There are two types of 'eye shine'.
As Chris said it is partly simple physics. By the nature of the eye and
its focus on the back of the eye when light is shone into it there is a
reflection from the back of the eye. As is the case with anything being
illuminated, at least some of the light is reflected back along the same
path and will appear to cause the eye to 'shine' (compared to the rather
non-reflective coat of hair, feathers etc).
But there is another factor. Some animals (including some birds such as
owls) which are active at night, have a layer at the back of the eye
called a 'Tapetum Lucidum'. This layer contains mineral crystals which
have the property of reflecting almost all the incident light back along
the same path as the incident light. (Look up 'Corner reflector' on
Wikipedia) This layer is behind the layer of light sensitive cells,
doubling the sensitivity of the eye. (The cells have two chances of
capturing photons; coming and going). When you shine a light into an
animal or bird with these eyes they really do appear to shine very
brightly. However the reflection is not perfect and some light is
reflected back somewhat 'off axis'. In this case, because the reflection
is so strong, you can still see a reflection even if your eyes are well
off the same axis as the torch beam.
In contrast, in animals (and birds) without a Tapetum Lucidum, the light
is reflected of the Fundus at the back of the eye. This is not a good
reflector but some does get reflected back along the axis. Because the
reflection is so much weaker your eyes need to be much closer to the
axis of the torch beam to see the effect.
The colour of the eye reflection from the Tapetum Lucidum depends upon
the properties of the crystals in the layer. It can vary more or less
across the full visible spectrum. However in animals without that layer
the reflection is always red. The red colour is mainly due to the
haemoglobin in the blood vessels at the back of the eye. (This is the
basis for the red eye effect in people. We don't have a Tapetum Lucidum.)
Andrew
On 18/11/2013 12:18 PM, Chris Corben wrote:
Roger
It's very simple physics. Almost anything looks brightest when the
light source is close to your eyes. But in addition, a bird's eye is
pretty much retro-reflective. That is, the light reflected by it
mostly goes back along the path it came from. If you think about it,
you are looking at the inside of a sphere, so wherever the light comes
from, it is reflected back in the reverse direction. The same
principle is used to make road markings shine brightly at night. The
paint on a road is filled with tiny glass spheres, so that from
whatever direction the light arrives, some portion of it is
retro-reflected. Since the headlights of a car are not too far off the
line of your sight, a lot of that light comes back to your eyes, and
the markings look bright. If the paint was just plain gloss paint, it
would be much more reflective, but nearly all the light from the
headlights would be reflected away from the driver, and the paint
would look essentially black. As does smooth ice, and for the same
reason.
Owlet Nightjars eyes are not nearly so bright as White-throated
Nightjar's eyes. But even a White-throated can be seen at much greater
distance with a light which is close to your eyes. It makes such a
difference, that you can wear a headlamp and see the eyes of things
like owls at reasonable distance even though the headlamp is not very
bright. A side benefit of this is that the lower light levels will
scare the bird a lot less, so you can actually gain by having a lower
intensity light if it is close to your eyes. A headlamp is perfect for
that!
Incidentally, if you know to look for it, you can see eye-shine of
animals in all sorts of unexpected ways. A classic case is to get to a
place where there are frogs on the surface of the water. If you get
the sun straight behind you (eg in early morning or late afternoon),
you will be able to see their eye-shines surprisingly well, especially
if you use binoculars.
Using binoculars with a headlamp is a great way to find all sorts of
creatures at night. Frogs, snakes, geckoes, spiders, small mammals and
even bats in the right situations. Just use your binoculars to look at
the spot of light from the headlamp.
Cheers, Chris.
On 11/17/2013 08:39 PM, Roger McNeill wrote:
A few weeks ago Gus McNab was over and we were discussing
spotlighting and I mentioned how I have a good population of Owlet
Nightjars on our block but I never see them at night because their
eyes don't eyeshine, despite wandering the woods after hours.
He told me (politely) how wrong I was and the issue was that I was
not holding the torch in the right place to see it. What I needed to
do was walk around like a unicorn with the torch beam emenenting from
between my eyes. I (polietly) said that is faseniting, thinking that
this was surely some ploy to make me look like an idiot...not that
help is required...and thinking how that could possibly be true?
Last night, I had a visiting bird-o who wanted to see Nightjars and
other things so we decided to wander the tracks and see what we could
find. First try was for White-throated Nightjars...two birds
pearched up for us, brilliant eyeshine. A koala started calling back
at the house so we wandered back, yep bright eye shine...we then
decided to walk down to "owlet-nightjar grove" and I had three birds
respond and two come in, one quite close. I put the torch on the
bird and as expected no-eyeshine...but then I tried Gus'
recomendation and move the torch between my eyes and wow, its eyes
shown back bright red like a Christmas tree. Amazing! Gus you were
right, but I have no idea what the explanitaion was or why this is
the case! Thanks for the tip...wanted to share publically.
Cheers,
Roger
Roger McNeill
Samford Valley, SEQ
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