There were 2 Dunlin in this pond who I photographed (I posted a picture here before this one here). You could be right--However, there is a very dark line at the bottom of its belly which could be the edge of the dark patch of the Dunlin. Also, the beak appears to be larger and a little downcurved. Finally, the Dunlin image appears to be larger than the bird at the front even though its further distance from the camera. These were the facts I used to think its a Dunlin--all these observations are easily disputed. There were white-rumped sandpipers present in the pond so your interpretation of the image is possible. Unfortunately, I was using a 500 mm lens on a cropped-sensor camera and the distance was quite far---so I could not tell with the naked eye one way or another.