Extinct Hawaiian Ibis Had Unusually Small Eyes and Limited Visual Capacity, Study Says

Apteribisan extinct species of ibis that once inhabited the Hawaiian Islands occupied a niche similar to that of the New Zealand kiwi: a nocturnal, flightless bird that relies on tactile signals from its beak to detect prey, according to a new study by scientists at the University of Lethbridge, Flinders University and the Smithsonian National Museum of Natural History.

Apteribis. Image credit: Sarah Citron, University of Lethbridge.

Evolution on islands often generates specialized lifestyles rarely observed in continental species.

Ocean island biota, however, are subject to extinction following human colonization, resulting in an incomplete understanding of the lifestyle of species that evolved prior to colonization.

For example, the Hawaiian Islands were home to a unique and diverse assemblage of endemic species, most of which became extinct following human colonization.

“Apteribis was a relative of the Australian white ibis, commonly called the ‘trash chicken’ because of its habit of digging through trash cans with its beak,” said Sara Citron, a doctoral candidate from the University of Lethbridge.

“But in reality, ibises are beautiful and distinct birds: they have exceptionally long, elegant bills and striking colors that set them apart from other shorebirds.”

“Their elongated bills are essential to their diet. By inserting their bills into mud, shallow water, or soft soil, they detect subtle vibrations that reveal the presence of hidden prey, such as small invertebrates.”

In the study, Citron and his colleagues examined the skulls of 25 of the 28 living ibis species in museum collections.

They used advanced imaging technology to create 3D reconstructions of the birds’ brains and compared them to fossil species.

“What we discovered was astonishing. All the visual system imprints on the skull – such as the eyes, the optic nerve and the area of ​​the optic tectum where light is processed by the brain – were significantly reduced in Apteribis compared to its living relatives,” Citron said.

ApteribisA reduced visual system suggests a nocturnal lifestyle, which is an unprecedented trait in ibises.

The bird likely roamed the Hawaiian landscape under cover of darkness to feed and possibly breed, while resting during the heat of the day.

“Such extreme reductions are only known in a few birds, including the elusive Australian night parrot or the kiwi and kakapo of New Zealand,” said Aubrey Keirnan, a Ph.D. student at Flinders University.

“All of these species rely very little on sight and are active mainly at night.

“Hawaiian species were targeted because island evolution often produces bizarre anatomy,” said Dr. Andrew Iwaniuk of the University of Lethbridge.

“The moment we looked at the skull, we could see that the eye sockets, the spaces where the eyes are, were much smaller than they should have been.”

“Hawaii is the most isolated archipelago on the planet and originally there were no predatory mammals,” said Dr Vera Weisbecker from Flinders University.

“Similar to the situation in New Zealand, large birds like ibises were safely on the ground and eventually lost the ability to fly.”

“An accurate view would not have been a particular advantage.”

The other driving factor Apteribis in the dark was probably their prey.

“The Hawaiian Islands were once home to an extraordinary diversity of flightless snails and crickets,” said Dr. Helen James, curator of birds at the Smithsonian National Museum of Natural History.

“Crickets and flightless snails are nocturnal and would have been more abundant back then, exactly the kind of prey that might hunt a bird like Apteribis adopt nocturnal foraging.

“Possible causes of the extinction are changes in climate and vegetation on the Hawaiian Islands, as well as the first arrival of humans.”

“The New Zealand kiwi is often considered a unique curiosity among modern birds,” Dr Iwaniuk said.

“But this extinct ibis shows that similar forms evolved elsewhere.”

“It reminds us how much diversity has been lost and how many ecological roles disappeared before we had a chance to study them.”

The study was published in the journal Integrative and Comparative Biology.

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Sarah Citron and others. 2026. Comparative anatomy supports the evolution of nocturnality in the extinct Hawaiian ibis Apteribis. Integrative and Comparative Biology 66: icaf159; doi: 10.1093/icb/icaf159