Recent Work - Studying the Dunlin
Dunlin at Jones Beach 9/23. Juvenile showing typical molt pattern of back preceding coverts, tertials, and scapulars in part. Dunlin at Jones Beach 9/23. Rump appears white, but close inspection seems to show a dark streak (if that's not a shadow). 
Juvenile Dunlins begin their molt before leaving the breeding grounds. With luck, you may find an early arrival that still has most of its juvenal feathers. Note the black streaking on the belly. A not so bright juvenile. But note the rusty scapular feathers. 
A juvenile that has begun replacing juvenile back feathers with gray feathers of first basic plumage. A juvenile with back feathers mostly replaced, but note the retained scapular feather (like the Jones Beach bird). Belly streaking has faded.
Obviously a Dunlin, in alternate plumage. Note the length of the bill.  Juvenile mostly in first basic. Note that this image gives the impression of a white rump. 
Curlew Sandpiper for comparison. This one has a shorter bill than in some Dunlins.   Curlew Sandpiper for comparison. Note the small head relative to the above images. To me, the structure looks more balanced than the front heavy Dunlin.