How High Can Birds Fly?

[00:00] In 1973, an airliner struck a bird

[00:03] called a Ruples Griffin vulture, which

[00:05] on its own isn't that weird. Planes hit

[00:07] birds pretty regularly during takeoffs

[00:09] and landings. But this collision

[00:11] happened at a cruising height of over

[00:13] 11,000 m. That's way above the height at

[00:16] which most birds fly, which it makes me

[00:19] wonder, what is the highest a bird can

[00:21] actually fly. Hi, I'm Cameron and this

[00:24] is Minute Earth. Birds don't tend to fly

[00:26] higher than they absolutely need to for

[00:28] the same reason you don't sprint when

[00:30] you could walk. It's difficult and

[00:32] tiring. So, we can't necessarily get the

[00:35] answer to this question through

[00:36] observation. I mean, I guess we could

[00:38] drop a bunch of birds out of airplanes

[00:40] and see what happens, but our AdSense

[00:42] revenue definitely isn't going to cover

[00:43] that. Plus, we're not monsters. So,

[00:46] let's use our understanding of

[00:47] aerodynamics, scaling laws, and biology

[00:50] to science our way to an approximate

[00:52] answer. There are two things that limit

[00:54] how high a bird can fly. Its ability to

[00:57] stay aloft as the air pressure

[00:58] decreases. And on a much more basic

[01:01] level, its ability to stay alive as the

[01:03] temperature and amount of oxygen

[01:05] decreases. So, first, let's figure out

[01:07] which bird could survive at the highest

[01:09] altitude. Oxygen supplies birds the

[01:12] energy they need to stay warm, but at

[01:13] higher altitudes, there's less oxygen

[01:15] available and the temperature is much

[01:16] colder. So, a bird's ability to survive

[01:19] high up in the air depends on how

[01:20] efficiently they use oxygen and how well

[01:22] they can retain body heat. This paper

[01:24] measured the oxygen use of a handful of

[01:26] birds and found that very generally

[01:28] their overall oxygen use increases with

[01:30] mass. We can then adjust according to

[01:33] other traits like how much energy their

[01:35] flight muscles require and how much

[01:36] insulation their feathers provide. From

[01:38] all of this, we can calculate the

[01:40] altitude at which each bird should

[01:42] suffer from hypothermia. Let's call this

[01:44] their popsicle point. If we then compile

[01:46] a data set of flying birds and plug

[01:48] their data into these equations, we can

[01:50] see a general pattern emerge. Larger

[01:52] birds can theoretically survive at

[01:54] higher altitudes than smaller birds.

[01:56] There are exceptions, of course. This is

[01:58] biology after all, but our calculations

[02:00] suggest that there are a bunch of birds

[02:02] that could potentially survive above

[02:04] 10,000 m. And the largest bird in our

[02:06] data set, the wandering albatross, might

[02:09] be able to survive as high as 17,000 m.

[02:12] But remember, we also need to figure out

[02:13] if any of these birds could actually

[02:15] stay aloft at such high altitudes.

[02:17] Because the air is less dense the higher

[02:19] you go, less air is available at higher

[02:21] altitudes to push upward against a

[02:23] bird's wings and create that lift. A

[02:25] bird's ability to stay a loft high in

[02:27] the air depends on its weight, size of

[02:28] its wings, and the shape and angle of

[02:30] attack of its wings. That's a factor

[02:32] called the lift coefficient. Combining

[02:34] all of that tells us how much lift a

[02:36] bird's wings should generate in still

[02:38] air at a given altitude. Simple enough

[02:40] at first. Uh, but while weights and

[02:42] wingspans and whatnot are easy enough to

[02:44] measure, the wing shapes and angles

[02:45] aren't because a bird's wing shape

[02:47] changes as it flies. I'll save you the

[02:49] long explanation of my rationale here

[02:51] and just say that this is about where I

[02:53] go out on a bit of a limb. The lift

[02:54] coefficient for the birds in our data

[02:56] set peaks at about 1.5 or so, and that's

[02:58] when they're taking off or about to

[03:00] stall. In other words, when the bird is

[03:02] trying hardest to generate lift. And

[03:05] since staying aloft is likely a struggle

[03:06] at a bird's maximum altitude, this is

[03:08] probably a pretty good estimate of the

[03:10] lift coefficient at this point. From

[03:11] there, we can find the lowest air

[03:13] pressure at which each bird could

[03:14] generate sufficient lift to keep its

[03:16] mass aloft. And then use our friend, the

[03:18] barometric equation to convert those

[03:19] numbers to altitudes to estimate the

[03:21] highest point each bird in our data set

[03:24] should be able to actually maintain

[03:26] flight. Let's call this their lift

[03:27] limit. In general, the smaller birds

[03:30] have the highest lift limits. The

[03:32] hulking mute swan would struggle to

[03:33] generate lift at a mere 3,800 meters,

[03:36] while the puny sand martin should be

[03:38] able to glide at nearly 19,000 m. Of

[03:41] course, air moves and it's not uniformly

[03:43] dense at given altitudes, so there's

[03:45] definitely some wiggle room here, which

[03:47] will be a surprise tool that's going to

[03:49] help us later. But in any case, a bird

[03:51] with a higher lift limit should be able

[03:53] to fly higher than a bird with a lower

[03:55] one. Now, let's combine our lift limit

[03:57] data with our popsicle point data. We

[03:59] can see that lots of birds like the

[04:01] missile thrush can theoretically fly

[04:02] super high but would freeze long before

[04:05] they got there. And then there are a

[04:07] bunch of other birds like the wandering

[04:08] albatross that could likely survive at

[04:11] super high altitudes but wouldn't be

[04:13] able to actually maintain flight up

[04:14] there. That leaves us with a small

[04:16] cluster of birds with relatively high

[04:18] popsicle points and high lift limits.

[04:21] Mathematically, these should be the

[04:22] highest flying birds. And for the most

[04:24] part, they're geese. The grey lag goose,

[04:27] the bean goose, the Canada goose, and

[04:28] the barheaded goose should be able to

[04:30] fly as high as 8,000 meters or so,

[04:32] according to our calculations. And this

[04:34] matches up pretty well with what

[04:36] scientists have actually observed. Like

[04:38] during its migration over the highest

[04:39] mountain range on the planet, the

[04:41] bar-headed goose can reach altitudes of

[04:43] over 7,000 m. And then there's the white

[04:46] str, which based on its popsicle point

[04:48] and lift limit, is our predicted highest

[04:50] flying bird. It could potentially fly up

[04:53] to about 10,500 m. In reality, it

[04:57] doesn't fly anywhere near that high. But

[04:59] remember, birds don't necessarily fly as

[05:01] high as they might be physically capable

[05:03] of. But wait, what about the Ruples

[05:05] Griffin? A bird we know for a fact can

[05:08] fly higher than 11,000 m. Our math

[05:12] suggests that it is lift limited a lot

[05:14] lower than that, about 8,200 m. But this

[05:17] is where theoretical calculations fall

[05:19] short without some additional real world

[05:20] knowledge. See, the Ruples Griffin likes

[05:22] to soar on thermals, warm columns of

[05:25] rising air that can help birds exceed

[05:27] their mathematical lift limit, sometimes

[05:29] even thousands of extra meters up into

[05:31] the air. Other birds are also known to

[05:33] ride thermals, but none of the other

[05:35] high popsicle point birds ride such

[05:37] supercharged thermals. So, the Ruples

[05:39] Griffin is likely the bird capable of

[05:41] the highest flight. With the right

[05:42] thermal, it might even reach its very

[05:45] generous popsicle point of 15,000

[05:47] meters. Turns out that bird might have

[05:49] had a lot of climbing left to do.

[05:55] You might have noticed that this video

[05:57] is chalk full of all sorts of

[05:59] calculations that I basically ripped my

[06:01] hair out trying to make sure I got

[06:02] right. It would have been great if I had

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