There are at least two races or sub-species of the Rock Sandpiper. These being the Pribilof Rock Sandpiper and the Aleutian Rock Sandpiper. Both species are similar in size, body profile may make the Pribilof species seem more chunky and heavier. The Aleutian species has a darker plumage in both breeding and in non-breeding seasons and shows a duller reddish-orange back and a darker face in the summer months and a noticeable darker grey body in the winter period.
The Rock Sandpiper is probably the hardiest member of the sandpiper family and can still be seen in the northern regions of the continent. In the winter months Rock Sandpipers can be seen along the coastlines from Southern Alaska to Northern California.
ABA - American Birding Association This site represents an organization that maintains official records of all birds species that have been proven to have been seen inside the perimeters of the North American Continent and the surrounding bodies of water. Regular revised versions are posted to keep the bird list current at all times. This is the list used by all serious birders over their lifetime. You may be aware of the movie called the "Big Year". It was with this list that all the competing birders used in an attempt to set a new record as to how many bird species that could be seen by an individual birder in one calendar year.