In article , Robert Ehrlich
I can't believe that, except for very low heights. The elevator
experience
you mention mention is for such heights, or at least when something (the
building itself) is very near.

The best processing system (e.g. the bird's brain) cannot infer anything
from missing or non significative input. In the case of climbing, the
only
information on which you say they rely is the change in the apparent size
of ground features. I didn't do the computation, but I bet that the
change
during one full turn is below the optical resolution of a bird's eye. In
this domain, we are better equiped than they are, our eyes are larger.


You don't look down to see height changes, you look out at an angle. You're
not looking for changes in the size of objects, you look for changes in
angles. It's just like we judge height on final approach to landing. I can
judge the strength of thermals visually up to 1000 meters or so and I bet
the birds can do a lot better.

Bill Daniels