Subject: The Avian Eye
Date: Feb 18 21:42:26 1999
From: Wm Mulligan - wmmulligan at home.com


>From "EyeNet", a publication of the American Academy of Ophthalmology:

Unquestionably, the avian visual system is the finest of any species on
earth. Most birds have visual mechanisms that far exceed our own abilities
although not all birds have better acuity. The diving birds, such as the
cormorants, have high degrees of accommodation(up to 50 diopters)[humans at
age 40 have about 4 diopters], and other species even enjoy corneal
"accommodation" by having the ability to change the base curve of the
cornea. Many species have tetrachromatic[4 color]visual mechanisms using
additional visual pigments and colored oil droplets in their photoreceptors.
Birds have especially good movement sense with the ability to see movement
as slow as 15 degrees per hour(equivalent to the motion of the sun across
the horizon).

Perhaps the most interesting adaptations in the avian visual system include
the multifoveate retina and the supreme vision of selected species such as
the raptors. In some species, such as the large eagles, acuity is at least
four times our own and may be as much as eight times our own. The owls must
also be able to see well in limited light, and evolution has responded to
this challenge with the very large eyes and densely packed photoreceptors,
although some owls also have supreme hearing to help. Some owls have been
documented to see over two hundred yards with only a few candle power, which
is about equivalent to hunting mice on a moonless night with cloud cover.

-Ivan R. Schwab, MD

Dr. Schwab also noted that in some owls, the weight of the eyes exceeds that
of the brain.
I suspect this might also be true of some humans.

Wm Mulligan wmmulligan at home.com
8423 California Av SW Seattle WA 98136
206-932-4340 FAX 206-937-6619