When identifying terns, it is safest to rely upon a combination of field marks. The Common Tern is most similar to the Roseate, Arctic, and Forster's Terns. The Forster's tern has a distinctive black eye patch except in the breeding season. The Common Tern has a black eye patch that continues across the nape. During the breeding season, the Forster's Tern has paler upper surfaces to the primaries that contrast with the rest of the wing while the Common Tern has a dark wedge in the primaries; its tail has white outer edges and dark inner edges, the opposite of the Common Tern; its underparts are white not gray like the Common Tern. The Arctic Tern is quite similar in all plumages but has a shorter, stubbier bill, shorter legs, and a longer tail. In the breeding season, it has darker gray underparts, an all red bill, more translucence in the primaries, crisp black trailing edge to the primaries, and lacks the dark wedge in the upper surface of the primaries. By late summer, Common Terns can have all red bills and the underparts turn whiter. At this time of year the most useful identification feature is that the Common Terns begin molting flight feathers before migrating to the wintering grounds while the Arctic Terns do not molt until arriving on the wintering grounds. Therefore, terns with missing flight feathers, and dark, worn outer primaries contrasting with fresh inner primaries will be Commons. Juvenile Common Terns have bicolored bills, brown-washed upperparts, and dark gray secondaries while juvenile Arctics have black bills, grayer upperparts, and paler secondaries. Basic and immature plumaged birds are best separated by structural differences and wing patterns. The Roseate Tern can be told from the Common Tern by its longer all white tail, darker bill (except in late summer), paler underparts and upperparts, and lack of a dark trailing edge to the undersurface of the primaries. In juvenile plumage, Roseates have dark foreheads, dark legs, and scaly backs while the Common Terns have white foreheads, pale legs, and brown-washed backs. The larger Sterna terns (Royal, Elegant, Caspian) have entirely orange or red bills, much larger bodies, broader wings, shorter tails and black legs. In Alaska, Aleutian Tern has a white forehead.