When the first lucky observers spotted Flaco, the Eurasian eagle-owl, in Central Park in February, they knew they were seeing an owl. Owls are probably the most easily identified birds, with their upright bodies, big round heads and enormous front-facing eyes — it’s hard for even young children to mistake them for any other creature.
Owls may be easy to recognize, but they’re not easy to spot. In the day, they often hide right under our noses, camouflaged against the bark of trees or tucked into tree hollows, and in the night, sail off into the darkness. “Finding owls is hard,” said David Lindo, a bird guide known as the Urban Birder. “It’s often a question of diligence. You’ve got to commit yourself to it. You’ve got to try and work out where they are and then religiously search the trees.” If you’re quiet, still and patient, like owls themselves, then you may have the privilege of seeing one.
Owls are hard to observe for some of the same reasons we love them. With their inconspicuous presence and almost soundless flight, they point to the value of not standing out in the world but fitting into it.
They wear the look of the land around them to meld into it, a strategy known as crypsis. Masters of camouflage — streaked like grasses; mottled, speckled and striped like tree bark; pale like snow — they baffle the eye of both predators and prey.
The ear tufts of some species disrupt the round identifiable shape of an owl head, so it blends better with its woody surroundings. To complete their disguise, some species of owl will act the part of the tree or field that’s concealing them, staying utterly still, letting their tufts sway in the wind in a way that can mimic branches or grass. Many birds protect themselves by flying away from danger, but for owls, especially small owls, the go-to defense is to sit still and be quiet, relying on their camouflage to vanish.
Owls are cryptic not just in sight but also in sound. Many species move through the world in secret silence, the better to hear and hunt their prey. Most birds make a lot of noise when they fly. Not owls. A mouse or vole scuttling along the ground never hears what’s coming.
The quiet flight of owls is an act of biomechanical stealth that still challenges science. Only recently have biologists and engineers begun to sort out the details and to use what they’ve learned to design quieter things, among them wind turbines, fans, trains and airplanes.
I once experienced the stealth of a great gray owl up close. Percy was the male in a resident pair of great grays at Skansen, an open-air museum in Stockholm. The zookeeper let me into the spacious aviary planted with trees and boulders and told me to stand by a railing. At first the big bird stayed in a far corner of the enclosure. I could barely make him out against the tree bark, and even in this enclosed space, his partner was invisible. But when the zookeeper brought out a bowl of frozen mice, Percy launched, and with slow, silent wing beats, flew over to the railing and landed soundlessly just two or three feet from me.
Google “owl silent flight,” and you’ll find a dramatic video of an experiment by BBC Earth some years ago comparing the flight noise of a pigeon, a peregrine falcon and a barn owl. The crew filmed all three birds with a slow-motion camera as they swoop down a flight path outfitted with six super-sensitive microphones. It’s an eerie setup, a bird flying in the dark, with a spectrogram of the sound it makes hovering in the black. The differences are dramatic. The flapping of pigeon and falcon show up on the spectrogram as big spikes of sound. The owl’s acoustic wave is flat.
Owls do make noise when they fly, but the sounds they make are faint compared with other birds. This is in part because they have big wings for their body size, so their flight is buoyant and slow, which makes it quieter. But it’s the marvelous and unique feathers and structure of an owl’s wings that really hush its flight.
In 1934, Robert Rule Graham, a British pilot and bird lover, identified three wing feather features that suppress sound in owl flight. There was a 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. There was a belt of wispy vane fringes on the wing’s trailing (or rear) edge. And lastly, there was a soft layer of velvet coating the whole wing.
These three mechanisms are indeed key to an owl’s quiet flight, but many of the particulars have emerged only recently. 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 recently discovered that when the airflow hits the comb-like serrations, they break up the turbulence, effectively suppressing the swoosh sounds of flapping (the tests were done on a specimen wing and have not yet been replicated on a live owl). When an engineer tested the number of “teeth” in the comb on the wing of a Eurasian eagle-owl like Flaco, he found it has a number that effectively reduces turbulence.
The wispy vane fringes at the tips of an owl’s wings and tail also help to prevent wind eddies. But it’s the velvet that plays a critical role in squelching sound by reducing the frictional noise between feathers.
Feathers tend to produce a lot of noise. Rub together two tail feathers of a red-tailed hawk and you get a fair amount of frictional sound, like Velcro unzipping or a piece of sandpaper rubbing against a surface. This friction generates an audible “signature” with every flap of a bird’s wings.
Rub together the feathers from most owls, and you won’t hear much. That’s because the feathers are coated with a fine layer of plush fibers called pennula, which shroud sound and give owl wings that soft, velvety feel, like rabbit’s fur. An owl’s wing feathers separate slightly from one another in flight, so air flows over each feather, with the pennula providing a gap between adjacent feathers so there’s none of the friction or rubbing there is in most birds.
Collectively, the serrated comb, the pennula and the wispy wing tips unite to form a single soft surface without sharp, noisy edges.
Designers of Japan’s Shinkansen bullet train have used the understanding of the comblike, serrated wing of an owl to model a structure for the train that diminishes the loud sounds from vibrations. City, University of London, has developed an airfoil, a bladed structure used in wings and fans with “finlets,” which mimic both the comb and the trailing edge of an owl wing to reduce turbulence and improve efficiency. To make modern wind turbines less disruptive to their surroundings, a renewable energy company designed turbine blades edged with special serrations modeled on those of an owl’s wing feathers.
To my mind, the real gift of an owl’s stealth is the example it sets for how to be in the world. Owls invite us to move through life more quietly, to still ourselves, to notice sights and sounds that might otherwise go unnoticed. For owls, quiet invisibility is a defense or disguise. For us, it’s a privilege, one that — if we’re lucky — may yield an owl sighting.