1/ The downwind turn fallacy should have become extinct with the advent of GPS showing groundspeed. Easily compare actual airspeed with actual groundspeed. Actual numbers are hard to dispute, even for a Flat Earther.
2/ Our bird theorist needs to get in an aeroplane and fly in ground effect, over varying surfaces, including swells.
That is, if a dreaded downwind turn doesn't get the better of him first....