Circling for rodents?

Bill Daniels wrote:

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.


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.
Nevertheless we can't decide if a glider or a bird is climbing when watching
them from below just by watching the change of their size during a short
time, except when they are very low. However I agree that after watching
a bird for a long time, as it changed from a beautiful thing with discernable
separate feathers at the tips to a vanishing little point in the sky, I can
say that it was climbing.

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.

On the other hand, do you think you'd be able to spot a mouse from
3000ft? No problem for some birds of prey.
--
Mike Lindsay

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

Bill Daniels wrote:

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

Well, the changes in (apparent) size of objets is nothing else than a change
in an angle. I agree that looking for such an angle just below the glider is
not what will maximize the change for a given height change. If your method
is by watching the change in the angle of the directions of some fixed ground
feature and the horizon, it can easily be shown that the maximum rate of change
is obtained by looking at some feature at 45 degrees below the horizon. In this
case, the change rate, in radians per climbing meter, is 1/(2*height), at 1000
meters the rate of change is of 1.7 minute per meter, in order to see a 1
degree change when climbing at 2 m/s, you have to wait 35 seconds.
Difficult but workable. I should try it in my next flight, although I think I
will not be able to perceive changes below 10 degrees, when looking at 45 degrees.
Looking toward a more horizontal direction should provide better senitivity, as
the fixed feature and the horizon are together in the visual field, but the
rate of change of the angle is much lower. When looking in the same direction
as on final approach, i.e. the direction of the 1/10 slope, the rate of change
is 1/(10*height) radians per climbing meter, 5 times lower than at 45 degrees,
you have to wait nearly 3 minutes climbing at 2 m/s to see a 1 degree change
at 1000 m.