We also have 6 alpina, and 1 arctica. From this sample it seems that both C. ferruginea and C. alpina in basic plumage are streaked on the breast, sides of neck, and face. The difference is in the pattern and density of the streaking. The difference, on the scale of the individual feather, is on Dunlin the dark streak along the feather shaft is shorter and broader, and set on a generally gray feather which pales towards the edge. On CUSA the streak is finer and the rest of the feather is paler - almost white.
Dunlin also tend to be plainer on the face as well.
On CUSA the streaking is finer, cleaner and more distinct. Of the 3 subspecies of Dunlin in our specimen collection the subspecies arctica, and alpina more closely resembled CUSA in terms of face, neck, and breast streaking. On some specimens the streaking is obvious and can extend down to the base of the vent. On others the streaking was much reduced along the flanks and on maybe 5 individuals appeared absent entirely. However on these specimens the apparent lack of flank streaks may have been due to displacement of feathers during the preparation of the specimen.