Sterna hirundo

From the excellent blog 10000birds.com re 1st summer Common Terns:

Shape and structure are unmistakably those of Common Tern, but coloring is radically different: the carpal bar is darker and more extensive, the forehead is entirely white, the underparts are mostly or wholly white, the bill and legs are blackish, and the tail streamers are even shorter than those of second-summer types. These are first-summer Common Terns, and they are unusual birds themselves in North America, as most stay south during their first summer.