Monday, November 8, 2010

Oops!... I did it again

I think I caught another Bicknell's Thrush today!

This morning it was obvious that migration was good overnight, for I caught many birds with only a few nets open. The vast majority were Catharus thrushes, with 23 Swainson's Thrushes, 2 Gray-cheeked Thrushes, 1 Veery, and the above individual. For me, this is another candidate for Bicknell's Thrush.

This bird has no obvious eye ring or supraloral mark, so we can quickly rule out Swainson's Thrush. It's not a Veery either, for it lacks the reddish tone on the upperparts, and has more spotting on the breast than Veery does. It could be a Gray-cheeked, except the colors are a little warmer than on the average Gray-cheeked. Also, it has a buffy wash on the throat, unlike Gray-cheeked. And the wing formula seems better for Bicknell's than for Gray-cheeked. (See also previous post for a discussion of this and other characters.)

Then if I tell you that the wing cord was 93 mm (almost too short for Gray-cheeked, about average for Bicknell's) and that the tail measured 61 mm (definitely too short for Gray-cheeked but about average for Bicknell's Thrush), I think you see where I'm going with this!

Another thing I noticed on this bird, having read up on Catharus thrushes since last week's intriguing capture, was the color of the legs. Pyle mentions for Bicknell's Thrush "legs flesh with a purplish tinge or a brownish-dusky wash, darker than toes" while his description of the Gray-cheeked Thrush's legs is "pale flesh with a dusky wash, paler than the toes" (Pyle 1997). In the hand, I did see a purplish tinge, and while I didn't take any photos of the legs specifically, I think we can appreciate in the third photo that the toes of this bird are lighter than the legs.

Obviously, this time I did collect a tail feather, and you can see quite well which one it is, from the third photo.

It will very likely be months at the very least before the feather has been analyzed, and we have a bit more certainty which species this is.

Assuming for a minute that both birds really were Bicknell's Thrushes, could it be that this species has been under-reported in Central America? Another, perhaps more likely possibility is that a small percentage of Catharus thrushes cannot safely be identified to species, even in the hand.

Another surprise find today was this bird, White-throated Thrush. Normally a middle elevations species (like the White-crowned Manakin of last week), not a bird expected at sea level. Like the manakin, this bird is not listed on the checklist included in the banding protocol for the site (Ralph et al. 2008), although both Stiles & Skutch (1989) and Garrigues & Dean (2007) mention post-breeding altitudinal migration in this species.

Cited literature:
Garrigues, R. and R. Dean. 2007. The Birds of Costa Rica: a Field Guide. Cornell Univ. Press, Ithaca, NY.
Pyle, Peter 1997. Identification Guide to North American Birds, Part 1 Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.
Ralph, C.J., M. Widdowson, B. Widdowson, B. O'Donnell & R.I. Frey. 2008. Tortuguero Bird Monitoring Station Protocol, unpublished draft version January 2008.
Stiles, F. G. and A. F. Skutch. 1989. A guide to the birds of Costa Rica. Cornell Univ. Press, Ithaca, NY.

1 comment:

Oliver Komar said...

John, seems like convincing arguments for Bicknell's. It could be informative to provide some more comparative photos with Gray-cheeked Thrush, for both the wing formula and the feet/leg color.