Sea Blue   (The eye of the predator)

Double Crested Cormorant (Nannopterum auritum) - Royal Palm (Anhinga Trail), Everglades National Park, Florida

 

I came across this guy sitting on the Anhinga Trail boardwalk railing, and in my usual manner I took an ensurance shot at a respectable distance and then moved closer for another. Keeping my eye on the viewfinder I moved closer and closer taking shots along the way until my camera malfunctioned and I could no longer focus. After a few seconds I realized it wasn't a malfunction, but the problem was that I was inside my 5 foot MFD, so I set the lens to its closest focus point in macro mode, and then used my feet to focus (i.e. set the 300's focus at the 1.5 meter MFD, and then used the camera to subject distance to focus). If I had thought to put on some of the X-tubes I had in my pocket, I could have got a really good look at its eye.

I called this shot "Sea Blue" because (aside from the pun) a decent number of diving sea birds have a blue iris, which I believe is the best color to see prey in a blue, shallow water environment. I haven't seen that explanation in any texts yet, but it sounds right to me.

 

Update

I've been in contact with several "experts" on the topic of iris color, and they say that the retina is what birds see with (duh), and the iris is all about aperture and has nothing to do with vision.

But I on the other hand (pun intended) think they are over simplifying, and continue to believe that nature has reasons (other than fem cormorants dig mates with big blue eyes), and that there is in fact a visual benefit to these shallow diving birds having blue eyes.

One possibility may be the manner in which sophisticated spectrophotometric light detectors often bump up (adjust) the "dark current" to raise the light gathering diode current above the "dark signal" to even out the signal, reduce noise, and obtain a more even spectrum. So my theory is that the iris is not 100% opaque, and lets through a small amount of blue light to raise the retina's receptors above the threshold level thereby making any additional light coming through the pupil detectable (i.e. see-able).

Old time astronomers also used a similar trick with film, when they pre-exposed their sheet film with small, even amounts of light to break the silver halide trigger threshold to make film used to record distant stars more sensitive to light.

Of course the other popular eye color for some of the deeper divers (loons, grebes, murre, Harlequins, Mergansers, Long-tails, etc.), and the more land based Night Herons is red, and we all know that red has the most sensitive color receptors (cones) on the retina, and is the color light we like to use to see things where it's really dark (like 50 feet under murky water).

 

So what do you think?

Is it possible that iris color increases visual acuity in low light?

No definitive study has yet to be done on the topic,

and I think it might be a good topic for an ornithologically inclined PhD thesis.

So go for it, and prove me wrong! :{)

 

Note: This is the first image in a series of images,

unfortunately, in order to see the other pics on the "new" improved Flickr, you'll have to scroll up the comment section to the top of the comment list, or click on the following link to

view the enlarged image of this guys eye

Done