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Curve-billed Thrasher Identification

Curve-billed Thrasher Identification

The AOU checklist committee recently rejected a proposal to split the Curve-billed Thrasher into two species: the “Palmer’s” Thrasher (palmeri group) in Arizona and West Mexico, and the nominate or “Eastern” Curve-billed Thrasher (curvirostre group) in the rest of the bird’s range.

"Palmer's" (Western) Curve-billed Thrasher, Desert Botanical Garden, Scottsdale, AZ. Photo by Patrick Coin (Creative Commons 2.0).Nominate (Eastern) Curve-billed Thrasher, Colorado, by Fort Photo (Creative Commons 2.0).

Although very similar, the two groups can usually be distinguished by sight. In the photos above, note that the eastern bird (right) has a much whiter background color to the breast, resulting in stronger contrast with the breast spots; it also shows sharper and bolder white highlights in the wings and tail.  The stronger throat pattern, with a more distinct dark line bordering the white throat, may also be significant.  However, the much colder, grayer tone to the plumage overall is likely an artifact of photo lighting.

Interestingly, one of the committee members who voted “yes” on the split did so in large part because of differences in the call notes between the two forms, which I hadn’t seen discussed anywhere before:

YES. I now favor splitting palmeri – the clincher for me is that palmeri has distinct call note differences, a clear upslurred whit-wheet, as opposed to a two note whit-whit in which both notes are the same.

I have investigated this difference, and it seems to hold up across (at least) most of the species’ US range.  The vast majority of the call recordings I could find from well inside the range of “Palmer’s” Thrasher showed the same typical pattern: two upslurred whistles that started at the same pitch, with the second one ending much higher:

"Palmer's" Curve-billed Thrasher call, Catalina State Park, Pima County, AZ, 2/23/2008.

Whereas the call of eastern curvirostre-group Curve-billed Thrashers consist of nearly identical notes, both upslurred across a wide frequency range like the second note of the “Palmer’s” call:

Nominate eastern Curve-billed Thrasher call, Carlsbad Caverns National Park, Eddy County, NM, 3/30/2008.

Both groups of Curve-billed Thrashers give versions of this call with 3 or more notes, particularly when they are excited.  When the eastern curvirostre group does so, as you can see in the spectrogram above, all the notes tend to be similar.  When western palmeri birds extend their calls, the first note is usually of the stunted variety.  The third note (and any subsequent notes) tend to be like the second, but a little softer, so that the second note ends up getting the emphasis: “wit-WEET-weet”:

"Palmer's" Curve-billed Thrasher calls (3-noted version), Alamos, Sonora, 7/3/2010.

Some Curve-billed Thrashers in southeast Arizona give multi-note calls that are difficult to classify.  Here’s a bird from a few miles south of Eloy in Pinal County, where I believe the palmeri subspecies would be expected:

Atypical Curve-billed Thrasher calls recorded south of Eloy, Pinal County, AZ, 2/22/2008. Recording by Andrew Spencer.

Here’s some more from the same individual bird:

The two-note versions of this individual’s call tend to seem like the reverse of the typical palmeri pattern, with the second note quieter and less extensively upslurred than the others.  One might suppose this could be an intermediate bird, since the palmeri and curvirostre groups apparently overlap in southeast Arizona, but most educated guesses that I’ve seen have placed the overlap zone farther east, between Tucson and the New Mexico border.  I don’t believe this bird was identified visually to subspecies, so it remains a question mark for now.

Just to whet the appetite of the curious, here’s a Curve-billed Thrasher call from the Oaxaca valley in southern Mexico, which preliminary DNA studies showed as being distinct from either the palmeri or the curvirostre group (though apparently more closely allied with the latter).  Note again the “WEET-wit” pattern, which is the reverse of palmeri’s:

Obviously, more sampling is needed to fill in the many gaps in our knowledge of the Curve-billed Thrasher and its vocal variation.  Amateur recordists of the southwestern US and Mexico, this is your cue.

AOU Checklist News!

AOU Checklist News!

The North American Checklist Committee of the American Ornithologists’ Union has published the results of its deliberations on the first round of proposed changes from 2009, and it has updated the slate of proposals currently under consideration.  Here’s a quick summary of the changes that affect species splits north of Mexico.  (I won’t get into all the changes to scientific names, even though those topics are just as interesting in my opinion — you can click through to read about those yourself.)

Proposal accepted

This split will become official once the next checklist supplement is published in the July 2010 issue of the Auk.

  • Split Pacific Wren from Winter Wren. As I reported earlier, this split did indeed pass, and unanimously at that.  However, note that the names “Pacific Wren” and “Winter Wren” are not final.  The committee is considering an addendum to the proposal that would split eastern North American birds from Eurasian birds and change the names of the American species to “Western Winter-Wren” and “Eastern Winter-Wren.”  Stay tuned.

Proposals rejected

In most cases, a 2/3 vote of the committee is required for a proposal to pass.  These proposals failed to muster that level of support:

  • South Hills Crossbill. The proposal to split South Hills Crossbill (Type 9) from Red Crossbill failed on a vote of 6 “yes” votes to 5 “no” votes, with three of the “no” voters indicating that they would be open to changing their minds if presented with more data.  Two of those voters preferred to deal with the North American Red Crossbill complex as a whole, rather than splitting one type at a time, piecemeal.  Thus, most of the committee appears to accept that the different call types of Red Crossbill are likely good species, but I think it may be a while before those species appear in your field guide.
  • The split of Western Scrub-Jay. The proposal to split the interior “Woodhouse’s” Scrub-Jay (woodhousei) from “California” Scrub-Jay (nominate californica) failed on a vote of 7 “yes” votes to 5 “no” votes.  Many members of the committee felt that more data were needed from contact zones.  The tagalong proposal to split the southern Mexican subspecies sumichrasti into yet a third species gained even less committee support.  Vocal differences between woodhousei and californica have been reported, and you can expect those differences to be discussed in a future post on this blog.

New proposals

The checklist committee never sleeps.  The following splits of North American species are now under consideration:

  • Split Black Scoter (Melanitta americana) from Common Scoter (Melanitta nigra).  I wrote about this split recently.  This proposal was originally submitted in 2006 and failed to pass at that time, but the recent publication of Sangster (2009) has revived it.  Personally, I think it’s a clear-cut split, but we’ll see if the committee agrees.
  • Split Curve-billed Thrasher (Toxostoma curvirostre) into two species: the western palmeri group and eastern curvirostre group.  The proposal makes no recommendation regarding the resulting English names.  The proposal cites various genetic data, which I won’t comment on, but it also cites vocal differences, including differences in calls.  I’m a little skeptical of these differences, but I’ll investigate them in the future and report back on what I find.
  • Split Whip-poor-will (Caprimulgus vociferus) into a nominate eastern species and the southwestern arizonae species, on the basis of subtle but easily diagnosable differences in song, differences in egg coloration, and (most importantly) a hot-off-the-presses study demonstrating that the vociferus and arizonae groups may be as genetically distinct from one another as either is from the Dusky Nightjar (C. saturatus) of Costa Rica and Panama.  I haven’t been able to track down the article text yet, so I can’t say what I think of it.

As you can see, vocal differences are playing an ever-more-prominent role in taxonomic decisions.  Look for more on this topic from me in the future.

Beware the Bendire’s

Beware the Bendire’s

In the middle of the afternoon a few weeks ago I was sitting in the public library in Sierra Vista, Arizona–a wonderful facility, by the way–escaping the heat of the day to download my bird sound recordings, recharge my batteries, and check my email, when my attention was drawn to a bird hopping directly toward me on the ground.  No, it wasn’t in the library–it was on the other side of the floor-to-ceiling plate-glass windows, out in the desert garden.  But it might as well have been in the library, because it ended up hopping right up to the window, less than four feet from my face, so that even without binoculars I could clearly see its pale yellow eye, its short, almost straight bill, its nearly-unmarked breast with a few spots front and center…in short, every field mark necessary to confirm its identity as a Bendire’s Thrasher–a scarce species I had seen only three times before.

And then, through the window, I heard it begin to call: a slightly noisy, one-syllabled note reminiscent of the “cheep” call of the American Robin.  Naturally, I raced right out to record it:

cbthjuv4c-ndp2009-15-06

The call note clinched the ID.  It was completely different from the characteristic call of the very similar-looking Curve-billed Thrasher, the loud wit-weet! that is a trademark sound of the southwestern deserts:

Curve-billed Thrasher call. Catalina State Park, Arizona, 2/23/2008 (08-19).
Curve-billed Thrasher call. Catalina State Park, Arizona, 2/23/2008 (08-19).

I was particularly excited to record the Bendire’s because their one-syllabled call note had been described in the literature, but as far as I could tell, never recorded — I didn’t know of any commercially available recordings that included it, nor had I ever seen published spectrograms of it.  The Macaulay Library didn’t have it, and in the two different mornings I’d spent recording Bendire’s Thrasher in Arizona and California, I’d never even heard it, much less caught it on tape.  So I was pretty stoked.

I relayed the details of my sighting to my friend Andrew Spencer, who was due to arrive in southeast Arizona shortly after I returned to Colorado.  He was equally excited about this rare recording opportunity, and he made a point of stopping at the Sierra Vista Public Library as well.  That morning I got a message from him that relayed some good news and some bad news.  The good news: he’d found the bird right where I said it would be, acting just as I’d said it would, giving calls just like the ones I’d recorded.  The bad news?  It had been accompanied by, and fed by, two adult Curve-billed Thrashers.  It wasn’t a Bendire’s at all–it was just a juvenile Curve-billed.

Sure enough, when I went to look at the Bendire’s Thrasher chapter in Kenn Kaufmann’s Advanced Birding, there was the description of a juvenile Curve-billed Thrasher, matching my bird to a T.  I had forgotten that Bendire’s has a pale base to the lower mandible, which my bird had lacked.  And Kaufmann even mentions the single-syllabled call note of the juvenile Curve-billed.  Blast.

The morals of this story:

  1. Juvenile Curve-billed Thrashers look and sound far more like Bendire’s than I ever thought possible;
  2. Even the most seemingly slam-dunk IDs can be wrong;
  3. I still don’t know of a recording of the call of Bendire’s Thrasher.  Anyone?  Anyone?