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How High Can Birds Fly? - Video học tiếng Anh
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How High Can Birds Fly?
How High Can Birds Fly?
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คำบรรยาย (204)
0:00
In 1973, an airliner struck a bird
0:03
called a Ruppell's Griffon vulture,
0:05
which on its own isn't that weird.
0:07
Planes hit birds pretty regularly during
0:09
takeoffs and landings, but this
0:10
collision happened at a cruising height
0:12
of over 11,000 m. That's way above the
0:16
height at which most birds fly, which it
0:18
makes me wonder, what is the highest a
0:20
bird can actually fly?
0:22
>> [music]
0:22
>> Hi, I'm Cameron and this is MinuteEarth.
0:25
Birds don't tend to fly higher than they
0:26
absolutely need to for the same reason
0:29
you don't sprint when you could walk.
0:31
It's [music] difficult and tiring. So,
0:33
we can't necessarily get the answer to
0:35
this question through observation. I
0:37
mean, I guess we could jump a bunch of
0:39
birds out of airplanes and see what
0:40
happens, but our AdSense revenue
0:42
definitely isn't going to cover that.
0:44
Plus, we're not monsters. So, let's use
0:46
our understanding of aerodynamics,
0:48
scaling laws, and biology to science our
0:51
way to an approximate answer. There are
0:53
two things that limit how high a bird
0:55
can fly. It's ability to stay aloft as
0:57
the air pressure decreases and on a much
1:00
more basic level, it's ability to stay
1:02
alive as the temperature and amount of
1:04
oxygen decreases. So, first, [music]
1:06
let's figure out which bird could
1:08
survive at the highest altitude. Oxygen
1:10
supplies birds the energy they need to
1:12
stay warm, but at higher altitudes,
1:14
there's less oxygen available and the
1:16
temperature is much colder. So, a bird's
1:17
ability to survive high up in the air
1:19
depends on how efficiently they use
1:21
oxygen and how well they can retain body
1:23
heat. This paper measured the oxygen use
1:25
of a handful of birds and found that
1:27
very generally, their overall oxygen use
1:30
increases with mass. We can then adjust
1:32
according to other traits like how much
1:34
energy their flight muscles require and
1:36
how much insulation their feathers
1:37
provide. From all of this, we can
1:39
calculate the altitude at which each
1:41
bird should suffer [music] from
1:42
hypothermia. Let's call this their
1:44
popsicle point. If we then compile a
1:46
data set of flying birds and plug their
1:48
data into these equations, we can see a
1:50
general pattern emerge. Larger birds
1:52
[music] 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
[music] Because the air is less dense
2:19
the higher you go, less air is available
2:21
at higher altitudes to push upward
2:23
against a bird's wings and create that
2:24
lift. A bird's ability to stay aloft
2:26
high in the air depends on its weight,
2:28
[music] size of its wings, and the shape
2:30
and angle of attack of its wings. That's
2:31
a factor called the lift coefficient.
2:33
[music] Combining all of that tells us
2:34
how much lift a bird's wings should
2:36
generate in still air at a given
2:38
altitude. Simple [music] enough at
2:40
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
[music] when they're taking off or about
3:00
to stall. In other words, when the bird
3:02
is trying hardest to generate lift. And
3:04
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. [music]
3:11
From 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
3:17
barometric equation to convert those
3:19
numbers to altitudes to estimate the
3:21
highest point each bird in our data set
3:23
should be able to actually maintain
3:26
flight. Let's call this their lift
3:27
limit.
3:28
>> [music]
3:28
>> In general, the smaller birds have the
3:30
highest lift limits. The hulking mute
3:32
swan would struggle to generate lift at
3:34
a mere 3,800 m, while the puny sand
3:37
martin should be able to glide nearly
3:39
19,000 m. Of [music] course, air moves
3:41
and it's not uniformly dense at given
3:43
altitudes, so there's definitely some
3:46
wiggle room here, which will be a
3:47
surprise tool that's going to help us
3:49
later. But in any case, a bird with a
3:51
higher lift limit should be able to fly
3:53
higher than a bird with a lower one.
3:55
Now, [music] let's combine our lift
3:57
limit data with our Popsicle Point data.
3:59
We can see that lots of birds, like the
4:01
[music] Mistle Thrush, can theoretically
4:02
fly super high, but would freeze long
4:04
before they got there. And then there
4:06
are a bunch of other birds, like the
4:08
Wandering Albatross, that could likely
4:10
survive at super high altitudes, but
4:12
wouldn't be able to actually maintain
4:14
flight up there. That leaves us with a
4:16
small cluster of birds with relatively
4:18
high Popsicle Points and high lift
4:20
limits. Mathematically, these should be
4:22
the highest flying birds, and for the
4:24
most part, they're geese. The Greylag
4:26
Goose, the Bean Goose, the Canada Goose,
4:28
and the Bar-headed Goose should be able
4:30
to fly as high as 8,000 m 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
Stork, which based [music] on its
4:47
Popsicle Point and Lift Limit is our
4:49
predicted highest flying bird. It
4:52
potentially fly up to about 10,500 m. In
4:56
reality, it doesn't fly anywhere near
4:58
that high. But remember, birds don't
5:00
necessarily fly as high as they might be
5:02
physically capable of. But wait, what
5:04
about the Ruppell's Griffon? A bird
5:06
[music] we know for a fact can fly
5:09
higher than 11,000 m. Our math suggests
5:12
that it is lift limited a lot lower than
5:14
that, about 8,200 m. But this is where
5:17
theoretical calculations fall short
5:19
without some additional real-world
5:20
knowledge. See, the Ruppell's Griffon
5:22
likes to soar on thermals, warm columns
5:25
of 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
5:34
high Popsicle Point birds ride such
5:37
supercharged thermals. So, the Ruppell's
5:39
Griffon is likely the bird capable of
5:40
the highest flight. [music] With the
5:42
right thermal, it might even reach its
5:44
very generous Popsicle Point of 15,000
5:47
m. Turns out that bird might have had a
5:49
lot of climbing left to do.
5:55
You might have noticed that this video
5:57
is [music] chock-full of all sorts of
5:58
calculations that I basically ripped my
6:01
hair out trying to make sure I got
6:02
right. [music] It would have been great
6:04
if I had a brilliant tutor sitting next
6:06
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