I think you know you've arrived as a shorebirder when you can scan a flock of peep naked-eye and pick out a White-rumped Sandpiper from the predominant (in the east) Semipalmated Sandpipers. When you can do that, you've clearly paid your dues, looking at thousands of peep over a period of years, and understanding that size and shape really are the keys to shorebirding. It's not an easy thing to do.
Luckily, White-rumped Sandpipers have a couple "silver bullet" field marks, as I call them, the kind of field marks that if seen are pretty unequivocal. There's the white rump, for one, which you can see in the photo below, but the obvious problem is that if the bird has its wings folded, you can't see the rump. But, it doesn't take much patience to ride a suspect peep with your scope or bins until it stretches or lowers its wings briefly to show the rump.
Another White-rumped silver bullet is the streaking extending along the flanks, another is the wing tips extending beyond the tail, and another, if you're close, is the reddish base to the lower mandible. All those things are missing on Semipalmated Sandpiper.
So, what to do with this information? Use it to find a White-rumped Sandpiper the standard way, and then study your white-rumped until you get comfortable with the slightly larger size, more attenuated rear end, fuller chest - I dunno, they look like the athletes in the flock.
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