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A few evenings ago, while walking through the forest at dusk, my daughter stopped us in our tracks with a loud "shush.” As we stopped to listen, she pointed towards a mewing sound on the trail just ahead of us—and there, two fledgling Barred Owls perched on a wooden railing. As we carefully approached, first one, then the other, flew noisily upwards. Each beat of their still-fuzzy wings whirred like a fan blade slicing the air as they slowly and awkwardly flapped up to a nearby tree branch.

It won't be long before these downy juveniles grow into their adult feathers and gain the amazing ability shared by many owls around the world: to fly almost soundlessly through the trees. When most birds fly, the air turbulence created by wing flapping produces sound, and, typically, the larger and faster a bird is, the noisier its flight. But not owls. Even large species like the Barn Owl or Great Horned Owl can fly virtually silently—a quality that has long fascinated scientists.

“Owls have a suite of unique wing and feather features that enable them to reduce locomotion-induced sound,” says Krista Le Piane, a graduate student at the University of California, Riverside who recently presented her work on the evolution of silent owl flight at the Animal Behaviour Society conference in Ontario, Canada. They have large wings relative to their body mass, which let them fly unusually slowly—as slowly as two mph for a large species like the Barn Owl—by gliding noiselessly with little flapping. Additionally, the structure of their feathers serves as a silencer. Comb-like serrations on the leading edge of  break up the turbulent air that typically creates a swooshing sound. Those smaller streams of air are further dampened by a velvety texture unique to owl feathers and by a soft fringe on a wing's trailing edge. These structures together streamline the air flow and absorb the sound produced.

The sound-dampening structures didn't evolve by chance. Silent flight is clearly crucial for many owls' survival, and two long-held hypotheses attempt to explain this ability. The "stealthy hunting hypothesis" holds that owls fly inaudibly so that prey can’t hear them coming and have less time to escape. On the flip side, the "prey detection hypothesis" poses that silent flight aids owls in hearing and tracking prey. If you are trying to navigate to your next meal—like a mouse or vole scuttling quietly along the ground in the dark—you don't want your noisy wing beats impeding your own hearing ability.

Le Piane is investigating which of these two hypotheses explains the silent flight of owls. She has closely examined feather and wing structure in 70 owl species and analyzed their traits, accounting for their evolutionary relationships and history. For each species, she photographed the leading-edge comb, the noise-cutting serrated fringe on the owls’ outermost flight feather. Then, aided by an image analysis program, she measured each comb’s width and length, among other features. Generally, the wider the comb the less sound the wing makes as it flaps.

She then compared feather structure with dietary differences among species. Because owls are present on every continent except Antarctica, they occupy an array of ecological niches and eat many different kinds of prey. Under the stealthy hunting hypothesis, “we expect owls hunting organisms that hear well, like mammals, to have increased features associated with silent flight,” Le Piane says. “And owls hunting prey that don’t hear well, like insects, and prey that can’t hear them approach, like fish, should have decreased features that aid in silent flight.”

So far, that’s what she’s found: fish-eating owls have narrower combs—meaning less noise-quieting by their feathers—compared to owls eating other types of prey. Some, such as the Tawny Fish Owl and insect-eating Burrowing Owl, had little-to-no comb at all. “We also find that owls hunting mammals have increased comb width [more quieting] compared to owls hunting insects,” Le Piane says. These findings, she suggests, support the stealthy hunting hypothesis.

Under the prey detection hypothesis, Le Piane predicted that owls hunting at night, when auditory cues are most important, would have more noise-cancelling feather structures compared to owls like Snowy Owls and Burrowing Owls that are active by day. “That’s also what we find,” she says, in the form of greater comb widths in nocturnal versus diurnal owls.

So, it appears that both hypotheses are correct, depending on the owl. Owls that hunt prey with good hearing and owls that hunt at night—or both—have feathers that reduce noise. “I wasn’t expecting to find support for both hypotheses,” Le Piane says. Next, she'll broaden her analysis after measuring yet more owl specimens this summer at the Natural History Museum's ornithology collection in Tring, near London, England.

As for those rapidly growing young Barred Owls, on my next evening walk in the forest, chances are they’ll hear me—but as they flap their way through the trees, I probably won’t hear them.