> So, I believe that some of the downward force changes into horizontal force as the air-under-high-pressure moved away from the bird.

Except that air molecules can't do that without causing an opposite and equal reaction on other air molecules. If an air molecule is redirected in its course from vertical to horizontal, it must have done so by colliding with other air molecules, so as to preserve the total inertia of all air molecules in question. This is true even if magnetic forces come into play.

My guess is that the scale won't even register oscillations on a noticeable scale. Too bad I can't think of a way to find out if I'm right.