Many of us have stories of owls flying across the road in front of our cars, calling eerily outside our windows, or quietly staring at us from a tree on a misty gray day. We recall these encounters vividly, for owls are such fascinating and mysterious creatures. As nighttime hunters, owls have incredibly specialized eyes, ears, feathers, feet, and digestion, all of which contribute to their superior predatory ability. Their calls are unique too, and, though strange and foreign to our ears, they are an important adaptation for owls to communicate with each other.
Along with eagles, hawks and falcons, owls belong to the group of birds known as raptors. Raptors are carnivores, or meat eaters, and they share some similar adaptations, including binocular vision, sharp, hooked beaks, and strong, sharp talons on their feet. Most raptors eat small animals like mice and other rodents, rabbits, frogs, fish, songbirds, and insects, and there can be a lot of competition for these food sources. For this reason, owls tend to be nocturnal, or active at night. Staying awake while other raptors are asleep enables owls to catch and eat nocturnal prey, avoiding daytime competition with other raptors.
Due to the low light levels of nighttime, owls have special eyes for seeing in the dark. Owls’ large eyes are almost impossible not to notice, whether they’re the yellow eyes that most owls possess or the deep brown eyes of barred and barn owls. Their pupils can open very wide to let in a lot of light, much more than our eyes admit. If humans had eyes as large as those of owls, they would be the size of grapefruits! Owl eyes have many rod cells which improve night vision by allowing an animal to see shapes and movements, but they have fewer cone or color-sensitive cells, so they have limited color vision. Owls have large eyes that are fixed in place in their skulls, as in most other birds, and protected by bony rings. Owl necks are extremely flexible, with fourteen vertebrae compared to only seven in humans. The neck bones have extra-large canals for the blood vessels and nerves to pass through, so the neck can swivel without pinching off the blood and nerve supply to the brain. An owl can turn its head 270 degrees in either direction and keep a lookout without shifting position.
While owl eyes have impressive capabilities, an owl’s ears are probably even more important in helping the bird locate its food. Although some owls have feather tufts on top of their heads that look a lot like ears, in fact all owl ears are located on the sides of their heads, a little behind and below the eyes and hidden by the facial feathers. The feather tufts have nothing to do with hearing but are used in display, to express aggression or fear. Owls have enlarged ear canals, and the feathers of the facial disk can twist to funnel sounds into the ear. Some owl ears are asymmetrical, meaning that one ear is located a bit higher than the other. This allows an owl to use triangulation to pinpoint sounds made by their prey. Barn owls, which have some of the best hearing of any animal, can locate prey in complete darkness, even under leaf litter and snow.
Owls also can fly virtually silently so they can sneak up on their prey, and also so that the sound of their wings doesn’t interfere with their own hearing. The leading edges of an owl’s wing feathers have comb-like fringes instead of being smooth as in other birds. These, along with velvety surfaces on the wing and soft fringes on the trailing edges of feathers, help to muffle sounds by breaking up the air flow around the wing. Not only the wings but every part of the owl’s body is covered with soft feathers so they can fly without making a sound. Unlike other raptors, owl legs and feet are also feathered, often right down to the nails. Feathered feet may provide warmth as well as muffling sound, for owls of the north, like snowy owls, have extra fluffy leg feathering compared to some owl species in warmer climates.
Once an owl has located its prey, it flies towards it and then thrusts its legs forward, trapping the prey with its feet, often stunning it on impact. At the ends of its toes are sharp, hooked claws called talons. An owl’s outer toe can rotate sideways, creating an effective cage for trapping and carrying prey. Owls may kill the prey by piercing its vital organs with its talons, or it may kill it with a bite of its sharp beak. The size of the prey depends on the size of the owl. Smaller owls like screech owls and saw-whet owls feed primarily on mice and voles, while great-horned owls take more rabbits and hares. However, the diet of owls is very varied and ranges from flying insects and bats to frogs, snakes, fish, birds, and even other owls. With powerful feet, owls can kill prey that weigh more than themselves. It’s not uncommon for a great-horned owl to catch and eat a skunk nearly twice its weight, and they’ve been known to eat porcupines! Once the prey is caught, a special locking tendon traps it in the owl’s grip without the need for further muscle action, saving energy needed for lifting off and carrying the load in flight.
Most prey is carried to a perch, often a regularly used location, for eating. Small animals are swallowed whole, usually head first. For larger items, an owl uses its foot and beak much like a fork and knife. It holds the prey with its foot while ripping it into bite-sized pieces with its sharp, hooked beak. Once the food has been swallowed, the owl’s digestive system takes over. In the first chamber of the owl’s two-part stomach, the proventriculus, enzymes break down the soft tissues. From there the food enters the gizzard, where strong muscles crush and grind it further. The softer parts of the prey, now largely liquefied, pass into the small intestine, while the hard bones, nails, and teeth are compacted, along with some fur or feathers, into a pellet. About eight to twelve hours after eating, the owl regurgitates this pellet. Dissecting a pellet reveals what the owl has eaten – bones and skulls in such good condition that they can often be identified.
Another thing that owls do while perched is make their strange and mysterious calls. Voices that carry well in the darkness help owls communicate with one another. While some owls, like the great-horned owl or long-eared owl do actually “hoot,” as we are taught when we are children, many owls make other strange noises as well. The barred owl, which is common in wooded and swampy areas, is often described as sounding like a monkey or a barking dog. Put to words, this owl’s strange vocalization sounds like, “Who cooks for you? Who cooks for you all?” The much smaller eastern screech owl sounds more like a frog trilling or horse whinnying than a hooting owl. You can replicate its trill by attempting to whistle and hum at the same time. There is great variety in the calls of different owls and even of a single owl, and the reasons for their vocalizations are varied as well. Owls vocalize to establish territories, to invite female owls to inspect nest sites, to beg for food, to chase predators away, and to court one another.
Whether we observe an owl sitting eerily motionless on a branch, hear it calling in the night, or watch it silently hunting on the wing, our encounters with owls are always memorable. Their many unique and varied adaptations make them strikingly different from other birds and superbly equipped for their lives as winged nighttime hunters.
Heinrich, Bernd. One Man’s Owl. Princeton, NJ: Princeton University Press, 1993.
Sattler, Helen Roney. The Book of North American Owls. New York: Clarion Books, 1998.
Voous, Karel H. Owls of the Northern Hemisphere. Boston: The MIT Press, 1988.
Lewis, Dean. The Owl Pages. http://www.owlpages.com.