In some species—e.g., the terns of Nantucket—kettling behavior is evidently a way of staging a flock in readiness for migration. Pre-migrational turkey vultures kettle by the hundreds in the thermals that rise over Vancouver Island before they venture across the Strait of Juan de Fuca toward Washington State. At Hawk Mountain, broad-winged hawks form kettles in September before flying south. Kettling apparently serves as a form of avian communication—an announcement of imminent departure—as well as a way of gaining altitude and conserving strength.