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Not all birds need to see. Blind but perfectly healthy kiwis have been found living in New Zealand.

The flightless nocturnal birds may be evolving to lose their eyesight altogether, suggest the researchers. The blind kiwis seem able to survive just as well using other senses such as touch, smell and hearing, so maintaining good eyesight might be a waste of energy.

The blind birds were discovered during a study of 160 (Apteryx rowi) found in the Okarito forest on New Zealand’s South Island.

“We found a very high prevalence of birds with eye lesions,” says at the in Wellington. “A third of them had eye problems.”

But the biggest surprise was chancing upon three sightless birds. “The finding of completely blind birds in good physical condition was absolutely stunning,” says team member at the University of California, Davis.

“No other birds are known to have a free-living population of blind [individuals],” says Tennyson. But plenty of other animals, such as moles and cave-dwelling fish, have evolved blindness. “Vision is not essential for survival in all animals.”

The discovery could help explain how species lose their sense of sight, a process called regressive evolution.

The most likely explanation is that kiwis do not need vision because of where and how they live: they are active at night, and their habitat offers plenty of food and no predators, apart from introduced animals such as stoats.

“Kiwis 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,” says Tennyson.

Other researchers speculate that a gene called Sonic hedgehog might be responsible for the dumping of vision. This gene is important in development and has been implicated in other animals losing their sight, such as the Mexican blind cavefish.

Sonic hedgehog could potentially enhance the function of touch and smell sensors in the kiwis’ long beaks, at the expense of visual function.

“Eye degeneration can be seen as ‘collateral damage’,” as the birds adapt to their “nocturnal, lightless niche in which normal, functioning eyes are not necessary”, says at Hartwick College in Oneonta, New York. “That’s our best guess as to what’s going on in these birds.”

The blind Okarito brown kiwis are a great opportunity to study how visual systems evolve and change, says Tennyson. But only if they survive: they are , with about 400 left in the wild.

Journal reference: BMC Biology, DOI: