This means that any autumn or winter adults seen on this side of the Atlantic will be in fresh winter plumage. All other common Sterna terns show black from the eye extending right over the rear crown or nape. Forster s instead has greyish shading across this area, forming a grey shawl connecting the face patches. This is caused by black streaking or mottling on the nape and rear crown which seems to become more obvious with wear. The shading is thus a variable feature.
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.