If I had to guess, I would suppose that they are not recognising specific dunes (which would be constantly changing), but patterns in the dunes caused by prevai

If I had to guess, I would suppose that they are not recognising specific dunes (which would be constantly changing), but patterns in the dunes caused by prevailing winds. I am sure there is something analogous to the Polynesian voyagers recognising patterns in the waves and currents of the ocean.