C'mon, now. Binocular depth perception ends for humans at about 20 feet and
is only really useful up to arms length, yet we can still judge distance
well. Since the bird is moving, they can use dynamic field depth perception
that has nothing to do with interocular distance.

Close one eye and move your head back and forth or up and down. You will
see what I mean. Birds and other small animals are observed to move their
heads constantly to better judge distances.

Ever ride one of those glass elevators (lifts) on the outside of a tall
building? Did you notice how powerful the impression of climbing is?

I still claim that they can see themselves rise away from the ground.
Excellent vision and the experience to use it to the fullest is the likely
explanation. It's the simplest explanation and requires no internal vario.

Bill Daniels


"CV" wrote in message
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Bill Daniels wrote:
It's pretty well established that soaring birds have extremely good
eyesight. I think they just see the ground moving away when they are
climbing.

Good eyesight is one thing, precision at judging distance another,
the latter basically dependent on how far apart your eyes are.

No way can they judge their climb rate based on seeing the ground
moving away.

Thermals also have a lot of stuff in them like insects and seeds.
The birds are probably able to see these rising at a considerable
distance

That could be, and other visual cues as well.

CV