For Xenicibus xympithecus, a pre-historic, fowl-like species of bird, the ability to fight took precedence over the need for flight, a new study shows.

Research conducted by paleontologists at the Yale Department of Geology and Geophysics and the Smithsonian Institute has found that the extinct, flightless Jamaican bird species used its wings as weapons — the only known bird to have evolved wings solely for use in battle. The study, which was published online last Wednesday in the Proceedings of the Royal Society B, found that the hand bones of the fossilized remains of the bird were unusually thick. Ornithologists interviewed said the finding was intriguing.

“It’s very common for birds that do fight to use wings in battle,” Kenneth Campbell, curator of ornithology at the Natural History Museum, said. “But the development of the bone in this case is something I’ve never seen before.”

The unusual wing structure in the Xenicibus was originally thought to be a defect, Nicholas Longrich, who led the research, said, because only one specimen of Xenicibus was available for study. But when a team of researchers led by Ross MacPhee at the American Museum of Natural History found more specimens in the Red Hills Fissure cave deposits in Jamaica in the 1990s, Longrich said the strange bone size was found in all samples.

But the wing structure itself was not what clued Longrich in to the weapon function of the wings. The birds could have used them to walk or to hook onto branches, Longrich said, but because the bone structure has never been observed in any other species, a comparative analysis of function was impossible.

It was the fractured bones in several of the specimens he examined, in combination with an observed flexible joint structure, that Longrich said led him to believe the wings were used as weapons. One sample even had an arm bone that was completely broken in half, an injury that could only have been sustained if the bird intentionally smashed its wing against something.

“Since the bird couldn’t fly, it had to use its wings to defend itself,” he said.

Other species of bird, such as swans or Canadian Geese, use their wings in battle, he said. But none have wings specifically developed for fighting.

Two ornithologists from the Burke Museum of the University of Washington and at the Natural History Museum of Los Angeles interviewed said they agreed that the bone structure of Xenicibus is unlike that of any living species of bird.

Campbell said the fact the bird was flightless implies that it did not often need to fly away from predators. Islands, like Jamaica, tend to be relatively free of predators and thus devoid of the need to flee. Such an ecosystem might induce a species of bird to become flightless, he said, especially given that flying is “energetically expensive.”

This lack of predators in Xenicibus’ habitat probably implies that its biological weaponry was used for intra-species combat, such as in territorial or mating disputes, he said.

But its warrior wings did not help Xenicibus stand the test of time. Longrich said he does not know when or why the species disappeared but said he thinks the wings might not have been effective against certain predators, like humans.

“This looks a lot like what happens when humans get involved,” Longrich said of the species’ extinction.

He added that many islands in the Caribbean and elsewhere have unique indigenous wildlife that suddenly disappeared with the arrival of people, who destroyed habitats and hunted.

Xenicibus xympithecus is a member of the Ibis subfamily of birds, with over a dozen species existing worldwide today.