Friday, August 16, 2013

Fri-D: Adult vs. Juvenile Shorebirds

 [Above, juvenile Short-billed Dowitcher. Below, adult breeding, or alternate plumage, Short-billed Dowitcher. Forsythe NWR, August 13 2013. Click to enlarge.]

One of these days I'm going to write a post called "The Rules of Shorebirds," and list them all, but for now, here are two of the rules. First, shorebirds come in three plumages, plus transitions between them.  The top dowitcher is a juvenile, and I'm technically required to point out that correctly we would say, and spell, the plumage it's in as juvenal. The bird is juvenile, the plumage is juvenal. The bottom dowitcher is an adult in "breeding" or alternate plumage, worn breeding to be more precise since it's been wearing this coat of feathers since northbound migration last spring and they're looking a little tired, not as bright and neat as they were last spring.

So there's two of the three shorebird plumages. The third plumage is winter, or basic, typically characterized by very plain gray or gray-brown coloration. Juvenal, alternate (breeding), and basic (winter). Remember, there are transitions, too, so we sometimes say things like "an adult in worn alternate plumage molting in a few basic feathers."

So how are the two pictured Short-billed's different? The juv is brighter, with bright orangy feather edgings above, and neat pale edging to the wing "panel," i.e. the wing coverts.

What really needs to be said here is that if you are going to identify shorebirds on plumage, as opposed to structure, you need to be able to tell what plumage the bird is in! And if you are describing a shorebird, as in for details on an eBird report or report to a bird records committee, proper form dictates that early on you say what plumage you think the bird is in, and why.

Not that Short-billed Dowitchers are rare and you have to write them up. In fact, because they're not rare, they're a great bird to practice adult vs. juvenile, and now is the time to do it, since now that we're into August, juvenile Short-billed Dowitchers are beginning to show up.

That reminds me of another "rule" of shorebirds: on southbound migration, the adults migrate first, and appear as much as a month before the juveniles. In the mid-Atlantic, broadly speaking, this means there are only adults in July, and both adults and juveniles from August onward.

[Bonus juvenile: the right bird is a juvenile Semipalmated Sandpiper, the left bird is an adult. The differences are similar to what they were between the juv and adult dowitchers, no? Juv's generally have brighter, more broadly edged feathers, and neater plumage. Note in particular how the neat feather edgings on the juvenile make it look scaly; this effect is more pronounced on some species than others, e.g. juvenile Baird's Sandpipers are famously scaly looking above. But caution is advised; as we can see here, other shorebirds can look that way. This bird has another potential confusion factor with Baird's: the buffy wash on the breast and upperparts, which is characteristic of semis in very fresh juvenal plumage. The buff fades away after a couple weeks. Forsythe NWR, August 13 2013. Click to enlarge.]

No comments:

Post a Comment