More than 1,000 chimney swifts circled above East Mecklenburg High School before funneling into an old brick chimney in early October, creating a dusk spectacle that local observers say may become harder to see as the birds’ roosts disappear. The swifts gather in large flocks ahead of a roughly 3,000-mile migration to South America. At East Mecklenburg, the birds arrived in small groups before building into a churning mass overhead, their calls carrying across the school grounds. Large brick chimneys provide the birds with places to roost. As those structures disappear, so do some of the sites where swifts congregate before migration. The East Mecklenburg chimney remains one such gathering place in Charlotte.