I would guess that raptors navigate by ground features, so must have sight of ground to migrate, and probably can't fly without a horizon as reference (unlike geese, which can navigate above clouds and even fly in zero visibility). I fly a light survey plane mostly in West Africa over the Sahel and southern Sahara. The visibility is mostly determined by Harmattan conditions. At its worst, horizontal visibility (to human eye) can drop to under 1000m, the horizon is blurred or even occasionally sloped, and vertical visibility 3000ft or so. However, our survey camera can still take pictures through thick Harmattan, even when the ground is not visible, because its sensors are sensitive over a wider frequency band than the human eye. I believe most raptors can also see in UV frequencies, so I would imagine that the falcons in your study are able to see ground features more easily than pilots can, and are probably rarely troubled by visibility.
The whole area has very strong thermal activity. If I am flying medium level orthophoto (4000ft agl etc) over rocky areas the thermals are so strong that the camera stabilisation cannot cope and I have to abandon the flight. I can feel thermals easily often at 18000 ft. The falcons would have plenty of opportunity for thermal flying.
However, I suspect the main reason for the changes in migrational altitude are due to the very variable wind patterns at different altitudes. It is not unusual to have the wind blowing in opposite directions as little as 4000ft altitude change. I suggest you download a good GRIB viewer (I highly recommend MeteoEarth for Mac platform) and use it to look at the wind variations with altitude.
Last edited by Trim Stab; 12th Nov 2015 at 04:42.