The endangered Ruppell's griffon vulture is the highest flying bird ever recorded, and it can ascend to heights of 37,000 feet. That's the average height at which a commercial airplane travels (and a Ruppell's griffon vulture has indeed been sucked into a jet engine at 37,000 feet). It is also the height at which a human would pass out from lack of oxygen. The vulture doesn't face this problem because it has developed a particular type of hemoglobin, making their use of oxygen more effective. A close competitor is the bar-headed goose, a species that migrates over the Himalayan mountains at heights of 18,000-21,120 feet. It's been reported they've even been seen flying over Mount Makalu, the fifth highest mountain on earth at 27,825 feet.