Least Sandpipers (Calidris minutilla), Semipalmated Sandpipers (Calidris pusilla), Western Sandpipers (Calidris mauri) and Sanderlings (Calidris alba) all occur in the US and can easily be confused with each other, especially in winter.
Least Sandpipers, however, have greenish-yellow legs while the others have black legs.

1st & 2nd photo: Flamingo Visitor Center area, Everglades National Park, Florida, USA, 24 November 2012
3rd & 4th photo: San Carlos Bay, Bunche Beach Preserve, Fort Myers, Florida, USA, 22 November 2012
5th photo: Alfred A. McKethan Pine Island Park, Spring Hill, Florida, USA, 25 March 2022