A group of biologists has finally solved one of the great mysteries of biology. For decades, scientists have wondered how birds can have keen eyesight without blood vessels to supply oxygen to the retina. Blood vessels provide vital oxygen to the retinas of other animals, but the birds’ eye design is different.
Without blood vessels that scatter the light reaching the retina, the birds’ eye design enables sharper vision. How can the retinal cells function without dying from lack of oxygen? The answer, according to researchers at Aarhus University in Denmark, is anaerobic glycolysis. “Anaerobic” means without oxygen, and “glycolysis” refers to the process of breaking down the sugar glucose. Anaerobic glycolysis is less efficient than oxygen-based metabolism, and birds’ eyes need a lot of energy. How can they get enough energy with this less-effective method?
The birds’ eye design features a unique structure that no other species has. A comb-like network of blood vessels called the pecten oculi protrudes from where the optic nerve enters the eye and moves freely within the vitreous humor, the fluid filling the eyeball. The pecten oculi transports glucose into the vitreous humor while removing carbon dioxide and lactic acid that could harm the retina.
This discovery explains how birds like bar-headed geese can fly at elevations over 6000 meters, where oxygen is scarce. The DNA of these birds is coded to overcome specific environmental while maintaining sharp vision. Encoding requires an intelligent programmer who understands what birds need to survive. Bird’s eye design, like every scientific discovery, gives us a window into the creation of life on planet Earth.
— John N. Clayton © 2026
Reference: “Briefings” in American Scientist for March/April 2026, page 77.