two species

• Pulsatrix – spectacled owls, three species

• Pyrroglaux – Palau Owl

• Strix – earless owls, about 15 species

• Surnia – Northern Hawk-owl

• Uroglaux – Papuan Hawk-owl

• Xenoglaux – Long-whiskered Owlet

Skeleton and Muscles

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.

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.

Senses

Sight

A 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 echolocationto guide them in flight in low-light situations. Owls are known for their disproportionally large eyes in comparison to their skull. 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. The owl has nocturnal eyesight far superior to that of its average prey.

 

Hearing

Owls exhibit specialized hearing functions and ear shapes that also aid in hunting. They are noted for asymmetrical ear placements on the skull in some genera. Owls can have either internal or external ears, but those genera exhibiting asymmetrical ear geometry only have external ear placements. With ears set at different places on its skull, an owl is able to determine the direction from which the sound is coming by the minute difference in time that it takes for the sound waves to penetrate the left and right ears.

Like the eyes, which 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, densely packed to form a facial ruff, which creates an anteriorly-facing concave wall that cups the sound into the ear structure. This facial ruff is poorly defined in some species and prominent, nearly encircling the face, in other species. Owls have an audible range similar to that of humans, but far more acute in certain frequencies, allowing them to detect even the slightest movements of their prey. Once an 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.

Feathers and Flight

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. 
he surface of the flight feathers is covered with a velvety structure that 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 [4,5]. 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.

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.

Behaviour (Social, Reproductive, Hunting)

 

Social and Reproductive

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. Variations in breeding schedule may correspond to the weather, food availability, competition from other owls, disease, and availability of a suitable mate.

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Courtship rituals vary from species to species, but invariably involve calling. The male will usually try and attract a female to a suitable nest site and may use special courtship flights, calls and offerings of food. Copulation often follows the acceptance of food by the female. There is often mutual preening, with the pair perched close together.

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As a general rule Owls are monogamous - pairs are comprised of one male and one female, neither one of which has any involvement with other nesting 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. Many of the medium-sized and large species will unhesitatingly attack even a human that strays too close to a nest, often directing blows with the feet and talons at the intruder's face and eyes.

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Owls do not construct nests as such, 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 favours the Arctic tundra, will use a hollow in the ground which the female may attempt to scrape out and line with plant material. 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.

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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. Owls will generally try to reoccupy the same nesting territories in consecutive years.

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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. Incubation of the eggs usually begins when the first one is laid, and lasts, in most species, for around thirty days.

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Owl chicks hatch with the aid of an Egg Tooth - a unique protrusion on the beak, common to all birds, which drops off a week or two after hatching. 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.

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The food is delivered as many as 10 times a day to the nest by the male. Larger prey items are ripped apart and fed to the chicks piece by piece. Smaller prey can be swallowed whole by the chicks as they get older. Young owls begin producing pellets as soon as they begin eating whole prey, or prey parts with fur, bones and other indigestible parts.

Upon hatching, owl chicks are blind and have a thin coat of natal down. In 1-2 weeks, a heavier second coat of down appears, called the mesoptile. As early as 3-4 weeks, some species' chicks may leave the nest and clamber about. In tree nesting species, these chicks are called Branchers. The next stage of development will be fledging, or learning to fly.

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Most owls reach sexual maturity and are ready to reproduce about a year after they hatch. Some larger species, however, may not begin breeding until their second or third year. 

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Hunting

Most owls are nocturnal, actively hunting their prey only in darkness. Several types of owl, however, are crepuscular—active during the twilight hours of dawn and dusk.

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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.

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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.