Semipalmated Sandpiper (Calidris pusilla)

https://www.allaboutbirds.org/guide/Semipalmated_Sandpiper/id

http://www.thewarblerguide.com/blog/2013/8/18/easy-id-points-for-separating-white-rumped-and-semipalmated-sandpipers

least and semi (over two or three pages)
http://www.surfbirds.com/Features/coxpeeps1006/coxpeeps1006.html

http://www.birdzilla.com/birds/semipalmated-sandpiper/description.html

http://thebirdguide.com/identification/semipalmated_sandpiper/peep.htm

http://freidaybird.blogspot.com/2013/05/