Stilt Sandpiper juveniles

Rear bird shows off the plain upperwing pattern and white rump/uppertail coverts contrasting mildly with the plain gray tail. Yellowlegs have a similar pattern in flight, though shape is different and the tail is barred.

You can just see the yellowish legs of the front bird. This is worth noting, if just to help exclude the possibility of the similar Curlew Sandpiper.