Resultant Force - continued!
With regard to Sand Hills circling in their migratory flights in order to catch rising thermals created from heated fields; I refute this theory for several reasons.
First of all, I'm not denying that the cranes are receiving lift from thermals; I'm sure they do. My point is - if you care to think about it, very few days will provide significant thermals, that are rising and at the same time as well as 'drifting with the wind' in the same direction as the migratory routes.
Further, the size of the overlapping 'race track' described by the ever circling flock seems consistently large; while thermals are most often quite small, with the occasionally massive thermal lifting off now and then.
I think we might find the flocks are circling on days when atmospheric stability is the rule and as such - no rising air is possible.
Although I am horrified to envision various 'radar' emissions attacking our wildlife more or less unthinkingly, it occurs to me that existing Doppler Weather Radar data might advance our knowledge of Sand Hill migratory habits. Unfortunate an approach as it may be. library time is up bye