Thousands of purple martins Progne subis, the largest North American swallows, skim over reeds during pre_migratory roost in Ohio. Pre_migratory roosts are large communal gatherings that occur on large lakes in late summer before the birds start their migration to South America. Birds may use the roost for several weeks. There are dozens of known roosting sites in the eastern half of North America and the largest contain tens and even hundreds of thousands of birds.