A couple weeks ago I wrote a post about identifying shorebirds in general, and White-rumped Sandpipers in particular. If you missed that post, you might want to check it out.
So in North America we have 5 small sandpipers (not counting extreme rarities, i.e. stints) that are usually collectively referred to as peeps. Three of them are smaller: Least, Semipalmated and Western Sandpipers. Two of them are bigger, though certainly not big: White-rumped Sandpiper and Baird's Sandpiper.
Let's talk briefly about spring. In the mid-Atlantic, in spring, there are NO Western or Baird's Sandpipers. None. Nada. Zip. Well, ok, maybe one Western. Maybe. I should perhaps clarify that as late as late April I have seen Western Sandpipers in the mid-Atlantic. These, I believe, are birds that wintered locally, and it can be fun to watch them over the weeks as they transition from winter/basic plumage to breeding/alternate plumage. Those birds invariably leave by the end of April. So come May, no more Westerns. Trust me on this one. As to Baird's, in both spring and fall it is a mid-continent migrant, and we only get a smattering of them in fall, usually juveniles.
On to "fall," meaning southbound migration, which begins in the last days of June for shorebirds. The photo above was taken in the South Cape May Meadows, NJ on August 8, 2015. This bird has everything going for it as a Western Sandpiper, except maybe that its bill is not one of those has-to-be-a-Western-because-the-bill-looks-like-it-came-off-a-Dunlin. However, the bill is still fine for a Western, long enough with a slight droop. The first thing I noticed on this bird was that it was well on its way to basic plumage. It's gray. The competing i.d. for Western Sandpiper is Semipalmated Sandpiper, and we pretty much don't see semis looking like this in the mid-Atlantic, and certainly not as early as August 8. Why? Because these species don't complete their pre-basic molts until they get close to, or are at, their wintering grounds. Semipalmated Sandpipers do not winter anywhere in North America, other than a few in south Florida. You might want to make a note of that; please do not report a Semipalmated Sandpiper in the mid-Atlantic or northward from December through March. As in all birding mistakes, this is a forgivable sin, but what you saw was certainly a Western Sandpiper or maybe a Sanderling, and your eBird reviewer is not going to validate a semi in winter.
Other things that are good for Western on the photo above include a thicker looking neck and bigger looking head than we see on Semipalmated. If you want more confirmation, click on the photo below to get a larger view, and look at the lower scapulars on either side of the bird.
All this Western Sandpiper plumage talk pertains only to adults. Juveniles are different, although they too have rufous concentrated on the scapulars. That's a post for another time. I have not yet seen a juvenile Western Sandpiper this fall.
On to the other eastern peep rarity: Baird's Sandpiper.
Baird's and White-rumped Sandpipers are bigger than the other peep, and that is clearly obvious in the above photo. The bird on the right is a Least Sandpiper. Baird's and White-rumped Sandpipers also share the trait of long wings (being longer-distance migrants than the other peep), and their wingtips project well past the tip of the tail. They look decidedly pointy or attenuated in the rear. To my eye this is more obvious on Baird's than White-rumped. Baird's is also one of the "grasspipers," meaning its preferred habitat on migration is very short, often slightly damp grassy vegetation. You don't often see Baird's wading in water, but it can happen. . .and Least Sandpipers like grass, too. And so do Pectoral Sandpipers, which are also scaly looking above. But pecs have yellow legs, and so do Leasts.
By far, most of the Baird's Sandpipers we see on the Atlantic Coast are juveniles. I don't even have a photo of an adult Baird's Sandpiper to show you. Juvenile Baird's Sandpipers are beautiful birds, usually richly buff with a neat, scaly pattern above created by pale edges to the upperparts feathers. A final tip: Baird's Sandpipers look the part, in shape and in pattern. If you're not sure, it's probably something else. Consider other juvenile peep or Pectoral Sandpiper.
Finally, about the "boink." I wrote in my White-rumped Sandpiper blog that "I hit a bird and my mind went "boink" - different."
Boinks have to be earned. The way to be able to boink on an unusual bird is know the common species cold. In this case, rather than spending all your time looking for Baird's or White-rumped or Western Sandpipers, look at all the Semipalmated and Least Sandpipers. No two are alike, and they are also quite fun to watch as they probe and feed and squabble and fly and land again. Before you know it, you will start boinking on the scarcer species.