Sterna hirundo

From the excellent blog 10000birds.com

A small minority of Common Terns on Long Island during June resemble breeding adults, but exhibit one or more differences from definitive breeding condition: the lesser coverts are dusky, the forehead is white or flecked with white, the underparts are less extensively gray, the tail streamers are shorter, or the bill or legs show dusky markings. These birds are called “second-summer types” and include two year-old birds as well as some older birds that for whatever reason fail to attain full breeding condition.