Friday, November 5, 2010

Bicknell's Thrush!



















Two days ago, I caught what I think will easily be the hottest capture of the season: a Bicknell's Thrush! If accepted, this would represent not only a first record for Costa Rica, but for all of Central America!

Casually looking at it as I pulled it out of the net, I thought it was a Gray-cheeked Thrush - after Swainson's Thrush the second most common spotted Catharus thrush here in Tortuguero, Costa Rica. While processing the bird, however, I quickly noticed it was different from that species in several ways.

For starters, the wing cord was exceedingly short: 93 mm. Most Gray-cheeked Thrushes have a wing cord of >97 mm, many >100 mm. In fact, the whole bird was small, with a tail coming in at 65 mm (63-97 mm for Gray-cheeked; 59-74 mm for Bicknell's, according to Pyle). The body mass was 27.0 g. Then I noticed how warm the colors were compared to the average Gray-cheeked. I further noticed a contrast between the tail and the back. The bird also showed a buffy wash on the throat, unusual for Gray-cheeked.



















Realizing I was holding a Catharus thrush that didn't easily fit into one of the three species found here (Swainson's, Gray-cheeked and Veery), I decided to consult Pyle (1997) and check for Bicknell's Thrush.

Summarizing, I found that everything about this bird appeared good for Bicknell's, although some characters can also be shown by a minority of Gray-cheeked Thrushes.

I photographed the bird extensively (but in the excitement forgot to collect a tail feather), and when my banding shift was over, decided to consult with Peter Pyle directly through email. He agreed with me that the bird looks good for Bicknell's Thrush, but added that he himself has no personal experience with this species. He forwarded my photos and measurements to someone with that experience.

He did add that the fact that P7 appears to be the longest primary may be important. The illustrations in his Identification Guide had already drawn my attention to differences in wing formula, so I photographed that aspect of the bird as well as I could.

Compare and contrast the next two photos; first the putative Bicknell's Thrush, next a Gray-cheeked Thrush in a similar posture.  Both are HY birds, caught within days here in Tortuguero, Costa Rica.

Thrushes have 10 primaries, with the 10th reduced (Pyle 1997). So the outermost 'large' primary feather is P9. Note how P8 is the longest feather in the Gray-cheeked's wing, with P9 almost as long (below), while in our Bicknell's candidate (above), P9 and P8 are both shorter, and the longest feather is P7. Thus the wing is rounder, less pointed than that of Gray-cheeked Thrush.



















Here again is that Bicknell's Thrush' wing, with the appropriate page in Pyle as a background.



















If accepted, this would be the first time, as far as I'm aware, that this species has been recorded in Central America. It breeds in southeast Quebec and the Maritime Provinces of Canada, and eastern New York and northern New England, USA. Its wintering grounds are found in the Greater Antilles, particularly the Dominican Republic. There are three records from Cuba (BirdLife International 2008).

Since this is not an easy species to identify, I'm hoping my documentation will stand up to further critical attention. With Bicknell's Thrush's IUCN status as Vulnerable and its population trend Decreasing (BirdLife International 2008), this capture is likely to garner some attention.

Cited literature:
BirdLife International 2008. Catharus bicknelli. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4.  <www.iucnredlist.org> Downloaded on 5 November 2010.
Pyle, Peter 1997. Identification Guide to North American Birds, Part 1 Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.

Postscript 6 November 2010:
One of the Bicknell's Thrush researchers, currently in the field in the Dominican Republic studying Bicknell's Thrushes, comments that there may have been a previous Costa Rica record from 2009 or 2008. He also writes that my documentation will be further studied and that he will get back to me at the next opportunity with an informed opinion. If this turns out to be the second recent record for Costa Rica, I would personally find that even more interesting. It would be a stronger clue for possible range extension, rather than a record of a lone disoriented individual.

3 comments:

Scott Winton said...


We banded a couple Gray-cheeks in North Carolina the other day and I was curious about what it would be like to band and try to properly document a Bicknell's.

Great post!

Did you ever get any confirmation (or rejection) of the Bicknell's ID for this bird??

johnvandort said...

Hi Scott, thanks for your comment! I don't know if you noticed that a week later, I caught another one? The next blog entry is about that second bird. I collected a tail feather from the second bird, and handed it over to the station manager for DNA analysis. That, sadly, never happened, and after some time trying to move this forward, I gave up.
Researchers of the International Bicknell's Thrush Conservation Group looked at the photos, and thought that both birds looked good for that species, but warned that it is impossible to identify one from photos. They directed me to a paper they published in the Journal of Field Ornithology (Fey et al. 2008) providing a formula using wing cord and tail length to discriminate Bicknell's from Gray-cheeked. When I applied their formula to the measurements taken of these two birds, one fell safely within Bicknell's Thrush range and outside the Gray-cheeked range while the other was borderline, a possible BITH but still within range for GCTH. Their paper is available online, here: http://www.vtecostudies.org/PDF/Frey08JFO.pdf. This paper suggests that it is possible to separate the majority of BITH and GCTH in the hand, but with great care. Some populations of GCTH are warmer in tone than others, so any suspected BITH should always be measured extensively. Bicknell's Thrush is still not on the official bird list for Costa Rica, although personally I think it belongs there. Thanks again for commenting!

Scott Winton said...

Hi John,
Thanks for your reply (which I just now saw). I wanted to re-read your post (and the subsequent one; thanks for the heads up) to remind myself of how impossible these birds are to identify. I just saw a very chestnut brown-toned Gray-cheeked type catharus in a yard in NC that had me thinking Bicknell's. I was bummed I didn't have a camera, but this has reassured me that it wouldn't have mattered.