Flapping

When a bird flaps its wings they are still moving through the air so they still provide lift but it rotates the lift component forward to provide thrust. This counteracts the drag force and provides the bird with airspeed. At the same time it also increases the lift to counteract the weight causing the bird to climb or maintain its height.

There are two stages to a birds flap, the down stroke and the up stroke. Most of the thrust is provided with the down stroke but depending on the shape of the birds wing the up stroke can also provide some thrust. When the down stroke happens the wing is fully extended to maximize its effect on the surrounding air. However on the up stroke the wings are usually folded inwards to minimize the energy loss of bringing the wing back into the up position.


        Soaring

Sometimes birds don't need to flap their wings to stay in the air. They are able to stay aloft using rising air currents known as thermals or ridge lift. this enables some birds to evolve to have massive wings and become very large. When a bird gets as large as it can be it would have to flap to often and hard in order to provide enough thrust. So very large birds have adapted to have large wing area to weight ratios which maximizes their lift. this enables them to better stay airborne on thermals. Simply by staying alert and knowing where to fly they are able to stay aloft. A remarkable evolutionary feet.