This 2,500-acre preserve includes the Monomoy Islands, a fragile 9-mile-long barrier-beach area south of Chatham. Monomoy's North and South islands were created when a storm divided the former Monomoy Island in 1978. A haven for bird-watchers, the refuge is an important stop along the North Atlantic Flyway for migratory waterfowl and shorebirds—peak migration times are May and late July. It also provides nesting and resting grounds for 285 species, including gulls—great black-backed, herring, and laughing—and several tern species. White-tailed deer wander the islands, and harbor and gray seals frequent the shores in winter. The only structure on the islands is the South Monomoy Lighthouse, built in 1849.