What Bird Is That Forum

I saw these birds today at Conaskonk Point in Monmouth County, NJ. I took a bunch of pics of semipalmated sandpipers, but Merlin is seeing these as white-rumped. I have absolutely no idea how to differentiate the two, so any help would be greatly appreciated. (On the last pic, Merlin sees the back bird as white-rumped..)

I also have trouble with these two species unless they help out by spreading their wings or flying.
I took a look at the Mac library to check the typical ID marks for breeding plumage and found the following:

Orange at base of bill: almost always found varying from very distinct (bright and a good length of the bill) to subtle and quite difficult to see. Fairly distinct in most cases.

Long wings, stretching beyond the tail: at the right angle usually clear, but not always.

Streaking on flanks: apparent in most cases but sometimes hidden. I looked at SESA for this and it seemed about 5% to 10% of them can also have this.

Longer body: sometimes apparent but depends on posture ( as in last photo).

The streaking on the flanks of these birds suggests White-rumped but I don't see any orange or paleness at the base of bill.

I'm hoping one of our more experienced birders can provide more insights.