The highest altitude that I know of that a bird has been observed flying is
a record of a Griffon (I forget the species) at 27,000 feet. It was
published as a tiny note in Wilson Bulletin many years ago. There are
many records of waterfowl and geese migrating over mountain ranges
e.g. the Himalayas) at altitudes in excess of 20,000 feet.

Christopher W. Thompson
Research Scientist, Washington Dept. of Fish and Wildlife
Research Associate, Burke Museum and Dept. of Zoology, Univ. of WA