I've long had this pet theory that soaring birds have a fine vision that allows them to see thermals (warm ascending currents) directly, through changes in the air's refraction index. I've searched a bit, but no research seems to ever have considered the question. Just a wild theory, though.
Interesting. I don't think that would be possible - the technique shown here relies on a mirror to get interferometry between the two light paths. I can't see how interferometry would be available to soaring birds (between its two eyes, perhaps?). Although nature has come up with some very surprising and impressive inventions before, so I might be wrong.
I would guess they're just very good at spotting the conditions which lead to thermals.
EDIT: of course, you can directly see thermals via the shimmering / "mirage" effect. It's normally obscured by the background noise, but maybe soaring birds are attuned to that. That's much more plausible, but a different effect to that shown here.
You're right of course, it can't rely on collimated light like the normal Schlieren system. But the background-oriented Schlieren technique, mentioned elsewhere in this thread, is more reasonable to look at for inspiration, and it provides pretty much the same end result.
I suppose it can be thought of as a differential filter: only minute changes are interpreted to create a representation of the flow of air.
