We performed the first winter surveys of the Atlantic Song Sparrow (Melospiza melodia atlantica), a poorly known subspecies that is endemic to sand dunes of the mid-Atlantic coast. Novel findings include that the Atlantic Song Sparrow is a year-round resident or partial migrant, primarily flocks with the same subspecies despite occurring in sympatry with the eastern Song Sparrow (M. m. melodia) in winter, and exhibits age-specific habitat use in winter, with immatures greatly outnumbering adults in adjacent saltmarshes. These results are important for conservation planning and understanding the ecology and evolution of this unique subspecies across the annual cycle.