Mystery Bird: Blue-gray Gnatcatcher, Polioptila caerulea

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[Mystery bird] Blue-gray Gnatcatcher, Polioptila caerulea, photographed in Starr County Park, Texas. [I will identify this bird for you tomorrow]

Image: Joseph Kennedy, 5 April 2008 [larger view].

Nikon D200, Kowa 883 telescope TSN-PZ camera eyepiece 1/1000s f/8.0 at 1000.0mm iso400.

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

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

Now here's a little bird, barely bigger than the menacing thorns it's paused among. Its very size is a clue: there aren't many birds in North America this tiny, and only a few of those combine overall gray plumage with a long, slender tail.

Let's start with that tail. The mechanics of tail folding give us a good view of the bird's outermost rectrices, which are decidedly pale -- white, even, taking into account the shadow in the photo. And that's enough. I can't think of any miniature gray bird with a long white-edged tail except for a gnatcatcher. The diagnosis is confirmed by the short wing, the large head and staring eye, and slender, insect-grabbing bill.

But which gnatcatcher? These fussy mites are hard enough to identify in the field sometimes, but stuck with a single photo, deprived of the advantages of multiple views from multiple angles, unable to wait for a diagnostic call or two, I often give up and wait for the photographer to offer her own ideas.

This is one of those photos. The tail structure is difficult to assess, on my monitor at least, though the outer pair of tail feathers seems to be not drastically shorter than the next -- the difference looks to be roughly equivalent to the primary extension. The visible bit of the bird's back appears to show no striking brown, and the folded secondaries are pretty much the same dull blue-gray. The only black on the head appears to be a shadow.

But then we're confronted with that bill, which strikes me as noticeably long. All the same, the other features discernible in this photo suggest a Blue-gray Gnatcatcher, far the likelier of the "white-tailed" species found in the US. In the field, I'd wait for a better view of the bird's tail and for a bout of the distinctive high whining this very vocal species gives.

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I'm guessing it's a Blue-gray Gnatcatcher (Polioptila caerulea), based on the color, white eye-ring, and white outer tail feathers.