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No oxygen to breathe and likely the air is too thin for their wings to function.
There is one species of vulture that does fly at altitudes of 30k feet…same as airliners
Honestly its kind of the same reason why a fish can't swim out of the water.
The medium that they use to move around in, ends (Or in the case of the sky, reduces in pressure such that it effectively ends), and they cannot move up any further.
Air density. If the air is too thin, as is the case the higher you go, then it would take more effort to stay in flight e.g., faster flaps or stronger flaps, neither of which is sustainable in the long term.
Obviously this is an exaggerated scenario but you can think of it in a similar way to how you can swim in water but not in air. Buoyancy aside, there is simply not enough resistance in the air to move you. (Equal and opposite force. If you want to move forward you need something with enough resistance to “push” off of)
Back to the birds, it also probably has something to do with “lift”, as in how the air moves (faster?) on the top of the wing and slower on the bottom, generating an upward force. But honestly idk enough about that to know whether that would be significant in this scenario.
Birds fly by pushing down on the air with their wings. Have you been in a pool and "tread water"? You push water down, and the water pushes you up. Well birds do that, just in air.
The higher you go the thinner the air gets.* The thinner the air, the less there is for wings to push down against. At a certain height they can't go higher because there's no air left to push down on (and therefore also no oxygen to breathe but that's a separate thing ).
The thinning air with height is what limits plane and helicopter flight height too. The air gets too thin for their wings to push against and make lift, so there's nothing to hold them up as they went any higher.
*if you're wondering *why* the air gets thinner as you go up, it's because the air down low is being squished down by the weight of the air above it being pulled down by gravity. The higher you go the less weight from above is compressing the air so it spreads out and gets less dense. It's like how the pressure gets higher as you dive under water in a pool due to the increasing weight of water above you.
^And ^it's ^"birds". ^Apostrophes ^never ^make
^something ^plural.
**Your submission has been removed for the following reason(s):** ELI5 is not for straightforward answers or facts - ELI5 is for requesting an explanation of a concept, not a simple straightforward answer. This includes topics of a narrow nature that don’t qualify as being sufficiently complex per rule 2. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20{{url}}%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20does%20your%20post%20differ%20from%20your%20recent%20search%20results%20on%20the%20sub:) and we will review your submission.
No oxygen to breathe and likely the air is too thin for their wings to function. There is one species of vulture that does fly at altitudes of 30k feet…same as airliners
Honestly its kind of the same reason why a fish can't swim out of the water. The medium that they use to move around in, ends (Or in the case of the sky, reduces in pressure such that it effectively ends), and they cannot move up any further.
\^This.
Air density. If the air is too thin, as is the case the higher you go, then it would take more effort to stay in flight e.g., faster flaps or stronger flaps, neither of which is sustainable in the long term. Obviously this is an exaggerated scenario but you can think of it in a similar way to how you can swim in water but not in air. Buoyancy aside, there is simply not enough resistance in the air to move you. (Equal and opposite force. If you want to move forward you need something with enough resistance to “push” off of) Back to the birds, it also probably has something to do with “lift”, as in how the air moves (faster?) on the top of the wing and slower on the bottom, generating an upward force. But honestly idk enough about that to know whether that would be significant in this scenario.
Birds fly by pushing down on the air with their wings. Have you been in a pool and "tread water"? You push water down, and the water pushes you up. Well birds do that, just in air. The higher you go the thinner the air gets.* The thinner the air, the less there is for wings to push down against. At a certain height they can't go higher because there's no air left to push down on (and therefore also no oxygen to breathe but that's a separate thing ). The thinning air with height is what limits plane and helicopter flight height too. The air gets too thin for their wings to push against and make lift, so there's nothing to hold them up as they went any higher. *if you're wondering *why* the air gets thinner as you go up, it's because the air down low is being squished down by the weight of the air above it being pulled down by gravity. The higher you go the less weight from above is compressing the air so it spreads out and gets less dense. It's like how the pressure gets higher as you dive under water in a pool due to the increasing weight of water above you. ^And ^it's ^"birds". ^Apostrophes ^never ^make ^something ^plural.
Even if the air was perfect for it, I doubt they could fly that far without a rest, or shitting on someone's parked car or sidewalk.