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Birds use the same four forces of flight as airplanes to fly. When birds are not flapping their wings, you could compare them to airplanes: their wings are shaped like an airfoil. This shape creates lower air pressure on the top of the wing, pushing them up, just like how lift works on a plane.

In order to create thrust, birds flap their wings. As they push on the air, the air pushes them forward and up, generating both lift and thrust. When they lift their wings back up, they bend them slightly so that the wings don't push on the air as much on the way up. As with airplanes, this is slightly simplified and there are other factors like angle of attack that come into play as well.