As a human that can fly, occasionally even soar, using hang gliders and sailplanes, I have always wondered how birds find rising air. Soaring pilots use sensitive rate of air pressure instruments called variometers to confirm and measure rising air. Do soaring birds have this built in? Birds also seem to able to anticipate rising air. I can do this by seeing stuff in the air going up faster then I am(birds, bugs, other gliders). Birds do it much better though. turkey vultures and albatross can even exploit wind shears for dynamic soaring. I suspect that most soaring birds can actually “see” the air in movement and in detail, maybe even in infrared. I can glide to a parcel of air that I think might be going up ,but it’s still aerial guesswork. I have read a lot about micro-meteorology, soaring theory, and just plain flying, only to be humbled in the air. How do dey do it?
Birds evidently have a very keen ability to sense differences in air pressure. Homing pigeons have been shown to be able to detect a difference in pressure equivalent to only a 10 meter change in elevation.
Many birds have very keen vision and will be probably be able to detect thermals from a distance by the movement of other birds, insects, floating particles, etc. I don’t think they would be able to detect rising air unless there were some visible particles in it, and I have never heard that they might use infrared for this.
I used to skydive and while under canopy it was really pretty easy to tell when you were catching a thermal or up/down drafts and your rate of descent changed. It was a combination of a few things, you can feel the acceleration, there’s different sounds of the canopy and tension in the steering lines.
You also start to be able to read the terrain - hills mean air will behave this way, water means something else, etc.
That’s for a person in an unusual environment without a few million years of evolution and daily exposure to that mode of travel.
Now take a bird that has been getting around that way all of it’s life and has evolved to fly effectively. Those same basic senses are probably highly tuned (in addition to any ability to see into the IR spectrum or whatnot). I imagine it’s as natural to them as our ability to rapidly cross uneven ground without actively thinking about how to place our feet for each step.