ID on a photo is not always easy, since the size is often difficult to get, and one get only a limit number of view. Like this one, I am not quite sure of the size, which would help to eliminate some possibility. If the choice is among Roseate and Common, the photo certain favour Roseate in non-breeding plumage. However, I am not sure others are totally eliminated, like Arctic Tern. The bill looks short to me for Roseate. May be that is an effect of the angle of view.
HF Cheung
