[0:00] In 1973, an airliner struck a bird
[0:03] called a Ruples Griffin vulture, which
[0:05] on its own isn't that weird. Planes hit
[0:07] birds pretty regularly during takeoffs
[0:09] and landings. But this collision
[0:11] happened at a cruising height of over
[0:13] 11,000 m. That's way above the height at
[0:16] which most birds fly, which it makes me
[0:19] wonder, what is the highest a bird can
[0:21] actually fly. Hi, I'm Cameron and this
[0:24] is Minute Earth. Birds don't tend to fly
[0:26] higher than they absolutely need to for
[0:28] the same reason you don't sprint when
[0:30] you could walk. It's difficult and
[0:32] tiring. So, we can't necessarily get the
[0:35] answer to this question through
[0:36] observation. I mean, I guess we could
[0:38] drop a bunch of birds out of airplanes
[0:40] and see what happens, but our AdSense
[0:42] revenue definitely isn't going to cover
[0:43] that. Plus, we're not monsters. So,
[0:46] let's use our understanding of
[0:47] aerodynamics, scaling laws, and biology
[0:50] to science our way to an approximate
[0:52] answer. There are two things that limit
[0:54] how high a bird can fly. Its ability to
[0:57] stay aloft as the air pressure
[0:58] decreases. And on a much more basic
[1:01] level, its ability to stay alive as the
[1:03] temperature and amount of oxygen
[1:05] decreases. So, first, let's figure out
[1:07] which bird could survive at the highest
[1:09] altitude. Oxygen supplies birds the
[1:12] energy they need to stay warm, but at
[1:13] higher altitudes, there's less oxygen
[1:15] available and the temperature is much
[1:16] colder. So, a bird's ability to survive
[1:19] high up in the air depends on how
[1:20] efficiently they use oxygen and how well
[1:22] they can retain body heat. This paper
[1:24] measured the oxygen use of a handful of
[1:26] birds and found that very generally
[1:28] their overall oxygen use increases with
[1:30] mass. We can then adjust according to
[1:33] other traits like how much energy their
[1:35] flight muscles require and how much
[1:36] insulation their feathers provide. From
[1:38] all of this, we can calculate the
[1:40] altitude at which each bird should
[1:42] suffer from hypothermia. Let's call this
[1:44] their popsicle point. If we then compile
[1:46] a data set of flying birds and plug
[1:48] their data into these equations, we can
[1:50] see a general pattern emerge. Larger
[1:52] birds can theoretically survive at
[1:54] higher altitudes than smaller birds.
[1:56] There are exceptions, of course. This is
[1:58] biology after all, but our calculations
[2:00] suggest that there are a bunch of birds
[2:02] that could potentially survive above
[2:04] 10,000 m. And the largest bird in our
[2:06] data set, the wandering albatross, might
[2:09] be able to survive as high as 17,000 m.
[2:12] But remember, we also need to figure out
[2:13] if any of these birds could actually
[2:15] stay aloft at such high altitudes.
[2:17] Because the air is less dense the higher
[2:19] you go, less air is available at higher
[2:21] altitudes to push upward against a
[2:23] bird's wings and create that lift. A
[2:25] bird's ability to stay a loft high in
[2:27] the air depends on its weight, size of
[2:28] its wings, and the shape and angle of
[2:30] attack of its wings. That's a factor
[2:32] called the lift coefficient. Combining
[2:34] all of that tells us how much lift a
[2:36] bird's wings should generate in still
[2:38] air at a given altitude. Simple enough
[2:40] at first. Uh, but while weights and
[2:42] wingspans and whatnot are easy enough to
[2:44] measure, the wing shapes and angles
[2:45] aren't because a bird's wing shape
[2:47] changes as it flies. I'll save you the
[2:49] long explanation of my rationale here
[2:51] and just say that this is about where I
[2:53] go out on a bit of a limb. The lift
[2:54] coefficient for the birds in our data
[2:56] set peaks at about 1.5 or so, and that's
[2:58] when they're taking off or about to
[3:00] stall. In other words, when the bird is
[3:02] trying hardest to generate lift. And
[3:05] since staying aloft is likely a struggle
[3:06] at a bird's maximum altitude, this is
[3:08] probably a pretty good estimate of the
[3:10] lift coefficient at this point. From
[3:11] there, we can find the lowest air
[3:13] pressure at which each bird could
[3:14] generate sufficient lift to keep its
[3:16] mass aloft. And then use our friend, the
[3:18] barometric equation to convert those
[3:19] numbers to altitudes to estimate the
[3:21] highest point each bird in our data set
[3:24] should be able to actually maintain
[3:26] flight. Let's call this their lift
[3:27] limit. In general, the smaller birds
[3:30] have the highest lift limits. The
[3:32] hulking mute swan would struggle to
[3:33] generate lift at a mere 3,800 meters,
[3:36] while the puny sand martin should be
[3:38] able to glide at nearly 19,000 m. Of
[3:41] course, air moves and it's not uniformly
[3:43] dense at given altitudes, so there's
[3:45] definitely some wiggle room here, which
[3:47] will be a surprise tool that's going to
[3:49] help us later. But in any case, a bird
[3:51] with a higher lift limit should be able
[3:53] to fly higher than a bird with a lower
[3:55] one. Now, let's combine our lift limit
[3:57] data with our popsicle point data. We
[3:59] can see that lots of birds like the
[4:01] missile thrush can theoretically fly
[4:02] super high but would freeze long before
[4:05] they got there. And then there are a
[4:07] bunch of other birds like the wandering
[4:08] albatross that could likely survive at
[4:11] super high altitudes but wouldn't be
[4:13] able to actually maintain flight up
[4:14] there. That leaves us with a small
[4:16] cluster of birds with relatively high
[4:18] popsicle points and high lift limits.
[4:21] Mathematically, these should be the
[4:22] highest flying birds. And for the most
[4:24] part, they're geese. The grey lag goose,
[4:27] the bean goose, the Canada goose, and
[4:28] the barheaded goose should be able to
[4:30] fly as high as 8,000 meters or so,
[4:32] according to our calculations. And this
[4:34] matches up pretty well with what
[4:36] scientists have actually observed. Like
[4:38] during its migration over the highest
[4:39] mountain range on the planet, the
[4:41] bar-headed goose can reach altitudes of
[4:43] over 7,000 m. And then there's the white
[4:46] str, which based on its popsicle point
[4:48] and lift limit, is our predicted highest
[4:50] flying bird. It could potentially fly up
[4:53] to about 10,500 m. In reality, it
[4:57] doesn't fly anywhere near that high. But
[4:59] remember, birds don't necessarily fly as
[5:01] high as they might be physically capable
[5:03] of. But wait, what about the Ruples
[5:05] Griffin? A bird we know for a fact can
[5:08] fly higher than 11,000 m. Our math
[5:12] suggests that it is lift limited a lot
[5:14] lower than that, about 8,200 m. But this
[5:17] is where theoretical calculations fall
[5:19] short without some additional real world
[5:20] knowledge. See, the Ruples Griffin likes
[5:22] to soar on thermals, warm columns of
[5:25] rising air that can help birds exceed
[5:27] their mathematical lift limit, sometimes
[5:29] even thousands of extra meters up into
[5:31] the air. Other birds are also known to
[5:33] ride thermals, but none of the other
[5:35] high popsicle point birds ride such
[5:37] supercharged thermals. So, the Ruples
[5:39] Griffin is likely the bird capable of
[5:41] the highest flight. With the right
[5:42] thermal, it might even reach its very
[5:45] generous popsicle point of 15,000
[5:47] meters. Turns out that bird might have
[5:49] had a lot of climbing left to do.
[5:55] You might have noticed that this video
[5:57] is chalk full of all sorts of
[5:59] calculations that I basically ripped my
[6:01] hair out trying to make sure I got
[6:02] right. It would have been great if I had
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