Had only a brief look at this bird and clicked of a few fairly distant shots that are enlarged here. Because of the absence of streaking on the chest throat and and the generally diffused look including apparent absence of moustachial stripe, my preliminary call was Nelson's. Knowing that this would be an unusual record and just having a nagging uncertainty, I sent the shots to one of the very helpful and wise experts in the area, and he came back with the Saltmarsh verdict. Worn Saltmarsh juveniles sometimes don't have the striping you'd expect (though none of the books tell you this!). You can see a little on the sides in some of the pictures and that is apparently sufficient. I do hope to see a worn Nelson's juvenile one of these days so I can compare the two - rather arcane I guess, but I'm curious!