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Solution by Tony Leukering
Sometimes, we don't consider foreshortening, the problem of correctly interpreting something that we're looking down the length of, instead of looking across it. This may very well have been the primary problem encountered by almost half the respondents this week.
We've got a couple of small shorebirds. They're reasonably close, well-lit, and showing many important features. One bird's head is mostly hidden, but other than that foreshortening difficulty, we've got a pretty good look at the quiz birds. The two birds do not seem to be all that much different in size; the front bird might be a bit smaller, but that might be attributable to the differing positions and postures. They both have black legs, but neither of them is showing the feet well enough to discern the presence or, particularly, the absence of webbing. We cannot really see the back bird's toes, and the front bird's legs are perfectly positioned to have the upper part of the foot (remember, that's the part between the toes and the joint halfway up the legs, which is the ankle) block our view of the critical areas around the toes (although, I doubt if we could tell even without those legs in the way, as we're looking from behind).
As is typical for such birds, we might want to spend some time determining the age(s) of the birds, as that can greatly assist our endeavors to ID the beasties. There are not any useful non-bird clues to the season in the picture, so we'll have to study the birds. Both birds have white tips to the primaries and most or all of the coverts and scapulars, though, so we don't have to note much more. For the record, though, neither bird sports two ages of feathers and they do not appear to be in active molt, other than the front bird seems to be growing a new tertial on each side (note the bright white edge to that one feather on the right side). Finally, these birds have strong patterns to their feathers and the scapulars are nearly entirely black.
Now, if you knew that the picture was taken in early October, and that it was taken in Weld Co., Colorado (by Steve Mlodinow in 2011), you might be able to guess as much about the age of these birds without seeing the birds or the picture and know as much as we do now that we've analyzed the plumage carefully. That is because adults of small shorebirds (other than, perhaps, Dunlin and Purple and Rock sandpipers) should be on (or very much closer to) the winter grounds by early October.
Because of the back bird's very long wings, we'll start with that one, as the wing length, itself, rules out most of our options. That bird must be one of either Baird's or White-rumped sandpipers, because anything smaller has shorter wings and anything larger would have longer legs and/or be considerably chunkier and/or have yellow or orange legs. We cannot see the bill base of the back bird, so we cannot see the color of the base of the mandible (a quite useful feature at separating these two when at close range). However, we can see the bird's buffy-brown nape, which is enough to rule out White-rumped.
The bill of the front bird is well-seen and is entirely black, ruling out White-rumped for that bird. But, what about the other small shorebirds? There are all sorts of things that are wrong for Least Sandpiper, not the least of which is size; it should be simply dwarfed by a Baird's Sandpiper, in addition to that thing about leg color. The bill is too relatively long for that of any of the stints other than Long-toed, but that species has the same problems that Least does, so let us consider that difficult duo, Semipalmated and Western sandpipers.
Before going well down the road of plumage minutiae (e.g., the long, pointed lower scaps of Western and the truncate scaps of Semi, neither of which really matches this bird), let's look at the bill. The bottom edge of it is virtually straight, ruling out Western, and the bill is just too long and fine, both at the base and at the tip, to be the bill of a Semipalmated Sandpiper.
Now, we've come full circle back to that foreshortening thing. Yes, the wings look like they do not extend past the tail, but we cannot accurately judge that feature by looking straight up the wings, because we have no true frame of reference, as we do on the back bird. Both birds are first-cycle Baird's Sandpipers (technically, at least the front bird cannot be a juvenile, because it has initiated its first post-juvenal molt -- the preformative molt).
CAUTION: Please note that the quiz asks for the name(s) of the species present in the quiz picture and that species names are not plural (except in cases like the two species of yellowlegs). So, even though two Baird's Sandpipers were photographed, the submission would still read "Baird's Sandpiper." Those providing such plural constructions in their submissions were not penalized this time, but note the wording of the rules. This situation has come up previously, often when geese are involved.
Five respondents provided answers including two species, four of which got one of the birds correct, though I don't know which one (but, I suspect the back one).
Incorrect species provided as answers:
Semipalmated Sandpiper - 9
Western Sandpiper - 2
White-rumped Sandpiper - 2
Congratulations to the 15 of 28 getting the quiz correct:
Bill Blackburn
Ben Coulter
George Cresswell
Patty McKelvey
Robert McNab
Megan Miller
Margaret Smith
Jeff Thompson
Dave Elwonger
Su Snyder
Sean Walters
Tyler Bell
Thomas Hall
Joel Such
Marcel Such
Answer: Baird's Sandpiper
