Above Scandinavia, on the Atlantic side of the Arctic Ocean, mackerel, cod, and other fish native to the European coast are migrating through increasingly ice-free waters, heading deeper into the Arctic Basin toward Siberia. Thousands of miles to the west, above Alaska, kittiwakes and other polar seabirds are being supplanted by southern birds following warm waters streaming north through the Bering Strait. And midway between, above Canada, sea ice-avoiding killer whales from the Atlantic are increasingly making themselves at home in a thawing Arctic.