[clydebirder]

In mid - late Summer the wing pattern is the best way to ID Arctic from Common Terns. Common Terns have a grey wedge on the upper primaries which is obvious from a fair distance away, the Arctic is much cleaner winged with no obvious grey on upper primaries. From underneath the Arctic Tern only has a small dark neat border to the trailing edge of the primaries whereas the Common has larger more diffuse dark markings on the underside of the primaries. Some Arctic Terns can have a diffuse dark tip to the bill which can cause confusion, tail streamers are longer on Arctic but in late summer can be broken off

cheers, John