Bird wing colour affects flight efficiency

Heating seabird wings indicates how darker feathers evolved to reduce energy expenditure on long flights
20 July 2021

Interview with 

Svana Rogalla, Ghent Uninversity

SOOTY-TERN

Sooty Tern Onychoprion fuscatus (syn. Sterna fuscata) flying in colony on Tern Island, French Frigate Shoals

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It’s well known that dark surfaces soak up more of the Sun’s rays than lighter shades, and now new research suggests some birds have evolved darker wings to take advantage of this heating effect and improve the efficiency of their flying. This is particularly relevant for seabirds like the Albatross that travel thousands of miles in a single journey. Harrison Lewis spoke to Svana Rogalla who’s been putting wings in wind tunnels to get a bird’s-eye view of the evolutionary advantages of darker feathers when it comes to flight...

Svana - There've been some studies suggesting that the heating of an airfoil, like a wing, can actually improve the flight efficiency. But until now there have been no studies on whether colour in birds can actually affect flight performance. So in my study, I found that darker wings will heat up more, and the hotter wing surface can actually increase the flight efficiency

Harry - And this is wild because you're saying that a darker wing is more efficient in terms of energy for flight, but most birds are completely different in colour.

Svana - Yeah, that's definitely true. So birds are amazing because they have all kinds of colours. These colours have different functions. So for example, very bright and shiny colours can serve for sexual signaling. But of course, not all birds are very colourful, so some are also more modal, so more like brownish grayish.

Harry - As a sort of camouflage, I guess?

Svana - That's camouflage, exactly. So in our study, we've looked at seabirds. Seabirds are mostly black and white, and we found that birds that are already efficient in flight have evolved darker wings. So already efficient birds can increase their efficiency even more by becoming darker and by flying under the sun.

Harry - Right, and how does one go about finding out if darker pigment in the wing helps with flight or makes flight more efficient? What are the steps?

Svana - What we did was to look at the wing in the wind tunnel. So we mounted the stuffed bird wing on a force balance, then we could measure the forces applying on this wing - so the lift and the drag, and does the heating of the wing affect the flight efficiency. So therefore we also mounted some light bulbs in the wind tunnel, so it would simulate a bird flying under the sun. And then we compared the heated wing with the non-heated wing, and we found that the heated wing is 20% more efficient.

Harry - And why is that?

Svana - So that's actually what we would need to test in the future, but we have some hypotheses. So it could be that the boundary layer, so the air flying around the wing, is delayed when the wing surface is heated. And, by the heating of the wing, it could be that the boundary layer is separated at an earlier stage, so that could make the flight more efficient.

Harry - When air passes over a curved surface, like a wing, it sticks to it. And in part, this is responsible for generating lift. But if it isn't optimal, it can actually hamper the efficiency of the wing's performance. Svana believes that heating the wing alters how long that passing air sticks to its surface, making the process much more efficient. So is one implication of these findings that we should be painting all our aircraft black?

Svana - Unfortunately it's not that easy. Of course, a plane will fly much, much faster than a bird. On the other hand, planes also store the fuel in the wings, so it's also not ideal to have the fuel in the wings heated up in that case.

Harry - That doesn’t seem ideal at all!

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