To Catch Voles Beneath the Snow, Nice Grey Owls Should Overcome an Acoustic Mirage

New analysis into the bizarre methods snow distorts sound affords new hints concerning the extraordinary searching methods of a winter phantom—the Nice Grey Owl.

From the Winter 2024 problem of Residing Chicken journal. Subscribe now.

Maybe no species of owl is as fantastically tailored for searching in snow because the Nice Grey.

Discovered all through the boreal forests of the Northern Hemisphere, Nice Grey Owls dine totally on small mouselike rodents referred to as voles. In winter, voles retreat to tunnels deep below the snow—however that doesn’t cease Nice Grays. Searching from an uncovered perch, an owl listens intently for its goal, then swoops down from above, punching by means of the crust of snow with its lengthy, highly effective legs. Capable of attain prey virtually 18 inches under the floor of the snow, Nice Grey Owls have been recognized to penetrate snow crusts thick sufficient to help a 175-pound particular person.

What hasn’t been clear, regardless of a long time of analysis about Nice Grey Owls, is how they do it—how do Nice Grays hunt prey animals now not than a ballpoint pen, which they will’t see, utilizing solely faint burrowing sounds greater than a foot below the snow to information them in plunging strikes with surgical precision?

Some intriguing new hints arrived just lately by way of quirky analysis carried out by an unlikely pair of scientists. One, a California biologist who had spent most of his profession finding out the sounds made by hummingbird feathers, had lengthy dreamed of working with owls; the opposite, a Canadian skilled on owl subject biology, had at all times wished to check sound.

Of their research, the researchers explored how Nice Grey Owls use a set of finely tuned variations for gathering sound and localizing its supply in deep snow. The Nice Grey Owl’s facial disc, a bowl-shaped circle of feathers that frames its face, is the most important of any owl species, amassing and directing even the softest sounds from the setting towards its ears. And like different owls that hunt by sound, its ears, hidden below feathers, are asymmetrical. A better ear opening on one aspect than the opposite enhances its potential to pinpoint a sound’s exact location.

To check the hunting-by-hearing capabilities of Nice Grays, the researchers used an array of microphones buried below the snow to hold out a posh and distinctive set of experiments within the chilly of Manitoba. Their analysis, printed within the journal Proceedings of the Royal Society B in November 2022, uncovered new hints concerning the bizarre methods snow muffles and distorts sound—and the Nice Grey Owl’s unbelievable potential to listen to a vole hidden within the snow, which it seems relies upon largely on the owl’s distinctive feathers.

Illustration of the details of an owl's feathers.
Feathers for Silent Flight. The combs and fringes on the perimeters of a Nice Grey Owl’s wing feathers break up the air currents throughout flight, decreasing the noisy turbulence that might in any other case be created by the flapping of wings. Graphic by Jillian Ditner.

Two Unlikely Consultants Crew Up

“I’m a hummingbird biologist. I do know this a lot about working with owls,” admits Chris Clark, holding up his thumb and pointer finger with a tiny hole between them.

Clark, a biology professor on the College of California, Riverside, started his profession finding out the mechanical sounds hummingbirds make with their feathers throughout show flights. About 14 years in the past, he turned fascinated with the alternative idea—how some birds (particularly owls) decrease the noise they’d in any other case naturally produce in flight. He initially struggled to get any owl subject analysis off the bottom, however in 2021, he was contacted by a nature documentary crew making a movie about animal sounds. And, the filmmakers talked about, they had been additionally working with a Nice Grey Owl skilled in Manitoba.

That skilled was Jim Duncan. Duncan has been finding out Nice Grays for nearly 4 a long time, and after he retired because the director of Manitoba’s fish and wildlife company in 2018, he began his personal nonprofit group, Uncover Owls, targeted on analysis, outreach, and conservation. Duncan recollects when, as a PhD scholar on the College of Manitoba, he carried out his first analysis on Nice Grey Owls and spent the night time in a snow hut referred to as a quinzhee. On a bitterly chilly February night time on the Taiga Organic Station in jap Manitoba, Duncan realized how snow muffled outdoors sounds.

“You don’t hear someone strolling as much as your quinzhee till they’re proper outdoors,” he says. “So it simply turned this nagging query in my thoughts: What sounds are these owls listening to, and the way are they utilizing them to catch meals?”

The main target of his PhD dissertation was elsewhere, and he lacked the coaching and gear essential to pursue that query, so he set the query apart— till, over 30 years later, the documentary crew launched him to an skilled on chicken flight and sound in California.

Quickly, Clark paid for a airplane ticket out of his personal pocket, packed each piece of heat clothes he owned alongside together with his acoustic evaluation gear, and headed for Manitoba. On the floor, the objectives of their collaboration had been easy: to check how snow would possibly take in and deform the sounds of voles and the way which may have an effect on Nice Grey Owl searching methods.

“Owl Ears” vs. “Mouse Ears”

Nice Grays use stealthy flight to shock their unsuspecting prey. Owls on the whole are recognized for flying virtually silently, however intriguingly, Nice Grays take these traits to the intense. Of all owl species on the planet, Nice Grays have the longest comblike serrations on the main edges of their wings, and the thickest velvety coating on their flight feathers—each evolutionary variations for silent flight that cut back wing noise to virtually nothing.

There are, says Clark, two main hypotheses to clarify why owls developed to fly quietly: “What I name the owl-ear speculation and the mouse-ear speculation.” The owl-ear speculation is that owls fly quietly to keep away from interfering with their very own potential to detect prey by sound; the mouse-ear speculation is that they’re attempting to keep away from being detected by potential prey.

“Though these hypotheses aren’t mutually unique,” explains Clark, “there are some instances the place they make totally different predictions, and the number-one case is when the setting itself blocks sound,” corresponding to when there’s a thick layer of snow on the bottom. The owl-ear speculation suggests {that a} snow-hunting owl ought to have particularly well-developed quieting options, in order that it will possibly hear its muffled prey over the sound of its personal wings. Beneath the mouse-ear speculation, nevertheless, quieting options could be much less vital, as a result of the snow would offer the owl with pure stealth.

However snow does extra to sound than merely dampen it. A snowpack is surprisingly complicated, half ice and half air, with totally different densities at totally different depths—all affecting the transmission of sound. High and low sounds go by means of snow in numerous methods, and due to the totally different speeds at which sound travels by means of air and ice, snow would possibly even refract sound, bending it in order that it appears to return from a special location than its precise supply.

Heading to a web site 60 miles northeast of Winnipeg, the place Duncan knew owls hunted, he and Clark positioned contemporary plunge holes from Nice Grays pursuing voles. They dug into the snowpack and slipped a water-proof speaker beneath, taking part in white noise or recordings of voles digging. Then, they pointed an acoustic digital camera that Clark lugged from his UC–Riverside lab on the snow. An acoustic digital camera makes use of an array of 40 microphones to localize the place a sound seems to be coming from, then superimposes this obvious supply location on a digital camera picture.

Working with digital gear in temperatures as little as –16°F introduced some difficulties: “I’ve solely skilled chilly temperatures like that a few instances earlier than in my life,” says Clark, the Californian. Each the laptop computer he used to run the acoustic digital camera and the audio system taking part in the sounds repeatedly froze up and stopped working, limiting the variety of trials they had been in a position to do. In the end, Clark and Duncan had been solely in a position to full six profitable trials with the buried speaker.

However even these six trials had been sufficient to offer some intriguing new insights into the challenges a Nice Grey Owl faces when searching in winter—and the way it overcomes these challenges.

Graphic of how an owl hears prey under the snow.
Pinpointing Prey in Deep Snow. Sound is bent because it travels by means of the snow, shifting its obvious supply by as a lot as 5 levels relative to the precise place of the prey and creating an acoustic mirage. Hovering instantly above prey reduces the acoustic mirage and permits owls to lock in on their precise place. Graphic by Jillian Ditner.

Refraction, Attenuation, and Acoustic Mirage

The outcomes from the acoustic digital camera offered detailed knowledge about how snow impacts sound in two methods: refraction and attenuation.

On this case, attenuation is the time period for the way in which a blanket of snow muffles sounds. Clark and Duncan’s outcomes confirmed that low-frequency sound is far much less affected than high-frequency sound, making Nice Grey Owls’ monumental facial discs—particularly well-suited for gathering low-frequency sound—a great adaptation for searching in deep snow. However the acoustic digital camera additionally confirmed the second, weirder manner snow impacts sound. Sound is certainly bent because it travels by means of the snowpack, shifting its obvious supply by as a lot as 5 levels relative to the precise place of the buried speaker— a phenomenon that Clark and Duncan dubbed an “acoustic mirage.”

This impact is minimized while you hear from instantly above the sound’s true supply, which helps clarify a particular side of Nice Grey Owl searching habits: hovering. Simply earlier than an owl plunges into the snow, it usually hovers in midair for a couple of moments, frenetically beating its wings. This tactic seemingly offers an owl an opportunity to lock in on a vole’s precise place from the purpose the place the acoustic mirage is minimized. Some fish-eating birds—corresponding to ospreys, kingfishers, and gannets—choose to strike straight down at their underwater prey for comparable causes, though they’re coping with water bending gentle as an alternative of bending sound.

A montage of owl photos as it focuses on something below.
Nice Grey Owls can plunge as deep as 18 inches into the snow to succeed in their prey. Photograph by Henry H. Holdsworth.

Clark sees two doable ways in which these snow sound results might play into Nice Grey Owl excessive variations for quiet flight. On the one hand, maybe the owls’ quieting options on their wing feathers particularly suppress low-frequency sound, making certain that sound from an owl’s personal wings doesn’t intervene with its potential to listen to the low-frequency digging sounds of the voles. Or, maybe (and that is the situation he thinks is extra seemingly) they might be particularly suppressing sound throughout hovering, in order to not intervene with an owl’s potential to focus on its prey precisely throughout this important closing second.

“After they’re hovering, you’ll be able to see the feathers behind the wing lifting up. That’s a sign that a part of the wing is stalling, which is when the air stops flowing easily over the floor of the wing and begins to kind lots of turbulence,” says Clark. Turbulence creates sound, which these wing options might have developed to counteract. Different birds that hover whereas searching, corresponding to kestrels and harriers, have velvety wing coatings like owls.

Each of those prospects are in step with the owl-ear speculation, not the mouse-ear speculation. Neither rationalization for the owl’s quieting variations is about serving to the owl sneak up on voles, which might’t hear the owl coming regardless, buried as they’re below a sound-attenuating blanket of snow. As a substitute, in accordance with Clark and Duncan, these variations be certain that Nice Grays can hear voles over the sound of their very own wingbeats as they lock onto their unseen prey’s place.

A snowy field in the evening light with a hole in the snow
Nice Grey Owls can plunge by means of a foot and a half of snow to seize a vole hiding under. Photograph by Jim Duncan.

Katherine Gura, a researcher on the Teton Raptor Heart in Wyoming and skilled on Nice Grey Owl ecology, who was not concerned with this acoustics research, was “thrilled” when she learn Clark and Duncan’s paper.

“This work serves as a superb instance of the fascinating questions we are able to reply by merging a robust data of the bodily properties of snow with wildlife ecology,” she says. “By testing how sound travels by means of the snowscape and linking these findings to Nice Grey Owl foraging methods and morphology, this research begins to unravel how this species developed its distinctive winter habits and traits.”

Gura says that Clark and Duncan’s analysis is an important first step for additional research on whether or not these snow-hunting acoustic variations of the Nice Grey Owl can maintain up over time, because the Earth continues to heat—and snowy winters soften away.

“This work opens the door for higher understanding how altering snow regimes doubtlessly will have an effect on Nice Grey Owls and different species that depend on subnivean [under the snow] prey,” says Gura. “Their potential to forage—and in the end persist—in a altering world stays unknown.”

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