Friday, September 14, 2012

"Fri-D" - Juvenile Shorebird Generalities


 [Juvenile Short-billed Dowitcher, Scarborough Marsh, ME las week. Click to enlarge all photos.]

If you've spent any time lately looking at shorebirds in the northeast, one thing you may have noticed is that they're mostly juveniles now that it's September.  If you feel a little sketchy about aging shorebirds, it's not all that hard. Start perhaps by checking out the big Sibley guide, page 181, for a typical progression of plumages as illustrated by Western Sandpiper. As Sibley notes, "Fresh juvenal plumage is characterized by uniform feathers, often with bright pale fringes, and scapulars and wing coverts that are shorter and more rounded than the same feathers in adults.

When looking at details like this, it is helpful to have a good grasp of feather tract names and locations. A particular use for this kind of information is found in the tertial feathers of the juvenile dowitcher above - if you can find them (laying over the wing tips at the rear of the bird), you will see they are heavly marked with bright cross-hatching and internal markings, which means this must be a Short-billed Dowitcher. Juvenile Long-billed tertials are marked only along the edges - but this only works on juveniles, so you still have to age them.

Anyhow, have a look at these three juvenile shorebirds for the commonalities of this age's plumage. All were photographed in Maine last week.

Why is it mostly juveniles now? Because the adults came through in July and August. In many shorebird species, adults depart the breeding grounds in advance of juveniles, and the bulk of adults stay ahead of the juvs on the southbound journey. When you see adult shorebirds in September or later, and you certainly will, you can guess that these individuals will be wintering more or less locally, at least not all the way down in South America. There are exceptions, of course - nature's like that.

 [Juvenile Black-bellied Plover. Caution - some juv black-bellies are spangled gold above (though not so much this one), suggesting golden-plover to the unwary.]

[Juvenile Semipalmated Sandpiper. Look how neat, even, rounded, and fringed the wing covert feathers are, creating a scaly effect typical of juvs.]

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