By Guillermo Rodríguez
Hudsonian Dunlin (C a hudsonia) records have been claimed several times in the UK & other countries in Europe, usually adults in mostly summer plumage in late spring and summer, when the pale head and bright back of Hudsonian is quite different from European birds (but, on the other hand, is difficult to separate from several Pacific taxa). However, Hudsonian Dunlin also differs from Dunlins that occur regularly in Europe (alpina/schinzii/arctica) in juvenile/first-winter plumage, when the likelihood of a vagrant is perhaps higher. Dunlins arrive in good numbers to the east coast of the United States in late September, only slightly later than Baird’s Sandpiper and American Golden Plover.
After careful study of a few hundred first-winter birds, I think some of the features typically shown by Hudsonian are relatively rare in European birds, and their combination on a prototypical individual might indicate a nearctic origin.
Moult: probably the most obvious and eye-catching difference at first glance (in September/early October) is that the birds arrive from the breeding grounds already in a remarkably advanced moult. Most birds have moulted head, breast, and most of the back feathers; the belly, rump and wing coverts are usually moulted slightly later. For instance, all 9 first-year birds seen in Massachusetts on September 24th were in moulted plumage; only one out of 76 was still in mainly juvenile plumage on October 1st; and only 3 out of 54 seen on October 8th showed partially (but still fairly advanced) juvenile plumage. Thus, the proportion of birds arriving with retained juvenile plumage may be well below 5%. At this time of year, most first-year Dunlins in Europe have some replaced scapulars, but usually not most of them and rarely if ever show a complete winter plumage overall (ie also including head, mantle and breast). Some Hudsonian Dunlins leave some back and rump feathers unmoulted until the late autumn and winter, showing a characteristic line of conspicuous rufous-fringed scapulars contrasting with the moulted greyer feathers, reminding in some way first-year Western Sandpiper.
Bill shape: Hudsonian Dunlins on the East Coast present limited variability in bill shape and length, at least compared to Dunlin taxa in other parts of the world. Most Hudsonians have a long bill, which curves down at the end, somewhat similar to Curlew Sandpiper. Some birds also show a sort of ‘drop’ at the bill tip. Even in the shortest-billed birds the bill shape is characteristic, and considerably different from the majority of Dunlins in Europe (although, obviously, European birds sometimes show similar bills).
Facial pattern: many Hudsonians present a characteristic facial expression: the brow is usually broad, and ends sharply just beyond the eye; in some birds there is a conspicuous pale area in front of the eye. The cap and nape are roughly concolorous, and thus more contrasting with the supercilium. In European Dunlins, the nape tends to be paler and so the supercilium seems to “merge” with it, contrasting with a slightly darker cap. As a useful comparison, these differences are reminiscent of the differences in facial pattern between juvenile Western and Semipalmated Sandpipers (with Hudsonian being more similar to Western).
Breast and flanks patterning: this feature has been considered as diagnostic in the past. Once they have moulted into winter plumage (but generally not in juvenile plumage!), around 70% of birds present sparse short streaks on the flanks of the underparts, beginning at the breast and sometimes reaching as far as the undertail. In most of these birds, the dirty grey patch on the breast partially extends towards the belly along the flanks. In most European birds, the underparts are neatly white, but some exceptional birds might show a similar streaked pattern to Hudsonian.
Some examples illustrating the variability of first year Hudsonian Dunlins.
Even in the shortest-billed individuals the bill drops down at the tip. Note also the characteristic pattern of the supercilium.
Note the similar bill shape and size in most individuals. Variability is relatively limited in the East Coast!
For comparison, look at this typical Dunlin from N Spain, still in mainly juvenile plumage in September:
and a slightly more advanced bird (October), with extensive replacement on mantle and scapulars but still retaining juvenile head and breast feathers.
Finally, check out this individual from the Canary Islands, identified as a (potential) Hudsonian on grounds of the flank streaking. The Spanish RC studied and eventually rejected this record. I think the bill does not look particularly good and the breast is too neat for Hudsonian, but undoubtedly a difficult bird!