Bird vision
<p>Aug 03, 2012 07:14 PM
 
pigeon = 2 eyes with wings.
 
ciliary muscle can change shape of lens rapidly  > than mammals.
 
birds:  largest eyes rel. to their size in animal kingdom.
 
Pecten oculi  comb-like struc of blood vessels in choroid... projects into vitrous
nourishes retins....  lifts blood vessels OFF the retina and away from it...
leading to extremely sharp eyesight in hawks.  unique to birds.
 
4 color receptors. Tetrachromatic... see chart... well down into UV  
 
can perceive in UV part of spectrum.
 
detect polarised light and/or mag fields.
 
more light receptors than mammals and more nerve connex.
 
Birds of prey... v high density of recpetors.
 
Eye of bird most closely resembles reptiles... not sphereical, flatter shap...
more of viz field is in focus.
 
lens pushed forward (similar to mammals but unlike reptiles) so that image size is increased on retina.
 
Most birds cannot move their eyes.
 
See photo of nictitating membrane:  horizontal windshield wiper!
 
Humans have a max density of 200K recetpors per mm.
Common Buzzard has 1M recept per mm.
House Sparrow has 400,000 per mm.
 
v high ratio of gnaglion cells/ photorecep.
White Wagtail has 100K ganglions and 120K photorecs.  (almost 1 to 1)
 
54% of birds  have SECOND FOVEA !!!  birds of prey_ kingishers, hummingbirds and swallows...
2nf fovea points sideways.
 
In diurnal birds, 80% of the recetpors are cones (90% in swifts).
bird vision:  peak at 370 nano  UV cone.
 
DIURNAL BIRDS OF PREY
 
Raptors large eyes for their sized.
tube shpae deye to produce larger ret image.
 
Many raptors have foveas with far more rods and cones than human fovea (65K in Kestrel vs 38K ih uman).
 
Buteo buzzards  6 to 8 X better vision than humans  (distance);
 
double fovea;  fwd facing
 
American Kesterel can see a 2mm insect from the top of an 18M tree.
 
 
 
 
 
 
 
 
 
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