An apparent set of conjoined twin birds — an incredibly rare find — has been discovered in Arkansas, authorities said.

The pair of barn swallows, attached at the hip by skin and possibly muscle tissue, is being sent to the Smithsonian Institution for examination and confirmation, Arkansas wildlife officials said Friday.

"I can't even say it's one in a million — it's probably more than that," said Karen Rowe, an ornithologist with the Arkansas Game & Fish Commission. "There's just very little to no records of such a thing."

The birds, found by a landowner in White County, fell out of a nest as a healthy sibling flew off to learn how to hunt with its parents, Rowe said. The birds first appeared to have only three legs, but further examination found a fourth leg tucked up underneath the skin connecting the pair.

Rowe said the landowner likely kept the birds for a day before calling wildlife officials. By the time officials arrived, the birds were not eating. One died early Friday and a veterinarian later euthanized the other.

Finding conjoined birds is rare because they likely die before being discovered, Rowe said.

X-rays of the pair found each bird was fully formed, Rowe said. He said the birds would have had to come from a double-yolk egg.

Barn swallows can live for several years, though the conjoined twins might not have lived that long even if they had been separated. Rowe said it would have been difficult to teach the birds to fly.