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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Chris Corben.
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