Bird Retinas Contain No Blood Vessels. They Run on Sugar Instead.

The bird retina is one of the most metabolically demanding neural tissues in the vertebrate body, and it contains no internal blood vessels. That combination should be physiologically impossible. It isn't.

Researchers have now confirmed that the inner parts of the bird retina operate under chronic oxygen deprivation, relying instead on anaerobic energy production. The study, published in Nature in January 2026, also solves a mystery that has been open since the 17th century.

For centuries, the prevailing explanation was that a structure called the pecten oculi, a comb-like, highly vascularized organ protruding into the vitreous body of the bird eye, supplies oxygen to the retina. The structure has been known since the 1600s, but its precise function remained speculative. The reason no one had settled the question sooner is straightforward: "no one had directly measured oxygen levels in the bird retina under normal physiological conditions. Doing so is technically extremely challenging," says senior author Jens Randel Nyengaard of Aarhus University. "You need to keep the animal under stable, normal physiological conditions while performing very delicate measurements."

Nyengaard's team used microsensors to measure oxygen levels in the retinas of zebra finches (Taeniopygia guttata), pigeons and chickens. What they found overturned the centuries-old assumption entirely. Spatial transcriptomics data revealed high expression of glucose and lactate transporters in the pecten. The structure serves as a metabolic gateway: delivering large amounts of sugar into the retina and removing lactate, a waste product of anaerobic metabolism, back into the bloodstream.

Birds maintain a vessel-free retina that supports high visual performance while avoiding the optical interference caused by blood vessels. Evolutionary evidence suggests this strategy emerged early in the dinosaur lineage leading to modern birds.

In conditions like stroke, human tissues suffer because oxygen delivery is reduced and metabolic waste accumulates. Understanding how birds solved that problem 150 million years ago may, eventually, have something to say about that.

The pecten oculi has been sitting in bird eyes since before birds were birds. It just took us 350 years to read the label correctly.

Read the full story at ナゾロジー (Nazology), May 20, 2026

Hot Take: Three and a half centuries of ornithologists, anatomists and vision scientists examined the same structure and reached the same conclusion, which is one way to achieve consensus and not, it turns out, the most reliable.