In article .com,
"John" wrote:

not to mention flying (for birds) is high metabolism work . . .. how
do they make it work that high

I believe I read long ago that there are migratory birds of some variety
that make a (nonstop?) migratory flight from the Indian Ocean across the
Himalayas, including flying higher than Everest. Apparently bird
metabolism allows some substantial sustained flight at altitudes above
where humans can even maintain life long term . . .?