I took a late-afternoon ride to check shorebirds in Cell 3 at Pointe Mouillee SGA. I parked at Roberts Rd and rode the bike over to Cell 3. Interestingly, I found a patch of mudflat in the NW corner of Cell 1 that could be productive for shorebirds later this summer. It's about an acre in size with water deep enough to support a couple dozen Mallard and several Blue-winged and Green-winged Teal.
When I finally got home I consulted my shorebird guides to compare Dunlin and Curlew Sandpiper traits. O'Brien, et. al. (2006) lists the Curlew Sandpiper (Calidris ferruginea) and Dunlin (Calidris alpina) as overlapping in length and weight, so size-wise, they are very similar. Dunlin subspecies C. a. hudsonia are known to show streaking along the flanks that are visible in eclipse (non-breeding) plumage (Paulson, 2005). Things to look for in a Curlew Sanpiper in basic plumage include: distinct white supercillium, white belly w/o spotting or marks on flanks, 'slender' look and 'taller' appearance with slightly thinner bill relative to Dunlin, and 'white rump' that is larger in coverage than a White-rumped Sandpiper. Wing projection also extends beyond the tail, whereas Dunlin wing projection is just short of the tail.
Things to look for in a basic plumaged Dunlin include: smoky gray / brown plain plumage with distinct hood appearance, lack of distinct supercillium, chevrons or spots along flanks, bulkier appearance, and dark rump w/ little white. From the images above you can see wing projection just short of the tail.
Late summer / early fall is typically the time when confusion between Dunlin and Curlew Sandpipers can occur. I found a couple of good articles that summarizes their differences and what to look for when trying to identify 'mystery' birds: http://www.dublinbirding.ie/pages/features/Curlew%20Sand%20ID/getting_to_grips_with_curlew_san.htm and http://www.oceanwanderers.com/BSmallSHorebird.html. Both are worth looking at, as well as the slew of photos that are available online. Just Google 'basic plumage Curlew Sandpiper'.
In the end I think there's no confusing this bird as anything but your basic Dunlin. But I'm glad I spent some time chasing it and learning a bit more about what to look for when that Curlew Sandpiper does show up at Pt. Mouillee. After all, we're overdue for another one...
Hayman, P., Marchant, J., and Prater, T., Shorebirds, An Identification Guide, 1986, Houghton Mifflin Co., Boston.
O'Brien, M., Crossley, R., and Karlson, K., The Shorebird Guide, 2006, Houghton Mifflin Co., Boston, New York.
Paulson, D., Shorebirds of North America, The Photographic Guide, 2005, Princeton University Press, Princeton and Oxford.