Since the time I input the first part of this and now the trees surrounding
my house are covered with snow. Such a transformation! And since we have done
our errands and have more than enough food we can just sit and enjoy it. And
think about owl vision. There was a nova program many years ago that had a
significant portion on how a Barn Owl catches mice. This material only begins
to scratch the surface on information on the vision of owls.

Jim Rosso
Issaquah

The Barn Owl, eds. Bunn, Warburton, Wilson; Buteo Books, 1982
pg. 33

Of the two types of photo-receptive cells, rods and cones, the former, which
are highly sensitive to light but not to colours, greatly predominate and are
packed together in huge numbers. They contain a substance known as visual
purple which is decomposed by light to produce minute electrical impulses.
Groups of rods are connected to cells known as bi-polar cells and groups of
these in turn are connected to ganglion cells whose axons transmit the
combined impulses to the brain. This process, whereby numbers of
light-sensitive cells join forces to produce a strong impulse in the brain,
is known as summation and contrives to make the eye extremely sensitive to
light at very low intensity, but it has one big disadvantage: the unique
information acquired by each individual rod is lost when the impulses mix
together in the bipolar and ganglion cells, with the result that only a
single message reaches the brain from each group. Thus sensitivity to light
is only gained at the expense of good colour vision and of visual acuity
(ability to see detail). Owls can probably compensate the effects of
summation to some extent by increasing the number of rods in the eye (the
Tawny Owl is said to have 56,000 per sq mm) and perhaps the huge size of the
eyes of some species facilitates the accommodation of such vast numbers.
We have said nothing so far of the other photo-receptive cells in the eye
known as cones. Though lacking visual purple and being of little use in
darkness, it is upon these that most keen-sighted diurnal vertebrates rely
for their colour vision and visual acuity. They are normally gathered
together to form a sensitive spot or fovea in the centre of the retina,
though some birds have an extra sensitive spot at the back of the eye so that
the brain can receive detailed information about areas in front of the head,
when the eyes are used together in binocular vision, and at one side when an
eye is functioning independently. Summation seldom occurs, each cone usually
having its own bi-polar and ganglion cells so that the brain is able to form
a detailed picture from information supplied by each cone individually.
It is hardly necessary to point out that it is a fallacy that owls cannot
see in daytime. On the contrary we may logically wonder, from our
understanding of the workings of the rods and cones, whether owls can see
rather better in daylight when their cones are able to function, albeit that
these are limited in number. Certainly if the owl can see colour in the
daytime it is highly probable that its visual acuity will be improved. We
cannot do better here than draw the readers attention to the extra detail
distinguishable on a colour television screen when compared with a black and
white set - the background of any country scene is alive with recognizable
species of wild plants that might never be noticed on a monochrome picture.
However, at times even humans may find it worthwhile deliberately to use
their rods at night. By looking straight at an object in the dark one is
bringing the cones into action, as a result of which one can frequently
discern nothing at all; averting the gaze somewhat and looking to one side of
the object it is often possible to see just a little of it. In doing this we
are simply pointing some of our rod cells at the object and using our
peripheral vision. We know also that, quite apart from the extra light
admitted as a result of the immediate opening of our irises when we enter a
dark room, a gradual increase of our sensitivity occurs as we become
accustomed or adapted to the change during the next few minutes.