Jan. 5, the day of the annual Christmas Bird Count, was freezing, snowy and windy. Not the sort of day you would expect to find a vagrant from the tropics.

But there on the beach at Squibnocket, Bob Woodruff spotted a species not usually seen north of the tip of Florida. How did it come to be there, dead in the sand thousands of miles north of its usual range?

Alas it may always remain a mystery; the one sure thing is that it didn’t fly. For their find was not a bird at all, but a lump of brain coral, Diploria strigosa, indigenous to the Caribbean.

Mr. Woodruff, of West Tisbury, described seeing first “something about the size of a baseball, protruding from the sand.”

He and his birding buddy, George Hartman, began to dig and eventually unearthed a lump roughly 16 inches in diameter and weighing some 20 pounds.

They took it away, photographed it and then set about trying to find an explanation. Mr. Woodruff, using his connections as a naturalist, enlisted the aid of an emeritus professor of Geological Oceanography from the University of North Carolina and seasonal Vineyard resident, Conrad Neumann.

Mr. Neumann’s particular expertise is in limestone, and coral, after all, is really living limestone.

At first Mr. Neumann doubted Mr. Woodruff’s identification of the find; there are cold water species which resemble the brain coral.

Then he saw the pictures and was convinced.

He also saw reddish staining on the coral, which may give a clue to its provenance, for it looks like rust.

They used ingots of iron as ballast in ships, he said. Could it be that a trading vessel, coming north from the Caribbean, used not only ingots but also lumps of convenient coral?

It seems the most likely explanation. The staining, of course could have other sources; maybe the coral was fossilized and stained in the process.

“But that seems very unlikely,” Mr. Neumann said.

And he said it was “almost impossible” the coral could merely have been bounced along the bottom of the ocean all the way here.

Could the coral be dated somehow, to get closer to the truth?

Well yes, he said, but for various technical reasons that would be very difficult, expensive, and not necessarily accurate.

“So we are left to theorize on the least improbable cause,” he said.

“And that is that it came here by the hand of man, and was either dumped here, or that the ship it was on was wrecked. We know there were schooners broken up off Squibnocket in the old days,” Mr. Neumann said.

But that poses another question: if it was the result of ballast dumping or a wreck, why was more of it not found?

So it remains a mystery. But should you be walking the beach around Squibnocket Pond in future, keep an eye out for any other lumps of the tropics, and hints at history.