They get their name by a patch of white above the base of their tail. It mostly shows when they are in flight. They tend to be found in mixed flocks of migrating sandpipers so that white rump sets them apart when the whole flock flies. There were several of them there but they never flew so I didn't get to see the white. As you can see, the wing feathers cover it up when on the ground.
White-rumped Sandpiper (Calidris fuscicollis)