About 700 species of birds occur in North America. In the fossil record, some 150,000 species of birds have been identified. Since there are less than 10,000 species living today, most of the birds that have existed on this planet are now extinct. The fastest bird known is the Peregrine Falcon. Birds fly at many altitudes, depending on where they are going and the purpose for their travel.
Most birds fly within a few hundred feet off the ground.
Birds migrating over large distances often select higher altitudes to take advantage of favorable winds. Some birds have been observed at very high altitudes. Geese can fly over the highest mountain, Mount Everest, at an altitude above 29,000 feet. In South America, the Condor has been observed at altitudes close to 20,000 feet. A vulture collided with a jetliner over West Africa at an altitude at an altitude of 37,000 feet.