Up-wash and down-wash
As a bird flaps, a rotating vortex of air rolls off each of its wingtips. These vortices mean that the air immediately behind the bird gets constantly pushed downwards (down-wash), and the air behind it and off to the sides gets pushed upwards (up-wash).
While trailing vortices are the price one must pay for generating lift, their primary effect is to deflect the flow behind the wing downward. This induced component of velocity is called down–wash, and it reduces the amount of lift produced by the wing.
In order to make up for that lost lift, the wing must go to a higher angle of attack, and this increase in angle of attack increases the drag generated by the wing. We call this form of drag as induced drag because it is “induced” by the process of creating lift.
These vortices are generally undesirable because they create a down–wash that increases the induced drag on a wing in flight. However, this down–wash is also accompanied by an up–wash that can be beneficial to a second wing flying behind and slightly above the first. The tip vortices cause additional down–wash (or down–flow) behind the wing within the wingspan.
For an observer fixed in the air, all the air within the vortex system is moving downward whereas all the air outside the vortex system is moving upward. A bird flying perpendicular to the flight path of another bird creating the vortex pattern will encounter up–wash, down–wash, and up–wash in that order. The gradient, or change of down-wash to up–wash, can become very large at the tip vortices.