Re birds and stereo

[P3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

Apparently birds all have some degree of overlapping vision with the
exception of penguins of the genus spheniscus but in pigeons it is only 20
degrees out of a total of 340 ,we have about 140 out of 180 owls 70 out 110
The American Woodcock has 360 vision with moe than 180 per eye so has
overlap behind it head as well as in front. But does that give them stereo
vision, according to "Animal Vision" R.H.Smythe M.R.C.V.S. "It is commonly
believed that stereopsis can only be achieved when there is incomplete
decussation of the optic nerve ,when all the fibres do not cross to the
other side of the brain , but a proportion pass to the same side . For us
it is about 50/50  acccording to walls for the horse with little overlap it
1/6 to 1/8 in the Rat and Opossum 1/5; the dog 1/4 the Cat with a flatter
face 1/3. While animals with lateraly placed eyes very few fibres are on
the same side although even a Rabbit has some . Fishes have none so have
complete decussation and "probably have no true binocular field of vision "
. In a diagram he shows complete cross over decussation for birds as well
and states that in the caption. So what about Owls ? he also goes on to say
that as Owls have no eye movement but move their whole head it cannot
converge its eyes so that an image of a mouse appears in the fovea of each
eye (assuming it possesed them apparently they dont all). He asks "Is it
likely, provided this particual posseses a fovea in each eye that sitting
in one place as it does ,it will be able to bring the image of the mouse
,as seen in either eye ,on to the point of greatest acuity (Fovea?) in each
retina simultaneously and thereby fuse the two images int one?. If not the
owl would see double ( so how can it decide which to go for) and it would
not have stereo vision . He suggests the reason an owl or presumably other
birds of prey pick up their food with their feet rather than beaks is a way
of overcoming this problem .
 I am not shure how serious he is and he also suggests that it can close
one eye in case of need !. I find this hard to believe .  P.J.Homer



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