My observations and comments on subspecies as well as molt are here:
a few photos here:
comments there on characters too
To get folks thinking of other possibilities about the Scarborough bird, I noted that Alaskan breeders differed in molt schedules. In my hurry on the road, I bolloxed the point and amended it with references in later posts. Here is the key reference again and the title gives you a hint: "Latitudinal differences in the breeding and molt schedules of Alaska Red-backed Sandpipers (Calidris alpina)" (Holmes 1971, Condor 73: 93-99). The point I *tried* to make is that the subspecies, pacifica, breeding in western Alaska, has a more extended, less compressed, molt than the other race breeding in northern Alaska, arcticola. Was it worth considering? Well, both should still molt on the breeding grounds or nearby staging areas, but could a vagrant not follow expectations?
Some aspects of the Scarborough bird are like those of an Alaskan breeder, e.g. pacifica or arcticola. It shows a white band below the breast streaks and ahead of the black belly patch. But other characters suggest it is one of our usual migrants, hudsonia: it has obvious streaks on the rear flanks and dark shaft streaks on the undertail coverts. I'm not sure what character trumps here and what the range of variation is. The possibility of a West Coast vagrant (or East Asian sakhalina?) seem open, though hudsonia is likely what this bird is.
The Scarborough bird is in wing molt and has replaced coverts and tertials. The old primaries and wing molt were easy to see with scope views in the field. Although many one-year-old birds look like they are in winter plumage (dull and gray), some become bright and acquire breeding plumage. Such a bird might appear like this bird. The heavily worn and old primaries on the Scarborough bird are typical of such younger birds in their second cycle.
To compare, here is a link to a northbound, though late, spring hudsonia Dunlin (I photographed the bird at Popham Beach 12 June 2012): https://flic.kr/p/cea7GQ
Note that this bird shows the breast streaks extending too the black belly patch (no white band) and shows obvious, though fine, streaks down the flanks. I also see that the amount of black and tone of the orange-rufous differs from the Scarborough bird, but that could be due to changes with wear. (Note: I wrote in my comments THEN about the complete molt of North American birds before migration; just so certain folks know ;-)
I am glad that people took notice of this bird. On August 8th, I wrote this warning to check late summer and fall (Sept) Dunlins that show breeding plumage because: "The North American, Pacific, and east Asian populations of Dunlin molt on the breeding grounds or at molt migration stopovers far to our north." (http://tinyurl.com/nwk8sag). The idea is that those appear down here in winter-like plumage. This was in reference to the Popham Beach bird that appeared to be a Greenland Dunlin (C. alpina arctica), which the Scarborough bird clearly is not.
Louis Bevier
Fairfield
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