The true owls or typical owls (family Strigidae) are one of the two generally accepted families of owls, the other being the barn owls and bay owls (Tytonidae). This large family comprises 230 living or recently extinct species in 23 genera. The Strigidae owls have a cosmopolitan distribution and are found on every continent except Antarctica.
Morphology
Typical owls (hereafter referred to simply as owls) generally share an extremely similar body plan, though they vary greatly in size, with the smallest species, the elf owl, being a hundredth the size of the largest, the Eurasian eagle-owl and Blakiston's fish owl. They tend to have large heads, short tails, cryptic plumage, and round facial discs around the eyes. The family is generally arboreal (with a few exceptions like the burrowing owl) and obtain their food on the wing. The wings are large, broad, rounded, and long. As is the case with most birds of prey, in many owl species females are larger than males. Because of their nocturnal habits, they tend not to exhibit sexual dimorphism in their plumage. Specialized feathers and wing shape suppress the noise produced by flying, both taking off, flapping and gliding. This silent flight allows owls to hunt without being heard by their prey. Owls possess three physical attributes that are thought to contribute to their silent flight capability. First, on the leading edge of the wing, there is a comb of stiff feathers. Second, the trailing edge of the wing contains a flexible fringe. Finally, owls have downy material distributed on the tops of their wings that creates a compliant but rough surface (similar to that of a soft carpet). All these factors result in significant aerodynamic noise reductions. The toes and tarsi are feathered in some species, and more so in species at higher latitudes. Numerous species of owls in the genus Glaucidium and the northern hawk-owl have eye patches on the backs of their heads, apparently to convince other birds they are being watched at all times. Numerous nocturnal species have ear-tufts, feathers on the sides of the head that are thought to have a camouflage function, breaking up the outline of a roosting bird. The feathers of the facial disc are arranged in order to increase sound delivered to the ears. Hearing in owls is highly sensitive and the ears are asymmetrical allowing the owl to localise a sound in multiple directions. Owls can pinpoint the position of prey, such as a squeaking mouse, by computing when the sound from the object reaches the owl's ears. If the sound reaches the left ear first, the mouse must be to the left of the owl. The owl's brain will then direct the head to directly face the mouse. In addition to hearing, owls have massive eyes relative to their body size. Contrary to popular belief, however, owls cannot see well in extreme dark and are able to see well in the day. Owls are also able to rotate their heads by as much as 270 degrees in either direction without damaging the blood vessels in their necks and heads, and without disrupting blood flow to their brains. Researchers have found four major biological adaptations that allow for this unique capability. First, in the neck there is a major artery, called the vertebral artery, that feeds the brain. This artery passes through bony holes in the vertebra. These bony holes are ten times larger in diameter than the artery that passes through them (extra space in the transverse foramina) which creates air pockets that allow for more movement of the artery when twisted. Twelve of the fourteen cervical vertebrae in the owl's neck have this adaptation. This vertebral artery also enters the neck higher up than it does in other birds. Instead of going in at the 14th cervical vertebrae, it enters in at the 12th cervical vertebrae. Finally, the small vessel connection between the carotid and the vertebral arteries allow the exchanging of blood between two blood vessels. These cross connections allow for uninterrupted blood flow to the brain. This means that even if one route is blocked during extreme head rotations, another route can continue blood circulation to the brain. Several owl species also have fluorescent pigments called porphyrins under their wings. A large group of pigments defined by nitrogen-containing pyrole rings, including chlorophyll and heme (in animal blood), make up the porphyrins. Other bird species will use porphyrins to pigment eggshells in the oviduct. Owl species, however, use porphyrins as a pigment in their plumage. Porphyrins are most prevalent in new feathers and are easily destroyed by sunlight. Porphyrin pigments in feathers fluoresce under UV light, allowing biologists to more accurately classify the age of owls. The relative ages of the feathers are differentiated by the intensity of fluorescence that they emit when the primaries and secondaries are exposed to black light. This method helps to detect the subtle differences between third and fourth generation feathers, whereas looking at wear and color makes age determination difficult.
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