What an Owl Knows: The New Science of the World's Most Enigmatic Birds

by Jennifer Ackerman

This book explores what new science has discovered about these enigmatic birds—their remarkable anatomy, biology, and behavior and the hunting skills, stealth, and sensory prowess that distinguish them from nearly all other birds.

Most owls are socially monogamous, pairing up to breed, but research suggests they’re also genetically monogamous—unlikely to engage in extra-pair copulations—highly unusual in the bird world.

Owls vary dramatically from species to species and even from individual to individual within a species. It’s one of the reasons I wanted to write about this order of birds—to explore the idiosyncrasies of different kinds of owls and what has been discovered about their evolution, species adaptations, and individual natures.

Owls may be known for their nocturnal way of life, but only about a third of owl species hunt solely at night.

Owls first appeared on earth during the Paleocene epoch, some fifty-five million to sixty-five million years ago. Tens of millions of years later, they split into two families, Tytonidae (barn owls) and Strigidae (all other owls).

But new research shows that owls are most closely related not to falcons or nightjars but to a group of day-active birds that includes toucans, trogons, hoopoes, hornbills, woodpeckers, kingfishers, and bee-eaters.

Now most owls share an array of remarkable features that distinguish them from other birds and give them a unique ability to hunt at night, including retinas rich in cells that provide good vision in dim light, superior hearing, and soft, camouflaged feathers tailored for quiet flight. Of the 11,000 or so species of birds alive today, only 3 percent have these sorts of adaptations that allow for stalking prey in the dark.

Some 260 species of owls exist today, and that number is growing. They live in every kind of habitat on almost every continent—from

DNA studies have revealed that Tytonidae, the scientific name for barn owls, is in fact a rich complex of at least three species, with a total of some twenty-nine subspecies. And there may be others existing in remote places that haven’t yet been recognized.

Cutting-edge imaging technology such as X-ray computed tomography (CT) scanning allows researchers to see inside the bodies of living owls, visualizing the anatomical structures that relate directly to behavior, and to peer through rock to see into fossils. DNA analysis is revealing relationships in the owl tree of life, challenging old concepts about who is related to whom and how closely. New “eyes” in the field—infrared cameras and other night vision equipment, radio tagging, and drones over areas as remote as the snowy landscapes of Siberia—are advancing new discoveries about owl behavior or confirming older observations by banders and biologists

Satellite telemetry is illuminating the movements of owls over short and long distances.

Nest cams are offering a look at intimate owl interactions at the nest that would otherwise be impossible to observe: the feeding of mates and young, for instance, and the squabbling between siblings.

Nest cams expose the sometimes nasty, sometimes charitable dynamics between siblings. Chicks in a brood can be selfish and competitive, to the point of siblicide. But some owlets display a remarkable form of altruism rare in the animal world. Nestling barn owls, for instance, are known to give food over to their younger siblings, on average twice per night.

Listening to owls remotely with sophisticated new audio recording devices has been a boon to owl research, helping scientists understand the interplay of different owl species without disturbing them.

In placing audio recorders in close to a thousand locations across 2,300 square miles of mountainous terrain to collect owl calls, they have discovered completely unexpected interactions between the aggressive Barred Owls, on the one hand, and the smaller, but still surprisingly feisty spotted owls, on the other—with significant implications for conservation.

Researchers are harnessing the olfactory powers of dogs to locate elusive owl species in places as far-flung as Tasmania and the Pacific Northwest. Specially trained “sniffer” dogs snuffle the pellets, those misshapen cigars made of leftover bits of undigested fur and bone, which owls eject onto the ground beneath their roosts and nests. The pellets are hard to spot, but they emit odors so the dogs can easily sniff them out, leading a researcher straight to the spots where the owls hang out.

As Sergio Cordoba Cordoba, an ornithologist studying neotropical owls, told me, “It can be really frustrating. Technology is a great ally, infrared cameras and telemetry, but we often still rely on sounds.

Researchers and birdwatchers often attract owls with “playback,” using audio recordings of owl territorial or mating calls to draw them in.

That owl seemed like a messenger from another time and place, like starlight. Being near her somehow made me feel smaller in my body and bigger in my soul.

This remarkable ability to move indigestible food up and out, against the usual direction, is called “antiperistalsis.”

But it’s an essential part of the digestive process: because the pellet blocks part of the digestive tract, an owl usually can’t eat again until it’s expelled.

For a long time, scientists thought owls didn’t scavenge—and if they did, it was a fluke. But lately camera traps have caught owls helping themselves vulturelike to carrion of all kinds—Eurasian

One testament to the hunting prowess of owls is their caching of surplus prey. Owls routinely cache, or hide, excess food in a nest, tree hole, or forked branch as a way of holding on to a glut of prey for later consumption. Caching most often occurs when female or young are satiated, and the male hides the leftovers.

But most have long, well-muscled legs, up to half the length of their bodies, with strong bones, especially in their feet.

They have four toes, three of which face forward in flight and sometimes in perching. But when owls need to grasp their prey, a special flexible joint allows them to swivel one rear toe forward to give them an extra powerful X grip. They can hold that grip without tiring thanks to a system of tendons in their feet that keeps the toes locked around prey without the exertion of muscles so they don’t have to put energy into holding it. This also benefits owls that capture prey “blindly,” beneath snow or leaves or in the complete dark, allowing them to lock on tight to their target even if they can’t see or judge its exact size or shape.

The flat, gray facial disk of a Great Gray Owl is like one huge external ear, a feathered satellite dish for collecting sound. Not all owls have the big, pronounced facial disks of Great Grays, Boreal Owls, and barn owls. It’s smaller in owls that rely less on sound for hunting—Great Horned Owls, Little Owls, pygmy owls. And in some species, like fish owls, it’s dramatically reduced.

The facial disk in owls that hunt primarily by sound is outlined with a ruff, or ring of stiff interlocking feathers that capture sound waves and channel them toward the ears, like people cupping their hands around their ears. Feathers in the back of the disk direct high-pitched sounds toward the ears, so the owl hears less noise from its surroundings and can focus on prey cues.

The use of the term eared in the common names of some owls is confusing. Long-eared and Short-eared Owls have tufts of feathers on the tops of their heads, called “plumicorns” (from the Latin for “feathered horn”), which look a lot like mammal ears. But these tufts have nothing to do with hearing

An owl’s actual ears are just openings in each side of the head, well covered with specialized feathers that allow sound to pass through.

In any animal ear, a little sliver of tissue called the “cochlea” collaborates with the brain in the hard work of hearing. The cochlea contains hair cells sensitive to the vibrations of sound, and its length in an animal is a pretty good measure of hearing ability. In most owls, the cochlea is enormous relative to body size and contains huge numbers of hair cells compared with other birds.

An owl’s auditory system shares with other birds another superpower we mammals don’t possess: it doesn’t age.

This suggests that owls, like other birds, have the capacity to regenerate their hair cells, keeping their hearing keen throughout life.

Some owls, such as Great Horned Owls and Eastern Screech Owls, have ears placed at about the same level on both sides of their heads like most animals do. But others—barn owls, Northern Saw-whet Owls, and Great Gray Owls—which rely heavily on sound for hunting, have one ear hole higher on one side of the head than the other.

The difference in the time of arrival of sound waves between his two ears, known as the interaural time difference, helps Percy gauge the exact azimuth (or horizontal location) of a sound.

Certain auditory neurons in an owl’s brain respond only when a sound is coming from a particular location. By comparing the responses to sound by neurons in the cochlea of both ears, the brain builds a kind of multidimensional map of auditory space. This allows owls to fix the location of prey with speed and precision.

The resulting auditory map allows owls to “see” the world in two dimensions with their ears.

The space-specific neurons in the owl’s specialized auditory brain do advanced math when they transmit their information, not just adding and multiplying incoming signals but averaging them and using a statistical method called “Bayesian inference,” which involves updating as more information becomes available.

The eyes of owls are forward facing because they’re so big, argues Martin, and an owl’s skull is so small and crowded with big, elaborate hearing structures that there’s nowhere else for the orbs to fit.

Owls have a narrower total field of view, but their binocular vision gives them an enhanced ability to determine their direction of travel and the time required to reach a target—all big advantages in zeroing in on prey, especially if it must be caught with split-second timing.

While it’s a myth that owls can rotate their heads full circle from a starting point facing forward, some species, like Great Grays and barn owls, can turn their heads almost three quarters of the way around, 270 degrees—three times the twisting flexibility humans possess. Owls have exactly twice as many neck vertebrae as humans do, giving them that much more flexibility.

That an owl’s neck can move swiftly and smoothly through those 270 degrees of rotation is due to some clever adaptations, a loose S shape that gives it flexibility, and a system of bones and blood vessels that minimizes disruption of blood flow through the neck to the eye and the brain when the head rotates.

Over evolutionary time, it seems, owls made a kind of sensory trade-off. They lost some of the genes involved in daylight and color vision. But their genes for nocturnal vision were enhanced and refined.

Most birds have a retina dominated by cones, cells that work best in bright light to help with color detection. The retinas of owls are packed with rods, which are much more sensitive to light and movement.

While they may not be waterproof, owl feathers have evolved ingenious adaptations for camouflage and quiet flight over millions of years. Their color alone is a wonder of specialization, dominated by natural hues, shades of brown, cream, and gray. The dark surfaces of their feather vanes (the webby part) are saturated with the pigment melanin, which gives them not only extra strength and hardness but also resistance to abrasion and to feather-degrading bacteria and parasites.

With all the benefits of dark plumage, why aren’t owls uniformly dark colored? Because it takes extra material and energy to produce dark feather parts, including deposits of minerals like calcium, cadmium, and zinc. (It’s not easy for owls to take in calcium—they digest the bones of their prey less efficiently than other birds of prey.) Also, darker colored feathers are heavier. The lighter colored parts of feathers weigh up to 5 percent less than the adjacent dark portions.

Big owls, like barn owls, tend to have strong feather shafts and ribs of dark barring against a lighter, paler background (which costs less to grow), whereas smaller owls have weaker feather shafts, reinforced by darker vanes, patterned with pale ovals and spots (again, to save energy costs and weight).

All birds make sound in flight. Noise arises from the drag of air over a bird’s body, from the vortices, or turbulent whirls of wind in a bird’s wake that generate sound waves, from air squeezing through the slits in feathers, and from feathers rubbing together. With every flap, bird wings rustle and flutter, whistle, hum, and whish. They drum and snap and clap. But the sounds that many owls make when they fly are so faint that they’re below the threshold of human hearing.

Owls may not be silent fliers, but they are nearly so. In part this is because owls have low wing loading—their wings are big in relation to their bodies—so their flight is buoyant and slow, as slow as two miles per hour for a big bird like a barn owl, which makes it quieter.

Robert Rule Graham, a British pilot, aeronautical engineer, and bird lover, identified three features that suppress sound in owl flight. First, he observed an unusual feature known as a comb, a row of fine hairlike bristles that extend forward along the leading edge of the wing (where it meets the oncoming air). He also noted a belt of ragged wispy vane fringes on the wing’s trailing edge (its rear edge) and, finally, a soft layer of velvet coating the whole wing.

In most birds, air flowing over the wing surface produces turbulence, air eddies that make noise. One team of researchers studying the leading-edge comb of a barn owl’s wing discovered that when the airflow hits the comb-like serrations, they break up the turbulence, effectively suppressing that swoosh sound I heard from the Harris’s Hawk.