The annual Sandhill Crane migration has been called one of the greatest spectacles of nature in the Southeast. Each winter, between 15,000 and 20,000 of these graceful birds converge upon the Hiwassee Wildlife Refuge just north of Chattanooga on their journey from the upper Midwest to Florida. “The cranes begin arriving in late October, but their numbers peak in early January,” said Kevin Calhoon, the Tennessee Aquarium’s assistant curator of forests.