Natural evolution could be making New Zealand's iconic bird blind as its natural habitat and way of life renders sight unnecessary for survival.

Three kiwi have been found to be profoundly blind in a South Island forest and an article in the New Scientist has suggested this could be an indication the flightless nocturnal birds may be evolving to lose their eyesight.

A study of 160 Okarito brown kiwi in the Okarito forest in New Zealand's South Island found a "very high prevalence of birds with eye lesions".

Te Papa Museum researcher Alan Tennyson said a third of the birds had eye problems.


The three completely blind birds had no other clear physical problems.

Tennyson said no other birds were known to have a free-living population of blind members.

However, he told the New Scientist, plenty of other animals such as moles and cave-dwelling fish, have evolved blindness.

"Vision is not essential for survival in all animals."

He said the most likely explanation the kiwi were becoming blind was because of where and how they lived.

"Kiwi are flightless and generally nocturnally active, and have very good senses of smell, hearing and touch, so it seems that vision is not essential for their survival, at least for some individuals," Tennyson said.

The New Scientist said other researchers have speculated that a gene called Sonic hedgehog might be responsible for the loss of vision.

The gene could also enhance the touch and smell sensors in the birds' long beaks.

Researchers suggested the loss of sight was "collateral damage" as kiwi adapted to their nocturnal lightless niche within which normal, functioning eyes were not a necessity.