Winter brings 2,000 Steller's sea-eagles to the coast of Hokkaido—almost a third of the species' population. Come spring, they head north to Russia to breed. With a wingspan of up to nine feet (two meters) and weight of up to 20 pounds (ten kilograms), the Steller's far outsizes North America's bald eagle.

Today Japan still considers Hokkaido its last frontier. The island's history of development is short, beginning only in the mid-1800s with the Meiji Restoration. Before that it was known as Ezo and inhabited by the indigenous Ainu people. With the restoration it received its new name and waves of new settlers, who planted rice fields wherever they could. But compared with Honshu, Hokkaido is still thinly peopled, with a fifth of the nation's land area but only 5 percent of its population. Parts of the island are as developed as any place in Japan, but pieces of it remain open and wild, serving as the winter haunts of some deeply intriguing creatures.

My plan was to travel to some of these wild corners, east from the domain of the red-crowned crane near the city of Kushiro, then along the Pacific coast to the Shiretoko Peninsula, a long, thin finger of wild land that juts into the Sea of Okhotsk, and from there back to the heart of winter crane country. My companions and guides would be Noriyuki Aoki, a young wildlife biologist, and Yulia Satsuki Momose, coordinator of the Tancho Protection Unit—a private foundation for the study of cranes—and one of Hokkaido's powerful voices for the preservation of wild land.

On a day that is fair and free of weathers, Yulia takes me to Kushiro Mire, an impressive sweep of boreal marsh 45,000 acres (18,000 hectares) in area, surrounded by hills and shaped, she says, "like the palm of a great hand." Japan's largest remaining marshland, Kushiro Mire was spared the fate of the nation's other wetlands because of the region's cool, foggy climate, which won't grow rice. It is the heart of the red-crowned crane's breeding grounds and the refuge where nearly all of the island's cranes congregate in winter. This time of year, the yachibozu, fen tussock, and kitayoshi reeds poking up from the snow are all brown, but in late summer their deep emerald green makes a vivid backdrop against which the tancho looks whiter than snow.

"Before the restoration, these birds were found in abundance in Honshu and Hokkaido," Yulia says. "Some of the cranes here in Hokkaido may once have migrated to warmer places in Honshu to overwinter. But hunting and the disappearance of their habitat forced all the cranes out of Honshu."

By the turn of the past century, even the cranes in Hokkaido were thought to have been lost to extinction. Somehow, a handful of the birds clung to a fragile existence deep in Kushiro Mire, roosting and feeding in the icy waters of small rivers. In 1924 they were discovered, and part of the marsh was designated a protected area. But three decades later the population still hovered at only 33 birds. Then, in the winter of 1952, Hokkaido suffered a bout of severe cold and blizzards. Local farmers found the cranes nearly starved and began to feed them corn and buckwheat. Every winter thereafter, they fed them, first at farms scattered around the countryside and more recently at four established feeding stations and several satellite feeding stations throughout eastern Hokkaido. As a result of the winter feeding, the number of Japanese cranes has risen to nearly 900.

Yulia came to Hokkaido more than 40 years ago when her late father, Shoichiro Satsuki, who had grown up on the island, returned to practice medicine in his native landscape. "He came back because he loved the land here," Yulia says, "and he threw his support to scientific studies of the ecology of eastern Hokkaido, especially of the crane and its habitat." When the family arrived, the eastern region of the island was mostly broad, reedy swampland. Over the past half century, however, Kushiro Mire has been radically reduced. "If you fly over the area, which I've done many times," Yulia tells me, "you see how little is left." Though part of the marsh was made a national park in 1987, it remains under threat from roadbuilding, the channeling of rivers, expanding farmlands, and a hundred other troubles.

As we travel east, away from the great marsh, the sun glances off snowy farm fields and remnant pockets of marshland. A rough-legged hawk hovers over a stand of reeds and sedges, white-tailed sea-eagles perch on the telephone poles, and everywhere circle scavenging black kites. Yulia points out the site of a new crane nest she found three years ago, tucked into a patch of vestigial marsh by the road. "We were so excited—there are not many new nests built. But so far this one has not been successful."

The land here marked the father; it has marked the daughter as well. Trained in the United States as an immunohematologist, Yulia has given over her professional life to the red-crowned crane. She and her husband, Kunikazu Momose, an ornithologist at the Yamashina Institute for Ornithology and an authority on Grus japonensis, study the bird year-round with the assistance of Hiroyuki Masatomi of Senshu University, taking censuses of its winter populations, conducting aerial surveys of its breeding grounds, banding individuals to learn about their movements and life cycle.