08/02/2024

One Bird’s Physics Trick For Flying At High Altitudes

9:50 minutes

A bird flying with a totally blue sky in the background
A turkey vulture (Cathartes aura) flying near Alcova, Wyoming. Credit: Jonathan Rader

If you’ve ever taken a trip to a higher elevation, you know that the air gets thinner as you go up. If you’re not acclimated to the altitude, it can feel harder to breathe. That thinner air also makes it more difficult for birds and airplanes to fly, because it’s harder to produce the lift forces in thinner air. But it turns out that turkey vultures have a way of dealing with that problem.

Researchers observed turkey vultures in flight at different altitudes and found that rather than flapping harder or more rapidly to deal with decreased lift, the turkey vulture exploits the lower drag in thinner air to fly faster, using increased speed to help balance the lift equation. Dr. Jonathan Rader, a postdoctoral research associate in biology at the University of North Carolina Chapel Hill and an author of a report on this research published in the Journal of Experimental Biology, joins SciFri’s Charles Bergquist to explain how flying things work to adapt to different flight conditions.

Segment Guests

Jonathan Rader

Dr. Jonathan Rader is a postdoctoral research associate in Biology at the University of North Carolina, Chapel Hill in Chapel Hill, North Carolina.

Segment Transcript

CHARLES BERGQUIST: If you’ve ever taken a trip to a higher altitude, you know that the air is thinner up there. And if you’re not used to it, you might want to take it easy. Maybe don’t plan to go running the first day you arrive in the mountains.

That thinner air also makes it more difficult to fly. But it turns out that a bird, the turkey vulture, has a way of dealing with that problem. Here to explain is Dr. Jonathan Rader, a postdoctoral research associate in biology at the University of North Carolina, Chapel Hill, and an author of a report on this research published in the Journal of Experimental Biology.

Welcome to Science Friday, Dr. Rader.

JONATHAN RADER: Hi, Charles. Thank you. It’s lovely to be here with you today.

CHARLES BERGQUIST: Thanks for being here. So for listeners who may not be totally up on their birds, what is a turkey vulture? These are not small birds.

JONATHAN RADER: No, they’re a large bird– I don’t know– about a foot and a half tall when they’re sitting on the ground, and they’ve got a wingspan of around 6 feet.

CHARLES BERGQUIST: Wow.

JONATHAN RADER: And they weigh somewhere between 3 and 4 pounds, about the size of a small dog. Most people have probably seen a turkey vulture. They’re common across all of the Americas. They’re all over North America, Central and South America. And as you’re driving down the road, they’re going to be the large black bird that’s just kind of hanging out, cruising around in the air, somewhere above the highway.

CHARLES BERGQUIST: When I see those vultures circling over the highway, what are they doing? Are they just looking for roadkill?

JONATHAN RADER: Yeah. When they’re circling like that, they’ve probably found a thermal, which is a highly localized, rising updraft of warm air. And this is one of the ways that vultures can take advantage of energy from the environment. They put themselves into one of those thermals. The air is rising, and they just hang out and rise with the air. It’s a really energetically inexpensive way to gain some altitude. And then they can glide around and look for roadkill or carrion laying on the landscape.

CHARLES BERGQUIST: They live in places, then, with very different elevations. How big a range of elevation do they need to deal with? What’s the span here?

JONATHAN RADER: Well, like I said, they live all across the Americas, which means that they live at the coast. They live in the highest mountains, in the Rockies, the Andes. So we’re talking about up to 15,000 feet of elevation change just on the land. And then they also have been observed flying at ridiculous heights. So airline pilots have reported seeing them as high as 11,000-12,000 feet in the air. There are some reports of seeing them higher, but unsure about those.

CHARLES BERGQUIST: Wow. We recently visited Colorado, and I felt that the air was thinner. But how much thinner is it at 11,000 feet up, or wherever these turkey vultures are flying?

JONATHAN RADER: Oh, at 11,000 feet, it’s almost 40% thinner.

CHARLES BERGQUIST: Wow. So if you need to be able to fly in all of these different conditions, what does that do to your flight capabilities? Is it 40% harder to fly at 40% less air?

JONATHAN RADER: Yeah. So the decrease in lift is kind of linear. If you take any individual flying thing, there are three main things that go into how much lift it can produce. It’s the size of their wings, the density of the air, and how fast they’re flying. And there’s some other things that go into it, but those are mostly comparisons among different kinds of flyers. And so if you’ve got a 40% reduction in air density, yeah, that means that you’re making 40% less lift. And so you have to do something about it.

CHARLES BERGQUIST: So how does the turkey vulture, then, stay aloft even in these low-lift conditions?

JONATHAN RADER: Well, that was the point of our study. We got interested in this when I was starting my PhD work, also at UNC Chapel Hill, with Ty Hedrick. And some other work has been done, showing that individual birds fly faster as they climb up in altitude. And we were interested in whether or not that same kind of pattern happens across different populations of birds that live at different elevations. So we started at home, in Chapel Hill, which is about 80 meters of elevation– 400-ish feet– and recorded a bunch of birds flying there. And then we went up to high elevation, in Wyoming, and recorded the birds there.

And what we found is that the birds in Wyoming, at high elevation, fly faster.

CHARLES BERGQUIST: Is that linear, too? I mean, do you have to fly 40% faster to make up for the 40% less lift?

JONATHAN RADER: No, this one actually works out better in the vulture’s favor. If you look at the lift equation, the amount of lift that you get out of an increase in speed is actually speed-squared. So they don’t have to increase linearly. A smaller amount of increased speed will give them the appropriate amount of extra lift.

CHARLES BERGQUIST: So this faster flight, does this come at a big metabolic cost for the birds, or do they have to consume more when they’re in higher elevations to deal with increased energy expenditure or something?

JONATHAN RADER: That’s the lovely thing about vultures. They’re kind of lazy. And so this increase in airspeed is a way of getting around having to do something energetically expensive, like flapping more or flapping harder. By just increasing their airspeed, they’re able to get around this lift problem without expending a lot of energy. And that’s great because vultures eat carrion, which means there’s not a lot of foraging available on the landscape.

CHARLES BERGQUIST: So wait. How do you get a higher airspeed without flapping faster or harder?

JONATHAN RADER: It turns out that the drag forces that are restraining the vulture flight also decrease with air density, and by about the same amount.

CHARLES BERGQUIST: So they don’t have to do anything to go faster. They just are going faster at these higher elevations?

JONATHAN RADER: Yeah, that’s how it works out. I mean, it didn’t have to be that way. We could have gone up to the high elevation and we could have found similar adaptation of higher-elevation birds having larger wings or something like that. But we didn’t find any evidence of that. And we didn’t find any evidence that they’re flapping more or flapping harder at high elevation.

CHARLES BERGQUIST: So you mentioned other adaptations that birds have for this flight problem. Do you need bigger wings or different feathers or something than low-elevation birds?

JONATHAN RADER: Yeah, exactly. Some other studies have found that high-elevation birds have proportionally larger wings when you compare them to low-elevation birds. And then, in some species of hummingbirds particularly, when they’re flying at high elevation, they’re just flapping harder. They’re putting out more muscular power in order to move enough air to keep themselves aloft.

CHARLES BERGQUIST: When we talk about birds needing bigger wings for dealing with different elevations, is that just a species-level thing– like, robins have smaller wings than vultures? Or do you find that vultures living in Wyoming have larger wings than vultures living in North Carolina?

JONATHAN RADER: Well, It happens across species. So high-elevation birds, in general– and this has mostly been studied in songbirds. There’s no evidence that there are differences in wing size among vultures– at least not turkey vultures.

CHARLES BERGQUIST: So a vulture is a vulture? You don’t have high-land vultures and low-land vultures?

JONATHAN RADER: Kind of, yeah. Yeah. But it’s a great question because– and exactly what you’re asking has been found in songbirds. High-elevation birds have bigger wings. Low-elevation birds have comparatively small wings. And then there are a couple of species that have been studied in detail across elevations. And the ones that live at higher elevation do seem to have bigger wings.

CHARLES BERGQUIST: Interesting. On a human level, do airplanes also need to speed up when they’re up high, or do human engineers use a different way to get around this problem?

JONATHAN RADER: No, the same airplane will increase its cruising speed at higher altitudes than when they’re flying at lower altitude.

CHARLES BERGQUIST: Staying with aircraft, I know that heat can lead to a similar thin-air problem. Do birds have to fly faster when it’s hotter out?

JONATHAN RADER: Yes. So in our study, we had to correct for a thing that’s called density altitude. And so there are a number of things that go into affecting air density. One of them is, of course, the elevation question that we were directly after. But also humidity reduces air density. Even though, on a humid day, it feels like you’re walking through pea soup, the air density is actually lower when it’s really humid. And then heat also reduces the air density.

CHARLES BERGQUIST: What about winds up there? I mean, it can get pretty windy at high elevations.

JONATHAN RADER: Yeah. And in fact, many of the days that we spent recording in Wyoming were quite windy. In fact, the wind speeds sometimes approached the flight speeds of the birds. And so we were interested in how the birds deal with that. And it turns out, in that case, they also increased their airspeed enough that they can still make some forward progress into the strong headwind.

CHARLES BERGQUIST: So where do you go with this next? What’s the next question that you want to answer about this problem?

JONATHAN RADER: So I’ve become really interested in just how lazy vultures can be. Like I said, they live a life that is resource poor. And that means that they have to be absolute masters at extracting as much energy from the world around them as they can, which means that I suspect that they’re exquisitely adapted to taking advantage of small air currents. We know that they take advantage of rising thermals in order to gain altitude.

So I think that a really interesting thing to do would be to track a bunch of different vultures flying around the landscape and try to build an energy budget for the birds for each day that they’re out foraging.

CHARLES BERGQUIST: Dr. Jonathan Rader is a postdoctoral research associate in biology at the University of North Carolina, Chapel Hill. Thanks so much for being with me today.

JONATHAN RADER: You’re very welcome. It’s been an honor to be here. Thank you.

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As Science Friday’s director and senior producer, Charles Bergquist channels the chaos of a live production studio into something sounding like a radio program. Favorite topics include planetary sciences, chemistry, materials, and shiny things with blinking lights.

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