Archivos de diario de agosto 2015

29 de agosto de 2015

A Very Interesting Warbler in Northern Mexico, June 2012

Jesus Salazar posted a series of images of an adult male warbler found in the Sierra La Paila, northwest of Saltillo, Coahuila, Mexico, on June 28, 2012:
This is a very interesting report, particularly because it might represent documentation of the endangered Golden-cheeked Warbler. The other possible identification is Black-throated Green Warbler. Whichever species it is, the bird would presumably be an early migrant at that location and date.

I have examined Jesus's images in great detail and unfortunately have come to the conclusion that this bird can't be definitively identified. That said, I think it may be useful to go into some detail on my analysis and reasoning. The reader might want to open a separate tab pointing to Jesus's observation to follow along with my notes on his six photos. I found it useful/necessary to actually download each of the six images, crop them to focus on the bird, then enlarge them to see details (e.g., using Preview program on a Mac).

Several aspects need to be considered here to have any hope at arriving at an ID.

—It seems clearly to be a male, but what age is it?
—What field marks are visible and which are not?
—How do the very local site conditions (vegetation, lighting) affect our view?
—How does the quality of the photos affect our view?
—Finally (after all else), what are the likely possibilities?

The bird was photographed in bright sunny conditions, leaving us with harsh shadows and lots of reflected light from the bird’s surroundings. Here’s what I see:

I judge both the extent of black on the throat and the width and boldness of the white wingbars to be sufficient to call this an After-Second-Year male. Second year (i.e. one-year-old) males of either species would typically have slightly less black in the throat, even with a few yellow or whitish feathers in the chin. The wingbars are hard to judge given that the images are not crisp, but to the extent that I can see them in the first four photos, they seem to lack obvious black shaft streaks. I’m not 100% sure of this but that’s my impression.

What we cannot see creates problems here. As Greg Lasley points out in his comments, no image clearly shows the color of the crown or upperparts. Images 1, 4, and 5 show nothing of the upper side. On Image 2, we may be seeing a tiny bit of green at the junction of the right side of the nape where it meets the right wing but it is so fractional and oblique that I’m not sure. A shadowed side view of the left side of the head in image 3 could be interpreted to show a blackish crown, but is inconclusive. A very pixilated image 6, with the bird facing us, shows a tiny fraction of the forecrown but it is not clear enough to make into green or black.

Underparts: The yellow on the “underparts” of a male Black-throated Green (BTGN) is brightest on the flanks (sides of the rump) and the vent. In an ASY male, this should be quite bright. In richly colored birds and good imagery, there is also usually a rather bright yellow tint (not bold color) in the center of the breast below the black throat. Images 5 & 6 seem to show a white vent area, but it is shaded and blurry in both, yet IMO if this were a BTGN, image 5 should show some fairly bright color behind and beside the legs. In image 2, I see no hints of yellow on the underparts that coincides with the pattern in Black-throated Green. Any yellow suffusion in image 2 is very slight and very uniform which suggests it may be an artifact of reflected bright sunlight off of the surrounding yellow-green foliage. I find the analysis of the underpart colors to be difficult and inconclusive, but if anything, I think it is more consistent with GCWA.

Face pattern: Images 1 & 2 seem to show a complete black eyeline which would be consistent with GCWA. No images show any dusky or green color in the auriculars which would be typical of the majority of BTGN. However, a complete black eyeline is possible in BTGN but this shows up in perhaps 10% or fewer of male BTGN. Examples of this are shown in these images on Flickr:
Typically, any male BTGN which shows a bold black eyeline will also have substantial dusky color in the auriculars or at least a complete dusky arc around the bottom of the auriculars. At a minimum, when a BTGN has a black eyeline, it is at least accompanied by a short blackish arc from the base of the bill extending down and back about equal to the position of the eye. Again, the first Flickr image of a BTGN above shows this. That lower arc of black is essentially never present in adult male GCWA; it appears at times in young birds or older, darker females. For the present bird, images 2, 4, and 6 seem to show a narrow black arc which would be diagnostic for a BTGN male of this age. The angle of these photos is severe in each case, rendering this mark a judgement call.

So, IF I have aged the bird properly and IF the underparts are, as I see it, not consistent with BTGN, then this is a GCWA. However, IF my perception of the whitish underparts (i.e. lack of yellow tint) is in error, and IF the partial lower black arcs below the eye are real (and diagnostic), then this is a BTGN.

What are the probabilities? I think a GCWA is MUCH more likely than any BTGN to show up in NE Mexico in late June. July records of southbound GCWA in Texas outside their breeding range are quite numerous and expected at a time when Black-throated Greens are still very rare in Texas (i.e., not typically expected until early August). June 28 is comfortably within the time frame for the beginning of the southbound migration of GCWA and a few other early migrants. Moreover, 2012 was a bad drought year during which there were several indicators of an early southbound push of migrants. I examined late June records of several species over the past 10 years in Texas on eBird. One of the typical early migrants in July is the Black-and-white Warbler (BAWW). Over the past 10 years, with the exception of 2012, there is just a single late June record of BAWW (2014 in Corpus Christi). By contrast, in 2012 there were 3 reports of BAWW in South Texas from June 27-29. Additionally, there were abnormal reports of early American Redstarts in 2012 (6/25 at Choke Canyon; 6/30 at Santa Ana NWR). And yet there were no early reports of BTGN that year anywhere in Texas.

In fact, in late June in Texas over the past 10 years, there are only two eBird reports of BTGN in Texas: One was listed in College Station on 6/28/2009 without comment; I would want to see photos of that bird but they apparently do not exist. Another was purportedly photographed by Dan Jones in a much-discussed record in Big Bend on 6/24/2013. I have studied his photos and personally I am convinced it is a HY male GCWA which seems to show an unusually dusky cheek. That leaves no uncontrovertible late June records of BTGN in Texas (or NE Mexico) for the past decade.

No decision on Jesus’s bird in the Sierra La Paila can be definitive unless there were other, better images. I think the available photographic evidence can be argued either way but the seasonal and geographic probabilities favor GCWA. To keep the database on GCWA squeaky clean--an endangered species with highly focused attention on it--I would prefer to leave this record out of the data set.

Jesus is to be congratulated for trying valiantly to document this occurrence. It is an effort worthy of emulation for any/every such report. I have personal experience trying to identify migrant warblers with yellow faces in migration in Mexico and I know how difficult the task can be!

Chuck Sexton
Austin, TX

Publicado el agosto 29, 2015 05:08 MAÑANA por gcwarbler gcwarbler | 3 comentarios | Deja un comentario