Turning
A bird turns by banking—the banking is not a consequence of the turning, it is the cause of it. Lift is perpendicular to the surface (plane) of the wings, so when the bird banks, there is a horizontal component of lift which causes the change in direction. (If instead of banking, the bird’s tail were to be used in the manner of a boat rudder, it would merely cause the bird to sideslip through the air: although the body would turn, the direction of motion would remain the same).
While a bird turning by banking is an easy thing to observe, more subtle is how the bird initiates and ends the banking, and thus the turn. To initiate a bank, the bird must increase the lift on the outer wing (or decrease it on the inner wing). To terminate a bank, the bird increases lift on the inside wing. To do this, there are a number of things it can alter on one wing or the other: angle of attach, camber, surface area, or stroke amplitude. The changes needed to accomplish these things are probably too subtle to be easily seen in the field.
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An Osprey banking and turning. When the bird banks, there is a horizontal component to the lift which changes the flight direction.
This Bald Eagle is executing a sharp turn and is clearly using its wings (and tail) asymmetrically. It looks as if the left wing is in a downstroke and the right wing is in an upstroke.