The size, black cap, strong bill and more positive flight recall Common or Arctic Tern, but the short, forked-looking tail and dark grey breeding plumage above and below are typically marsh tern characteristics.
The summer adult has white cheeks and red legs and bill. The crown is flecked with white in the juvenile, and the hindcrown is more uniformly blackish, though in the winter adult this too is flecked with white. The black ear-coverts are joined to the black of the hindcrown, and the space above is mottled with white, causing the black to appear as a C-shaped band. The sides of the neck are white; this sometimes continues across the nape. The collar is less sharply defined. All through the year the rump is pale grey.
In winter, the forehead becomes white and the body plumage a much paler grey. Juvenile Whiskered Terns have a ginger scaly back, and otherwise look much like winter adults. The first winter plumage is intermediate between juvenile and adult winter, with patchy ginger on the back.