This Willet appears to be in breeding plumage in December. Birds of North America Online describes the definitive pre-basic molt in western Willets as normally complete by the end of September, and I don't remember ever seeing one in this plumage in late fall or winter. Because the feathers look fresh, this bird presumably had its pre-basic molt in late summer like other Willets, but the new plumage was of the breeding rather than the normal non-breeding type.