Adults have silvery-white primaries and primary coverts which flash white at a distance. They may look particularly white at a distance, especially in bright light. Note that Common Terns show darker and greyer outer primaries that darken further with wear. Both adult winter and first-winter Common Terns show a prominent dark carpal bar along the leading upperwing coverts, visible both in flight and at rest. Although Forster s may show a faint bar, it essentially lacks this feature, adding further to the uniformity of the upperwing.
Commons also show a darker secondary bar in winter.
Like Common Tern, Forster s shows thick greyish tips to the under-primaries, but they seem to be most obvious when the feathers are fresh. Also, like winter Common, all ages of Forster s have grey shading to the tail. First-winters also show a series of blackish marks at the tip of the tail, most prominent from below. In autumn, both adults and first-winters show a black bill, but red at the base is gradually acquired during the course of the winter. The rate at which it appears varies individually, but even some first-winters gain extensive red by January.