Mystery Bird: Sedge Wren, Cistothorus platensis

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[Mystery bird] Sedge Wren, Cistothorus platensis, photographed in Minnehaha County, South Dakota. [I will identify this bird for you in 48 hours]

Image: Terry Sohl, 19 September 2009 [larger view]

Canon 50D, 400 5.6L lens.

Please name at least one field mark that supports your identification.

Review all mystery birds to date.

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Whoa; check out that beak. What is that thing?

After some poking around, I'm going to go with Sedge Wren. I'd like there to be a little more of a supercilium, and the beak seems a tad on the short side, but the plumage (what I can see of it) seems to match the Sibley illustration of the juvenile (which he indicates is "Jun-Aug") fairly well, and honestly, I can't think of anything else.

Very tricky shot. I'm looking forward to finding out what it is.

I think you got it again John, a Sedge Wren, Cistothorus platensis: brown upperparts with a light brown belly and flanks; light throat and breast; streaks on the back and a dark crown also with pale streaks; a faint line over the eye and a short thin bill...

The Patuxent Wildlife Research Center specifically notes that the bill should indeed be "short and thin" and with an "indistinct supercilium" so I am not troubled by those field marks...

I think the similar Marsh Wren (Cistothorus palustris) can be eliminated because it has a relatively longer bill, and an unstreaked cap; other wrens with indistinct superciliums are House (lacks streaking on the crown and back), Winter (more reddish-brown above, darker below), and Rock Wrens (more grey to the upperparts, longer bill).

There are at least 20 subspecies, extending from Canada through Central America even as far as the Falklands, but only one, C. p. stellaris, would be found in the northern US, so this is Cistothorus platensis stellaris