How do owls fly so silently?

Owl Fact #1: Owls have extra-stretchy blood vessels in their necks, allowing them to rotate their heads up to 270 degrees.

Owl Fact #2: Owls have asymmetrical ear openings that help them figure out exactly where sounds made by their prey are coming from.

Owl Fact #3: Owls are able to fly nearly silently … and that’s the topic of today’s post!

(For more owl facts, click here.)

Owl You Need Is Fluff (Fluff, Fluff is Owl You Need)

When birds fly, air molecules rush past their wings, forming big spirals. These air formations – called the wake in flight-jargon – are the source of the whooshing sound we hear as a bird passes by. But if the air molecules in the wake are jumbled around instead of neatly organized, we don’t hear a “whoosh” – instead, we hear nothing at all.

Owls are masters of acoustic stealth because their wakes are mostly scrambled, not spiraled. Owl wings have (1) a front side [leading edge] that’s serrated like a comb, (2) a back side [trailing edge] that’s fringed like the edge of the rug, and (3) a velvety down covering the whole wing like fuzzy carpet. All of this assorted fluff knocks air molecules around in random directions, allowing near-silent flight.

owlWingJaworskiClarkWake-scrambling features of owl wings (Credit: Jaworski & Clark)

Two hypotheses could explain why quiet flight might have evolved: (1) maybe owls can more easily hear tasty prey if their own noisy wings aren’t getting in the way, or (2) maybe owls are better at sneaking up on critters who can’t hear them coming. It turns out that both hypotheses are supported by comparisons between species of owls with different lifestyles – in other words, this superpower can be super useful in more than one way.

Hoot Off the Press

In a new study published today in Integrative Organismal Biology, R. Gurka from Coastal Carolina University and his colleagues describe, in unprecedented detail, the wake of the Australian boobook owl. (Boobook is the Dharug word for bird.)

Gurka and colleagues visualized the wake of three flying owls using a technique called particle image velocimetry, or PIV. By spraying particles of olive oil into a wind tunnel and lighting them up with a laser light sheet, the researchers were able to take snapshots of the physical structure of the wake at different locations around the owls’ wings. They then compared these snapshots to those taken during earlier PIV experiments on other species of birds.

Screen Shot 2019-04-29 at 12.55.41 PM

The disorganized wake of the Australian boobook owl, visualized using particle image velocimetry (PIV) (Credit: Lawley et al. [2019])

Through a series of aerodynamic analyses (check out the paper for details!), Gurka and colleagues demonstrated that the boobook owl’s wake is extremely messy – way more so than those of shorebirds and songbirds. And, as we’d expect, it doesn’t contain any of those noisy spirals of air molecules. Thanks to this work, we now have a much more quantitative understanding of how owls fly so silently.

Where Are We Hedwig Next?

As research into owl flight continues, it’ll be really interesting to see how the wake structures of other owls differ from that of the boobook owl. We know that owls that eat mammals (which can hear well) have more wing-fluff than those that eat fish and insects, and owls that hunt in the dark (and rely on hearing to do so) have more fluff than those that hunt during the day. How much more effectively do fluffier wings scramble air molecules, and how directly does that scrambling translate to even quieter flight?

Knowledge of the relationships among wing features, wake structure, and flight silence has huge potential for applications to technology. Imagine incomparably undetectable drones, wind turbines that quietly harness clean energy, and silent fans and air conditioners to cool your home – there are endless possibilities for owl-inspired design! That’s owl for now… but keep checking iobopen.wordpress.com for the latest in cool, organism-centered biology.

By Armita R. Manafzadeh

Armita R. Manafzadeh is a PhD candidate studying the evolution and development of joint mobility at Brown University. Her interests include functional morphology, vertebrate paleontology, and biomechanics.

 

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