When the birds touch down, they have no idea of the danger that lurks in the water. But soon they feel weak. Their eyes may close. They struggle to hold up their wings, then their heads. Eventually, they drown. Over the past three months, nearly 100,000 birds have died in this vicious sequence that scientists say marks the worst outbreak of avian botulism ever at the Klamath Basin national wildlife refuges, along the California-Oregon border. The die-off is centered at Tule Lake, an ancient, volcanic lake in Siskiyou and Modoc counties. It’s one of six federal refuges designed to provide sanctuary for the hundreds of thousands of birds, as well as other animals, that live and visit the remote region annually. Among the recent dead are both the local waterfowl, namely ducks, and the many migratory birds that stop for food and rest on their often-long journeys up and down the West Coast.