I am no expert on shorebirds especially in fall/winter plumage so first off could you follow this bird until may and get a good breeding plumage shot?
Here are my thoughts...
legs seem a little short for yellow-legs or stilt SP
bill is thick for a yellow-legs, seems more in line with a short-billed dowitcher, but stilt does have thicker bill with very slight dorwn-turn dowithcher has more of a straight bill, yellow-legs have a slight up-turn
I think we can rule out yellow-legs esp by the bill
The spots under the tail and behind the legs don't appear to be on stilt , in my Nat geo guide, Sibley is out in the truck so i will check later. Spots do seem common on Dowitcher.
My guess would be short-billed dowitcher but shore birds are not my best bird species, give me a few ducks and I'll nail each one.
So please find this bird in may for some breeding shots!