The verdict is not in on this bird. It's bill seems consistent with Semipalmated but bill lengths vary and books say that sometimes cannot be distinguishing. The full winter plumage is consistent with Western Sandpiper (WESA) becasue they molt this early in the year whereas Sempalmataed Sandpipers (SESA) don't reach full molt until on wintering grounds (which are not Duxbury, MA). The feet might be helpful but no photos show the feet.