First off, thanks for all the responses to the mystery bird quiz. Below is the photo again to refresh your memory.
The top bird and the bottom bird seem to be the best place to start in identifying these birds. Both birds are similar in size and immediately give the impression of one of the Calidris sandpipers. Both have a rather long, slightly drooping bill, grayish-brown head, white belly, and black legs. The bottom bird is showing a white underwing with gray tips to the flight feathers and the top bird is showing a gray upperwing with a white area through the middle of the flight feathers. The top bird gives the impression of being smaller, potentially being a Western Sandpiper. However, the bill is a bit too long to be a Western and the overall chunky look of the body doesn’t agree with the slim look of a peep. A Western Sandpiper in non-breeding plumage should have a colder gray tone to the feathers, while both of these birds are more gray-brown. Both of these birds are non-breeding Dunlin. Here is a photo of another Dunlin in non-breeding plumage for comparison.
Finally, let’s look at the center bird with yellow legs. This bird is similar is size to the two Dunlin; mostly gray and white; and has a black, drooping bill; but the yellow legs quickly rule it out as another Dunlin. It has grayish armpits and a white supercilium. The first bird that may come to mind is a Lesser Yellowlegs. This species fits most of the descriptions, except Lesser Yellowlegs have a very straight bill without any droop and they are a bit bigger than a Dunlin, roughly 2 inches longer in length and a 7 inch longer wingspan. Also Lesser Yellowlegs typically don’t show such a profound white supercilium. Greater Yellowlegs can be ruled out also, because once again the bird is not nearly big enough and a Greater Yellowlegs bill is slightly upturned, opposite of the mystery bird. The only other North American species that fits the description then is the Stilt Sandpiper. This species is uncommon to rare in PA, but occurs occasionally during migration. The Stilt Sandpiper has a very obvious white supercilium in the adult nonbreeding plumage, yellow legs, and a somewhat long, drooping bill. Take a look at this photo of the same Stilt Sandpiper as in the mystery bird photo but in a standing posture.
These photos were taken at the Kriner Road Water Retention Pond in Chambersburg, PA last October 25th. Ironically, there was also a Lesser Yellowlegs present so below are some photos that demonstrate the sizes and shapes of Dunlin, Stilt Sandpiper, and Lesser Yellowlegs. In the first photo notice the similar sizes of the Dunlin and Stilt Sandpiper, and the much larger appearance of the Lesser Yellowlegs. Also note the lack of an obvious white supercilium on the Lesser Yellowlegs. In the second photo, take a look at the noticeable size difference in the Stilt Sandpiper and the Lesser Yellowlegs, the lack of a white supercilium in the yellowlegs, and the shape of each of their bills.
There were many responses, which made this more fun. Most responses I got said they believed both the top and bottom birds to be Dunlin and they thought the center bird (with yellow legs) was a Stilt Sandpiper, so good job! Several responses thought the top and bottom birds were Dunlin, but considered the center bird to be a Lesser Yellowlegs. Some folks thought the top bird could be a peep of some sort, maybe a Western Sandpiper or White-rumped Sandpiper. Thanks so much for participating and I am very, very sorry for the poor quality of all of these photos, but I believe they were good enough to explain the identifications.
Sibley, David. The Sibley Guide to Birds. New York: Alfred A. Knopf, 2000. Print.