The amount of lift generated by a wing is determined by its shape, size, and speed. The shape of the wing determines how much air it can push down, while the size of the wing determines how much air it can push. The speed of the wing determines how fast the air is moving over it.
For a slow flying bird, the wing is larger and moves slower than the wing of a fast flying bird. This means that the slow flying bird's wing can push down more air and generate more lift.