>> Passerines have very poor depth perception since they lack binocular vision,
<<

Actually, I'll point out that binocular vision is highly over-rated as the
primary source of depth perception. There are many other clues that contribute
to the brain's generation of depth information, with the exception of when the
viewer is ~perfectly~ still.

As a person who totally lacks binocular vision, I have regularly flown an
airplane 6" off a runway for its length, swished baskets from all over the
court, smacked baseballs, caught footballs, shot straights in skeet, and never
had a moving auto accident. (They've always got me while I was stopped at a
light. <G>)

I really got fooled once, though. In the 50's, tired, waiting for a bus, leaning
(my head perfectly still) against a building at (I think) 4th and Pike where
there was a bus island. My line of view across the end of the bus island was on
the opposite corner of the street. What I saw was a traffic sign of the "No
Parking" variety on the end of the bus island. The message on the sign was
totally inappropriate for its location -- until I moved my head and realized the
sign was actually on the opposite (kitty) corner. That particular sign had been
made oversize, and from a perspective viewpoint its apparent size matched what a
normal sign would have been on the end of the bus island. That's how people
without bino vision get tricked.

As for passerines, with the small distance between their eyes, I doubt the
binocular effect would be that great anyway. Further, when we see what birds do,
and the speeds they do it at, and the skill with which they maneuver among the
trees, and the ease with which they land on just about anything, I don't think
passerines have much of a depth perception problem. Heck, think about swifts,
swallows and flycatchers operating as they do in three dimensions, catching bugs
we can't even see at 40mph. Depth perception? Terrific!

But it sure looks like we are building a good case for Varied Thrushes being
easy prey for both raptors and windows. Maybe they just have a smarts perception
problem.

Jerry


E-mail from: Jerry Blinn, 30-Oct-1996