Red-breasted Merganser: The Eyes Have It

You may have noticed that duck species often have very colorful eyes. In the case of the Red-breasted Merganser, their eyes are bright red. There is a good reason for colorful eyes. The red eyes function to improve their ability to see underwater in cloudy (i.e., murky) conditions. 

In this regard, think of how sunglasses improve your ability to see on a bright, sunny day - especially if they are polarized. In the same way, red eyes allow a bird such as the RBM or Common Loon to see parts of the color spectrum better. Some species that feed at the surface or plunge for food have red oil droplets in the cones of their retinas. This improves contrast and sharpens distance vision, especially in hazy conditions. Birds that have to look through an air/water interface have more deeply coloured carotenoid pigments in the oil drops than other species. This helps them to locate shoals of fish.

To give you an unscientific idea of how this works I have desaturated the same photo so that it is depicted as a grayscale image. If you click on any of the pictures so as to enlarge them on your screen, you can click back and forth between this photo and the next to see the differences between the two images. It works even better if you have a scroll wheel on your mouse to move back and forth between the photos.

This photo also appears to be grayscale, but is actually a copy of the isolated red channel from the image. Any part of the photo which is red (such as the bill) becomes much brighter while the loss of the blue and green channels causes other parts of the image to darken.

While this is an inexact example, the mergansers eyes give it advantages in hunting for fish similar to how the pictures change in contrast between the latter two.