Marine biologists in Brazil were stunned to discover a young humpback whale on Friday that had washed ashore on a remote, forested island in the Amazon River, at a time of the year when it should have already migrated thousands of miles to Antarctica.

Members of the conservation group Bicho D’Água found the whale after following vultures that were circling a mangrove on the island in the Amazon, Marajó Island, the group said. About 50 feet from the shore, scientists spotted the lifeless humpback — about 26 feet long — lodged in thick shrubs and brush. It had been dead for at least several days, government officials in Pará, a state in northern Brazil, told local news outlets.

While tens of thousands of humpback whales are estimated to live in the Atlantic Ocean off Brazil, nearly all of them have migrated south by this time of year, the summer in the Southern Hemisphere, to feed near Antarctica. But this humpback was found near the mouth of the Amazon River, some 4,000 miles from its expected feeding grounds, a baffling discovery that has stumped the scientists who found it.

“We imagine it was floating and the tide took it into the mangrove,” Renata Emin, the president of Bicho D’Água, told the Brazilian news site G1. “The question is, What was a humpback whale doing in the month of February on the northern coast of Brazil? It’s unusual.”