Judsonia, Arkansas, resident Danny Langford found the odd couple on July 17 after they fell out of their nest and onto his driveway. Barn swallows have nested near Langford's house for the past seven years, he said, before completing their annual migration to South America.
Arkansas wildlife officials who examined the young swallows said the phenomenon of conjoined twinning in birds is very rare.
"I can't even say it's one in a million—it's probably more than that," Karen Rowe, with the Arkansas Game and Fish Commission, told the Associated Press. "There's just very little to no records of such a thing."
But two barn swallows are not better as one. The birds stopped eating soon after they were found, and one died on July 18. A veterinarian later euthanized the surviving twin. Rowe told AP that she plans to send the bodies to the Smithsonian Institution in Washington, D.C., for further study.
The news, however, isn't flying with everyone. Gary Graves, curator of the Smithsonian's bird division, told National Geographic News that, while it's possible the birds are the result of a genetic abnormality that started in the egg, he thinks this scenario is unlikely.
X-rays of the bodies taken at the Little Rock Zoo did not show shared internal organs or skeletal elements, which are typically present in conjoined twins. Also, the birds were initially thought to share only three legs, but later examinations found a fourth leg tucked under the skin connecting them, AP reported.
"More likely it's something that happened in the nest," Graves said. One baby bird could have suffered a cut, for example, which the other stuck its foot into or became stuck to, joining them as the wound scabbed and healed.
"It could be stuck together by glue for all we know. At this point, there's nothing scientific you can say about anything," Graves said. "This could be something far less of a spectacle and far more mundane."