To refresh your memory, here is the photo from July’s Bird Quiz. It was taken in Colorado during the month of May. Don’t read any further if you want one last chance to identify these birds.

I bet you could you tell these were sandpipers without looking them up. That’s great; now you know where in the field guide to start reading. Of course, the fun begins when you’re asked, which sandpiper? These aren’t the simplest birds to pin down, but it’s actually much easier to identify these birds from a photo, rather than squinting through a spotting scope while they run around on a distant shoreline. You can do this!

Remember, I took the picture in May, in Colorado. May means migration, which gives us plenty of species to choose from. While these could be accidentals, straying off course to visit our lovely state, it’s unlikely. Plus, the photo shows two birds, and having two strays at the same place at the same time is even more unlikely. For sanity’s sake, we’re going to stick with the locals—birds that commonly migrate through Colorado. (Most sandpipers nest on the tundra, far north of here.)

When it comes to identifying sandpipers, we need to look at two main features—the length and color of the legs, and the size and shape of the bill. Both of these attributes influence where along the shore the birds feed. Short legs do best in shallow water, long legs can venture out further. Short bills probe for animals just below the surface of the sand or mud, long bills go after those buried deeper. All this is to ensure that, even with multitudes of species feeding on a mudflat, they don’t compete with one another.

Of course, there are other features: the size of the bird, any color variations (especially during the breeding season), and their behavior. If we’re lucky, we’ll hear their flight song, which helps immensely.

These birds are relatively small, as you can see by comparing them to the pond scum.

They have typical sandpiper coloring, with a tan chest and neck, tan/brown splotchy back, and white undersides with minimal spotting on the flanks.

As far as behavior, we can see that they’re feeding in the water. We don’t know if they’re bobbing their tails (as do Spotted Sandpipers), or probing the mud like a sewing machine (like Dowitchers), but that’s all right in this case, as we can immediately rule out these species on the basis of other characteristics.

The legs are pretty short. Right away, that eliminates all the long-legged waders. And black legs means we can rule out a bunch more. (Determining leg color is very helpful, but when a bird is wading in dark, sticky mud, it’s easy to be fooled.)

Now it’s time to look at the beak: medium length, fairly sturdy, more or less straight, although the bottom curves to give it a very, very slightly downward turn at the end.

We’ve narrowed our choices to three similar species that pass through Colorado: Western, Semipalmated, and Baird’s.

How do our candidates measure up?

Sibley describes Baird’s Sandpiper as having a back that’s “silvery with black spots.” Also, their wing tips project well beyond the tip of the tail. The bill is thin and straight. I don’t know about you, but I don’t think that describes these birds. Mark the page in the book and turn to the Western.

Breeding Westerns have pale rufous on their crown, auriculars (the feathers over their internal ears), and scapulars (shoulders). These birds are a lovely warm brown, but I have a hard time calling it rufous. Westerns are thick-bodied—sort of front-heavy—with relatively shorter wings. These birds have shorter wings, but they don’t look stocky to me. Finally, Western beaks are noticeably longer than the other two species under consideration, with a distinct downward bend. Nope. Next?

Semipalmated Sandpipers are plain gray-brown to rufous. They have a shorter beak than Westerns, and it’s straighter, too. It’s sturdy without being thick. When folded, their wingtips end just at the tail feathers. It’s looking good!

I was lucky to have an expert birder on this field trip, who confirmed my tentative ID, so I feel pretty confident about calling these Semipalmated Sandpipers. Did you figure them out ahead of time?