This photo was taken on the peat peninsula that juts south into Duxbury Bay, MA from the area known as High Pines. I had my first Nelson's in the same general area in 2010 (5/26).
Nelson's Sparrow is one of the two species formerly called Sharp-tailed Sparrow (Saltmarsh Sparrow is the other). The trick is telling the two apart and in the spring migration Nelson's are apparently not frequently seen, so a Duxbury sighting is, I'm told, useful record.
The trick is differentiating the migrating Nelson's from the more common Saltmarsh Sparrow which is what we usually have in the summer. These three shots show the dull blurred striping on the breast and the fact that the malar (lower cheek) stripe coloring blends into that of the throat and chest without much contrast. The back is essentially dull as well.