Whooping cranes, one of the world's rarest birds, have waged a valiant battle against extinction. But federal officials warn of a new potential threat to the endangered whoopers: wind farms. Down to as few as 16 in 1941, the gargantuan birds that migrate 2,400 miles each fall from Canada to Texas, thanks to conservation efforts, now number about 266. But because wind energy, one of the fastest growing sources of renewable energy, has gained such traction, whooping cranes could again be at risk - from either crashing into the towering wind turbines and transmission lines or because of habitat lost to the wind farms. "Basically you can overlay the strongest, best areas for wind turbine development with the whooping crane migration corridor," said Tom Stehn, whooping crane coordinator for the U.S. Fish and Wildlife Service.