Which bird can fly over Mount Everest?

That is thought to be the Bar-Headed Goose, although you may want to take that with a pinch of salt.

The Bar-headed Goose is thought to be one of the world's highest flying birds, having been heard flying across Mount Makalu (the fifth highest mountain on earth at 8,481 metres (27,825 ft)) and apparently seen over Mount Everest (8,848 metres (29,029 ft), although this is a second hand report with no verification).