How do bats birds and most insert overcome the force of gravity and fly?

Birds is use they wing to keep them up. They flap and up in air.

By moving their wings in a flapping motion, along with angling their bodies and internally measuring their speed, they are able to maintain a constant elevation. Some birds travel in V forms to ease the burden on the bird behind them, constantly swapping positions to keep any one bird from having a harder time than another.