On a few occasions, I have observed soaring birds fly quickly towards a point where they found a thermal - sometimes a distance of about a kilometer. The speed and direction of them indicated to me that they were flying to an objective and not just hunting randomly. If so, this means that they can detect thermals remotely using their senses. If we can figure out how they are doing this, there is a chance we can build instruments to replicate their method.

Having studied the possibility of using electrostatic methods (thermals advect space charge from the ground and dust devils are actively charged by particle collision), this is very short range and not likely the mechanism birds are using. I would guess it has to be visual.

Mike