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The vast marshes on the southwestern tip of the Alaskan peninsula must look like a buffet to a seagrass-loving goose like the Pacific black brant.

Right now virtually the entire population  – about 160,000 birds – is gathered in the sheltered and remote wetlands within the Izembek National Wildlife Refuge, feasting on the most extensive beds of eelgrass on Earth.

In the past, the Izembek was just a stopover in the brant’s autumn journey down North America’s western coastline. After a short stay to fatten up, the sated sea geese would lift off together and head south on a 3,300-mile, nonstop migration to Mexico’s Baja California.

But nature doesn’t follow that predictable course anymore.

Scientists have documented that increasing numbers of black brant are skipping that far southern migration and staying in Alaska instead. Fewer than 3,000 wintered in Alaska before 1977. In recent years, however, more than 40,000 have remained north, with as many as 50,000 staying there last year, during the most ice-free winter that Izembek had seen in more than a decade.

“The temperatures now in winter are much warmer,” said David Ward, a researcher at U.S. Geological Survey’s Alaska Science Center, who conducted the research along with scientists from the U.S. Fish and Wildlife Service. “In years past you’d often have ice that would build up in these lagoons, and the eelgrass would be unavailable for the winter period.”

“But now that’s changing.”

It is unclear whether this climate-change-driven adaptation spells trouble for the Pacific brant, one of the darkest of all waterfowl. The small goose sports a sleek black head and breast set off by white striations. A white “necklace” of feathers, different in each bird, may play a role in mating, which the brant do for life.

There may be some advantages in opting out of migration: Winter estimates of its population have edged up an average 4 percent annually from 2004 through 2013.