Ilija is correct on this bird. A couple keys in separating basic/winter plumaged Forster's from other small terns is the extend of the black mask around the eye and the bill size. At close range Forster's seem to have oversized bills that are particularly thick at the base when compared to other similar-sized terns (Common, Arctic, Roseate, Aleutian). After the first year, basic/winter plumaged Forster's will show a black patch around the eye that is completely surrounded by white, with no dark wrap around the back of the head. In the other small terns, the black around the eye wraps around the back of the head in all age classes. In the case of your bird, the weak dark wrap around the head (mottled rather than solid black) is a characteristic only shown by first-year Forster's Terns. When this bird goes through its prebasic molt this fall (should be happening right now) the head will be all white except for the black eye "patch."