Thousands of purple martins (Progne subis), the largest North American swallows, gathering on transmission tower and power lines near lake during pre-migratory roost in Ohio. The birds fly in from all directions and it is thought they gather here because it is an obvious landmark. Once gathered they will fly off en masse to their roosting site in a nearby reed bed to sleep. 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.