Monday, September 27, 2010

Curve-billed Thrasher in Manitoba

Curve-billed Thrashers have an interesting history of wandering in fall/winter. The phenomenon of fall dispersal has made it difficult to understand the migration patterns of this species. Amazingly one showed up this weekend in Manitoba’s Whiteshell Provincial Park – a long way away from their breeding range in Arizona, New Mexico, Texas and Oklahoma.

This bird was originally reported as Bendire’s Thrasher but, as we drove up to the dumpsters where the thrasher was feeding, we quickly realized that this was a Curve-billed Thrasher, the bright orange eye and the decurved all dark bill being the most salient features. You can see this in these two photos. You will also notice how worn the plumage is…

This bird was foraging right in front of our vehicle on the gravel pull-out. It was almost as though we were observing them in a tiny patch of their preferred desert habitat! We had ample opportunity to study the features although the amount of feather wear complicated the picture.

Curve-billed Thrasher subspecific taxonomy is a little complicated but two main groups are recognised - the eastern T. c. curvirostre group (breeds in areas like Texas) and the western T. c. palmeri group (breeds in areas like Arizona). If you study the photos below you can see that this bird had fairly extensive white tail corners (outermost rectrices tipped white), visible from above (first photo) and below (second photo), despite heavy wear, but most visible when the bird flew. This made me think that this birds might be of the eastern subspecies group.

The photo above also shows the spotting on the breast, which showed a fairly pale ground colour creating contrast between the breast spots and background. This also seems consistent with an eastern bird. The wing bars were hard to gauge, this bird being in such heavily worn plumage, but certainly, although they were buffy overall, there was white tips to the outermost greater coverts and outermost median coverts (as opposed to buffy tips as expected in western birds) as you can see in the first of the two photos above (although if you compare with the very first photo of this post you will notice that the white tips on the median coversts on the bird's other side have worn away). I did not manage a good look at the undertail coverts, another useful feature, as the bird was almost always on the ground.

If you are wondering why the bird is on one leg in both of the above photos, they appeared to have an injured right foot. This and the heavily worn plumage makes me think that this bird is not likely going anywhere soon and will stick near the food supply they have found. This bird also exhbited a curious sneezing behaviour - almost as though they were trying to cough up a pellet but repeated constantly.

It is also worth studying the interesting mix of dark and light flight feathers and tail feathers. Even at a glance the birds gives a somewhat striped appearance due to the mix of light brown and dark brown feather. This suggests multiple feather generations and thus an adult bird, although at first glance the buffy "wing bars" seemed to match Sibley's illustration of a juvenile.

On a bird this worn and this far out of range, it is hard to know how to interpret such features. The bird didn't seem all that happy either, though I guess the problem with a decurved bill is that you always wear a frown!

Amazing to see this bird so far out of range!


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