Mystery Bird: Western Sandpiper, Calidris mauri

tags: Western Sandpiper, Calidris mauri, birds, nature, Image of the Day

Rick Wright, Managing Director of WINGS Birding Tours Worldwide, writes:

Quick: big or little? Even without careful measurement of the diameter of the surrounding water molecules, it's obvious that this is a small bird, compact, even stumpy, and I'd venture to guess that it's equally obvious that we're looking at a shorebird. No plover shows a bill as long or as evenly tapered as this bird's, or upperparts as complexly patterned. Everything fits for a sandpiper, and for a small sandpiper of the genus Calidris.

Oh no: a little calid! We call them peeps, the Brits call them stints, and lots of birders simply call foul. But with a little thinking, and especially with a view as close as this, most small sandpipers really aren't bad. Even just recognizing this bird as a small Calidris narrows the field considerably, giving us a great headstart as we work towards an identification.

Let's begin with what the bird is doing. Since it's belly-deep in the water, we can be confident that this is neither a Sanderling nor a Least Sandpiper: the former trips happily along the tideline, never letting its toes get wet; the latter is a true mudpiper, generally preferring the edges of open wetlands where the water has receded to leave a duckweed-covered mess. Structurally, neither of those species is a good fit anyway: Sanderling is like a tennis ball with a golf tee stuck onto its face, while Least Sandpiper carries a delicately curved, fine-tipped toothpick in its mouth. Structure also easily rules out White-rumped and Baird's Sandpipers. While our quiz bird is decidedly chopped off at the rear and front-heavy, both Baird's and White-rumped are "swallow-tailed" on the ground, the extraordinarily long primaries extending noticeably beyond the tail, making them look nearly ternlike when perched. Temminck's Stint is just the opposite, the tail longer than the wingtip, and it is never as colorful above or as heavy-billed as our bird.

So we're down to Spoon-billed Sandpiper, Red-necked and Little Stints, Western and Semipalmated Sandpipers, and Dunlin. It's time to look more closely at that heavy bill, which rules out Spoon-billed Sandpiper and both Red-necked and Little Stint, but to my eye is very like the bill of a miniature Dunlin: deep at the base, relatively fine at the tip, and curved throughout its length. North American birders rarely consider Dunlin a confusion species with Western or Semipalmated, but British observers, carefully scanning the large flocks for a transatlantic stray, are often brought up short by a runt of that abundant species. But even underdeveloped Dunlins don't look like this. They're longer and fatter, with heaving shoulders and no neck, and a big square head; their bills are also usually longer and thicker at the tip, and on many individuals the curve is less even and less graceful than it seems to be here.

We're left with Semipalmated and Western Sandpipers. In the eastern portions of their range, female Semipalmated Sandpipers can have decidedly long and decidedly curved bills, overlapping with (or even exceeding) those of male Westerns. But even those long-billed birds typically show a "blobby" tip, the bill widening slightly just before the end. The tip of this bird's bill is fine and even--and it is, again, standing clear up to its belly in the water, a behavior not regularly seen in Semipalmated Sandpiper.

Just to be sure, let's look at the bird's plumage. Starting from the rear, we see fairly dull tertials, then the classic "zoned" upperparts of a juvenile Western Sandpiper: the wing coverts and lower scapulars are cold gray in tone, contrasting strikingly with the rich rufous and black of the upper scapulars. The mantle, just visible, is also gray. Not even a bright Semipalmated Sandpiper will show the contrast this bird does, presenting instead a regularly scaled pattern of chestnuts and browns. The few flecks of black far back on the white flank, barely seen beneath the folded wing, are also more typical of Western. This bird's face is paler than a Semipalmated's would generally be, though its cap does strike me as more contrasting than on many other Western Sandpipers.

These and a large handful of other subtle details can be helpful in identifying peeps. But note that in this case, all we really needed was our first, superficial observation: wet belly = Western Sandpiper.

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