Sandpipers at Punta Chame

Euclides Campos sends in two interesting photos of interesting sandpipers taken at the former shrimp farms at Punta Chame.
The first one shows two stilts: a Black-necked Stilt and a Stilt Sandpiper. Can you tell which is which? Notice especially the typical sloping forehead and pronounced, rounded crown. We say the sandpiper is an adult molting into basic plumage (fide the remains of barring in the underparts and the few leftover black feathers in the mantle and scapulars).
Then there's this bird, seen foraging with Semipalmated Sandpipers. It was identified in the field as a White-rumped Sandpiper (and the jizz seems right for that), but the chest seems too dark even for a juvenile. What do you think, freak lighting effect or Pectoral Sandpiper? Click on the photos for full size and chip in (via the comments).
[Update: consensus seems to lean strongly in favor of White-rumped Sandpiper]