The dark head and neck on a white body is a good indicator for Tundra but some late hatching Tundras can hold the dark body into the winter. Therefore it is best to look for more ID points. Trumpeter Swans generally hold the dark body into the late summer or following fall and early winter.
CAUTION- On many young swans and even some adults, the ID can be quite confusing due to the overlap of many field marks