Actitis macularius or Tringa solitaria?
At the time I photograhed this bird I was quite certain it was a Spotted Sandpiper, based on the behaviour, however, Geoff suggests in his comment that this is not Spotted but rather Solitary, so I scoured my books and polled some birder friends and I now I just don't know!
Here is a summary of the characteristics in favour of Solitary Sandpiper:
Tail barring, eye, ring, slate gray back colour, fully dark bill (should be dull orange in Spotted in fall) and posture with neck extended.
Characteristics in favour of Spotted Sandpiper:
Long tail, white throat (Solitary shows streaking on throat and upper breast), white in front of the wing, lack of spotting on back and stripe through the eye.