When identifying terns, it is safest to rely upon a combination of field marks. The Arctic Tern is similar to the Common, Forster's, and Roseate terns. It can be told from the Common Tern when perched by its longer tail projecting beyond the wingtips, shorter legs, and shorter bill. In flight, the wing patterns are distinctive. The Arctic Tern has extensively translucent primaries while just the bases of the inner primaries are translucent in the Common. The trailing edge to the undersurface of the primaries is thin and crisp in the Arctic but thicker and blurrier in the Common. On the upper surface of the wing, the primaries are evenly colored in the Arctic Tern while there is a dark wedge in the Common Tern. Because Common Terns begin molting during migration, there is often a mixture of old, new and missing feathers in the wing. The Arctic Tern does not molt during migration and appears evenly colored in the wing. In the breeding season, May and June, the Arctic Tern has an all red bill and darker gray underparts than the Common Tern. Juvenile Arctic Terns have grayer upperparts, dark bills, and white secondaries. Immature plumages are best separated by the underwing pattern and structural features. At all times of year except the breeding season, the Forster's Tern has a distinctive black eyespot. In alternate plumage, it has a dark-tipped bill, white underparts, gray inner edges to the outer tail feathers, and whiter primaries. Roseate Tern in definitive alternate plumage has a black or mostly black bill, paler underparts and upperparts, and lacks dark trailing edge to the primary tips below. Juvenile Roseates have crisp black scalloping on back, dark legs, and dark foreheads. The larger Sterna terns (Royal, Elegant, Caspian) have longer, thicker bills, much larger bodies, broader wings, shorter tails, black legs, and ragged crests at rear of head.