What is the most energy demanding system of a bird's brain? Need another clue? This system does not require blood to function properly. One more clue, the cones and rods, which number in the millions, use an amazing amount of nutrients and oxygen. They convert the light they receive into nerve impulses and then transmit these impulses to the brain. I'm sure you have it by now. We are talking about bird's eyes.
As you know your eyes and most mammals have blood vessels in their eyes and it takes a large amount of blood to supply all the nutrients needed. Now this creates a problem within the eye. These blood vessels block light from reaching the retina. As I'm sure most people know, birds have even better vision than humans. To accommodate the bird's eyes need for nutrients, oxygen, glucose and protein they have developed a structure in the eye called pecten.
The pecten, which is filled with melanin granules, is a tissue that is folded into the eye. The pecten projects from the retina into the vitreous. The pecten is supplied with blood vessels and supplies the necessary nutrients and oxygen to the retina.
The melanin granules (the same granules are used by humans to darken hair and skin) create a warming effect because of their dark color. The warming is believed to increase the nutrients to the vitreous and retina.
This same dark pigment may also create shade in the retina. This would stop stray bright light from entering the eye and possibly reducing glare. The reduction of glare helps birds detect movement and helps with their hunting.
Scientists are continuing to study the remarkable eyes of the birds. In the mean time you have to agree with me that the eyes have it!