[stouset]

There is a speed difference between birds and the ambient air. Birds don't just drift aimlessly with the wind, they flap their wings and gain airspeed. But from their perspective, the "speed" at which they're flying is entirely relative to the mass of air, and not the ground.

This isn't some mystery, it's the same way boats and planes work. Consider a plane flying at 100 knots of airspeed. If the mass of air they're in is moving perpendicular to the plane at 50 knots, the plane will track diagonally across the ground even though it's pointed forward. The plane won't experience side loads because it's tracking 50 units sideways (with respect to the ground) for every 100 units it moves forwards, the exact same as the "wind". If the plane is instead in a 100 knot headwind, it will be stationary with respect to the ground. It won't drop out of the air, but it also won't make headway to its destination either.

From a mechanics perspective, neither the plane nor the bird care about what the ground is doing once they're airborne. The only thing they care about is the mass of air they're aloft in.