"Peep" are the smaller sandpipers in the genus Calidris. In North America these include the regular Western, Semipalmated, and Least Sandpipers, as well as occasional vagrants such as Red-necked, Temmink's, Long-toed, and Little Stints.
In Oregon, Western and Least are common in spring and fall migration. Least is uncommon in winter, while Western is rare in winter, more likely on the coast. Semipalmated is very rare in spring and rare in fall (July to mid-September). Since Semipalmated is so similar to Western, how do you tell the difference? Let's start with a rather obvious Semipalmated Sandpiper photographed by Chuck Gates on July 30 at Prineville Reservoir, Crook Co., Oregon. They are not all this obvious; July birds are generally easier to identify than September birds.
"Obvious?" you ask. It looks like any one of a million little brown shorebirds.
What do I look at first? Well, I can't help it, I am drawn to the very short, very blunt-tipped bill. That's likely because this is a smaller-billed male. Black, or dark olive legs. The bird is quite pale, grayish and brownish, not bright rufous. It's a good candidate for Semipalmated Sandpiper. So, how do you convince the skeptics?
This is where you must know the correct location and terminology of the feather tracks of the wing. First find the scapulars.
The feathers circled above are the scapulars. They are brownish-centered with black subterminal anchor-shapes, and very broad, pale buffy (buff = the color of buffalo leather, a dull yellow) fringes around each one. On an adult or juvenile Western Sandpiper these feathers would be edged in bright rufous or chestnut.
The feathers circled above are the wing coverts. Again, notice these are brownish-centered, with black subterminal anchors, and buffy or grayish fringes. A warning, though, some individual juveniles can be lightly washed with orangish on scapulars and wing coverts, similar to adult Western Sandpipers. In such a case it becomes even more essential to separate juvenile and adult birds.
Why is this bird a juvenile? The July 30 date is better for an adult--they migrate south first. First of all, the scapulars and wing coverts are fresh and new. They have wide, bright, clean, crisp fringes all the way around the feather margins. Adults get new wing feathers in spring. Then they migrate north to the Arctic. Then they spend a month breeding and incubating eggs. When you see them on the return fall migration, the wing feathers are tattered and faded. The edges of the adult feathers are frayed and what colored margins remain are usually worn quite narrow. Secondly, the shape of the scapulars and coverts are quite rounded at the tip. Adult wing feathers are much more pointed. Thirdly, Adults are generally streaked on the sides of the breast, while Chuck's photo shows a nice even buffy wash across the chest. This is a juvenile bird.
So what? Well, certain look-alike plumages can be eliminated. Now that we know it is a juvenile, the similar adult wings of Western Sandpiper, Red-necked Stint, and Little Stint can be ignored. The gray lower wing coverts of juvenile Red-necked Stint are different from our bird, too. Looking only at the scapulars and coverts of this peep, the only bird we haven't eliminated is juvenile Little Stint.
The circled tertial feathers above are very long and narrow. When the wing is folded they cover most, if not all, of the primary feathers. In flight, these tertials are actually the elongated innermost secondary feathers, next to the base of the wing. They are black with pale edges that appear to be slightly orangish--not bright rufous as Little Stint, just lightly washed with cinnamon. The blunt-tipped bill is also different from the tapering bill of Little Stint.
There. We've carefully eliminated all the alternatives. By noting the condition, pattern, and colors of the scapulars, wing coverts, and tertials, we were able to age the shorebird and make an unequivocal identification.
Mike Yip sent me the following two Semipalmated Sandpiper photos.
I don't know who sent me this one that was taken August 8, 2003.