Owls are birds from the order Strigiformes, which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers adapted for silent flight.
Except for the polar ice caps and some remote islands, owls are found worldwide. They hunt mammals, insects, and other birds, although a few species catch fish. However, rodents are the most common prey; smaller species eat insects. The size range of owls is about the same as that of their day-active counterparts, the hawks, with lengths of about 5–28 inches and wingspans between 1–6.6 feet.
All owls have the same general appearance, which is characterized by a flat face with a small hooked beak and large, forward-facing eyes. The tail is short and the wings are rounded. They have large feet with sharp talons. Some owls are diurnal, meaning that they hunt during the day. This includes the great gray owl, northern hawk owl, and northern pygmy owl. Perhaps these birds are daytime hunters because their preferred prey, such as songbirds or small mammals, are also diurnal.
Owls have 14 neck vertebrae, which is twice the number that humans have. This unique anatomy helps owls turn their heads 270 degrees. Their vertebrae have holes that are about 10 times the size of the animals’ blood-carrying arteries which can easily pass through the vertebral holes when the owl turns its head. The eyes of an owl are not true “eyeballs.” Their tube-shaped eyes are completely immobile, providing binocular vision which fully focuses on their prey and boosts depth perception. Bony eye sockets support an owl’s eyes, and they cannot turn their eyes. Turning their neck, which they do well, is the only way to look around.
As most owls are active at night, their eyes must be very efficient at collecting and processing light. This starts with a large cornea and pupil. 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, and an owl’s eye possesses few of these, so most Owls see in limited color or in monochrome.
An owl has three eyelids: one for blinking, one for sleeping, and one for keeping the eye clean and healthy. The third eyelid is also called the nictitating membrane, and many other birds also have it, including other raptors and many ducks, anhingas, and dippers.
Owls rarely hunt in the rain. Their feathers lack the oil that some birds have to repel water. Soft, silent feathers are more important for stealthy hunting. Fluffy feathers soak up a lot of water making it difficult, if not impossible to fly. Unlike most birds, owls make virtually no noise when they fly. They have special feathers that break turbulence into smaller currents, which reduces sound.
Soft velvety down further muffles noise. Owls’ wings are very large in comparison to the owl’s overall size with a surface area that allows them to glide through the air more like an airplane than a bird so they can reduce the amount of flapping they need to do.
Their wings have what we call ‘primary feathers’ along the edges which muffle any sound by organizing the direction of the airflow passing over them so that any turbulence created is silent. The turbulence created is broken down into smaller currents which we know as micro-turbulence and this happens by air being divided through the fine comb-like feathers.
Owls are zygodactyl, which means their feet have two forward-facing toes and two backward-facing toes. Unlike most other zygodactyl birds, however, owls can pivot one of their back toes forward to help them grip and walk. Getting killed by an owl is gruesome. First, the owl grabs the prey and crushes it to death with its strong talons. Then, depending on the size of the meal, it either eats the prey whole or rips it up.
The owl’s digestive tract processes the body, and the parts that can’t be digested, like fur and bones, are compacted into a pellet, which the owl later regurgitates. Sometimes, those pellets are collected for kids to dissect in school.
Not only do owls eat surprisingly large prey (some species, like the eagle owl, can even grab small deer), but they also eat other species of owls. Great horned owls, for example, will attack the barred owl. The barred owl, in turn, sometimes eats the Western screech owl. In fact, owl-on-owl predation may be a reason why Western screech owl numbers have declined.
Owls lay up to 14 eggs per brood, depending on the species and the availability of food. Parents protect their babies, called owlets or nestlings, in a tree cavity or nest built and abandoned by other birds such as hawks and crows. Only the short-eared owl and the iconic snowy owl build their own nests, which they often construct by scraping dirt into a hollow in the ground at a high point such as the top of a mound, to allow them to keep watch for predators.
Parents work together to raise their young. The male delivers food to the female, who tears it up for the little ones. Owlets begin exploring outside the nest after seven to 12 weeks, depending on the species, and permanently go off on their own a few weeks later.
Owls can have either internal or external ears, both of which are asymmetrical which allows the owl to pinpoint the location of its prey. 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.
The owl turns its head until the sound reaches both ears at the same time, at which point it is directly facing the source of the sound. This time difference between ears is about 30 microseconds. Behind the ear openings are modified, dense feathers, densely packed to form a facial ruff, which creates an anterior-facing, concave wall that cups the sound into the ear structure.
The facial disk also acts to direct sound into the ears, and a downward-facing, sharply triangular beak minimizes sound reflection away from the face. The shape of the facial disk is adjustable at will to focus sounds more effectively.
- The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All Anura (tailless amphibians), and some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the nictitating membrane remains in the corner of the eye. Some mammals, such as cats, camels, polar bears, seals, and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia. [Back]
The Owl Pages