Thousands of purple martins (Progne subis), the largest North American swallows, fly off transmission tower and power lines to reed bed in nearby lake to sleep during pre-migratory roost in Ohio. Pre-migratory roosts are large communal gathering that occur on large lakes in late summer before the birds start their migrations to South America. Birds may use the roost for several weeks. There are dozens of known roosting sites in the eastern U.S. alone and the largest contain tens and even hundreds of thousands of birds.