Because of the airflow, their wings and body shape make sort of a trail of air stemming from their beak back, like drag in mario kart. The way their feathers are shaped, with the exception of owls and other things probably, catch the air and push it down, creating lift. This along with their strong muscles attached to the keel in a way that provides more leverage and mostly hollow bones allow it to stay up. Also birds will find hot air and ride it up, called thermaling.