Hi Dan -
Thank you for the very detailed and excellent explanation - really
appreciate it. I was curious about some aspects of it.
Does the much larger physical size of the human eye, relative to most
birds' eyes, compensate (if at all) for the smaller cone density? I.e.,
does the larger eye result in a larger fovea, which in turn
significantly increases the total number of cones, even though they're
at a lower density? As area increases proportionally to the square of
the radius, how much does that help (if at all)?
Or are there other qualitative aspects to the cones in birds' and other
animals' eyes that are contributing factors to their much better vision?
Thanks,
Nagi
On 2/9/2017 00:06, Dan Gleason wrote:
I don't know why a junco would not see and avoid such an object, but I doubt
that a blind spot is the answer. Here's why:
All vertebrates (including all birds, humans, etc.) have a blind spot in the retina of
each eye. This is due to the structure of the eye and how it forms in embryonic
development. The neurons called cones and rods in the retina (the cells that respond to
light) are organized such that the light-sensitive tip points to the rear of the retina.
The cell body points into the eye and interfaces with other neurons leading to the optic
nerve. The bundle of nerves must turn inward, everting through the retina to reach the
optic nerve. The point at which this happens leaves a small area of the retina with no
light sensors, and thus, a blind spot is formed. We never notice our blind spot because
the brain takes information from the surrounding cells and "fills in" the blind
spot with enough information to create a continuous image.
The blind spot is always in the peripheral field of view and never in the main
area of focus so it is not a liability. If an object's image extends beyond the
blind spot (as a rod would) the image created in that spot would extend across
the spot and be seen as a solid object. So I doubt that a normal blind spot
would have caused a problem for this junco. An abnormality is always possible,
but I suspect there is another reason for what you saw...and, alas, i dont know
what that could be.
An additional bit of intresting information — birds eyes are far superior to
mammal eyes in many respects. Color reception is better, some have multiple
fovea, resolution, etc. Cones are responsible for color reception and sharpness
and are densely packed. In the fovea (or macula). Our eyes have approximately
200,000 cones/mm2 and many fewer in the periphery. Raptors, flycatchers, swifts
and some other kinds of birds have 1,000,000 or more cones in the fovea. A
House Sparrow, which has poorer eyesight than many birds, still has over
400,000 cones/mm2 in the fovea; far superior to us. So birds see really well
and resolve much smaller detail than we can discern.
Dan Gleason
Owner, Wild Birds Unlimited of Eugene
Ornithology Instructor, University of Oregon
dan-gleason@xxxxxxxxxxx
On Feb 8, 2017, at 10:35 PM, "woodenapple@xxxxxxxx" <woodenapple@xxxxxxxx>POST: Send your post to obol@xxxxxxxxxxxxx
wrote:
Obolonians,
I observed an odd bird strike today in our N. Eugene/Santa Clara backyard. On our back
patio we have three half-barrel planters. The middle one has a welded sculpture of a
dragonfly which is perched on a 3/16" vertical metal rod about 30" long,
anchored in the soil. A Dark-eyed Junco flew up from next to the house, across about 10
feet of patio, and ran headlong into the metal rod, hard enough to leave feathers behind!
It sat stunned on the patio for perhaps 15 minutes and flew off. The rod is fully in the
open.
In analyzing what happened and why, my only idea has to do with the vertical
position of the rod and a supposition about Junco eyesight. I can't imagine
that a Junco would collide with a horizontal twig, which would be of a similar
size to the rod. Is it possible that a Junco has a blind spot directly in front
of it such that a thin, vertical object would disappear, whereas a similar,
horizontal one would be visible to one or both eyes?
Rudi
____________________________________________________________
Warning: Don't Use Probiotics Before You See This
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