Identifying shorebirds foraging on ice-glazed, wave-washed rocks in December is fairly straight-forward. Only four shorebird species regularly occur in Maine this late in the year: Sanderling, Dunlin, Ruddy Turnstone, and Purple Sandpiper. Sanderling and Dunlin can be ruled out by the habitat; they favor beaches and mudflats. Ruddy Turnstones will forage on rocks but with rare exceptions they've flown south by now, and, in any case, they have white unmarked undersides. Thus, our birds must be Purple Sandpipers.
Purple Sandpipers have orange legs, a dark bill with an orange base, and overall gray plumage. In the following video, watch how they fly up to avoid a wave, then quickly return to foraging. This behavior is typical for these hardy winter birds.