A Ruppell's vulture (Gyps rueppellii) collided with a commercial aircraft over Abidjan, Ivory Coast, at an altitude of 37,000 feet in November 1973. The impact damaged one of the aircraft's engines, but the plane landed safely. The species is rarely seen above 20,000 feet.
In 1967, about 30 whooper swans (Cygnus cygnus) were spotted at an altitude of just over 27,000 feet by an airline pilot over the Western Isles, UK. They were flying from Iceland to Loch Foyle on the Northern Ireland/Republic of Ireland border. Their altitude was confirmed by air traffic control. (obelus.com)
A hummingbird or sparrow released at 50,000ft would almost certainly asphyxiate before it reached a low enough altitude to breath. Those smaller birds could not 'glide' down because of their small secondary wing surface area.