No group of birds in the ABA area is as infamous for being difficult to identify as Empidonax flycatchers. The very utterance of that dreaded name can make a beginning birder quiver in fear, and even the most studied of experts have a healthy respect for the challenge they represent. In earbirding terms, “Empids” are just about the ultimate argument for sound over sight. While they are no longer considered identifiable by voice only (knowledge of their visual identification has progressed by leaps and bounds the past few decades), vocals are still the best way to distinguish the various species.
The purpose of this blog post is to draw attention to one of the least known Empidonax species, one that has yet to appear in the ABA area, but is high on the list of many a birder as a potential vagrant: Pine Flycatcher (Empidonax affinis). An inhabitant of montane forests from just shy of the Arizona border to northern Central America, it favors (as one would expect) pine-dominated woodland.
While Pine Flycatcher has yet to appear north of the border, the species is already somewhat infamous for an “almost” that appeared in 2008-2009. For several weeks, an Empid in Choke Canyon, Texas, caused a good deal of excitement when it was identified as a Pine Flycatcher, both on visuals and vocals. Many listers visited and “ticked” the bird before it was re-identified as a Least Flycatcher. Even though, in the end, the bird wasn’t the hoped-for first ABA record, Pine Flycatcher was now “on the radar”.
Pine Flycatcher is still a species that could show up some day. As it is also one of the most poorly known Empid species, both visually and vocally, I’m hoping this blog post will help anyone trying to identify them.
Visually, Pine Flycatcher isn’t one of the more distinctive Empids. It tends to have an overall greenish cast to the upperparts and whitish/dingy coloration to the underparts, but it is not terribly colorful, even by Empid standards. Structure plays a better clue in its identification, as it has long primary projection (like Hammond’s), but also has a long, and narrow, bill with a completely pale mandible. This last feature can be especially useful, as the most common way to see a Pine Flycatcher (at least on the breeding grounds) is to be looking straight up at it as it perches high in a tree.
Away from the breeding grounds, the most commonly heard vocalization of any Empid is typically its contact call. In many species this vocalization is transliterated as a “whit”, while in others it sounds more like “pip”. Pine Flycatcher falls into the “whit” category, but with an important caveat. Almost all of the other species that have a “whit” call are at a similar pitch, and the difference between the calls are down to subtle differences in note shape that give the call a different quality. Pine Flycatcher is distinctly lower pitched, which gives its “whit” call almost a “pip-like” sound. It is very distinctive once learned, and would be a major piece of evidence in favor of a potential vagrant.
In the graphic above (with thanks to Tayler Brooks for her help in its preparation), note how strongest (brightest) part of Pine Flycatcher’s call is nearly 1 kHz below the strongest part of the next lowest “whit”, that of Willow Flycatcher. It also has a smaller bandwidth (that is, covers a small range of frequencies), which contributes to the impression of a “pip” like quality. The other main factor determining the sound quality of each species’ call is the overall steepness of the note, with a larger rise over a shorter time imparting a “drier” (less musical) quality to the call. On the far right is Hammond’s Flycatcher, just for comparison. Hammond’s give a true “pip”, the effect of having a call that is cleanly overslurred, without an overall rising pitch. This graphic is also a good one to study to visualize the differences between the western ABA breeding Empid species (the intricacies of their call notes will likely be the topic of a future Earbirding post). You can listen to the various Empid call notes below:
The song of Pine Flycatcher is also distinctive. Though it is less likely to be heard in the US, it wouldn’t be entirely out of the question to have an overshooting bird set up territory somewhere like the Huachucas or Chiricahuas – a pair of Tufted Flycatchers nesting in Ramsey Canyon shows how the crazy isn’t entirely impossible! Like many other western Empid species, Pine Flycatcher has a song made up of three phrases, typically mixed together (especially at dawn). These are A) a bi-syllabic “PIT-wee”, B) a single syllabled “DIT”, and C) a burry “breee”. All of these phrases can be preceded by a variable number of quiet “pip” notes, especially phrase C, which nearly always has 3-4 “pips” included. The sonogram below has one of each phrase, given at a natural interval at dawn. You can listen to the recording the sonogram is made from here.
As you would expect, Pine Flycatcher has a whole suite of other, more rarely heard calls. These tend to be analogous to the various calls given by our more familiar species, including a twitter series (the first recording linked below), a “pweep” call indicative of high agitation, superficially similar to calls made by Great Crested or Nutting’s Flycatchers (the second recording), and one in the vein of Dusky Flycatcher’s “du-hic” call (no publicly available recordings). These calls are unlikely to be heard from a vagrant bird, but should you find yourself standing in some towering pine forest in Mexico you would do well to keep an ear out for them!
Finally, there has been quite a bit of talk about a potential split of Pine Flycatcher involving differences in birds found north and south of the Isthmus of Tehuantepec. The reasoning given is that the populations found on either side of the Isthmus have notably different songs. While there aren’t a whole lot of recordings of the full song of birds found south of Oaxaca, the ones that I have been able to find (such as this one by Kathi Borgmann, and one kindly sent to me by Knut Eisermann) are disappointingly similar to the birds I’ve heard from further north. While there do appear to be some minor differences in one of the phrases, it is perhaps more on par with differences among populations of Willow Flycatcher currently considered subspecies rather than the differences found between most full-fledged Empid species. However, that is a conclusion drawn from only two recordings. I’m quite willing to change my mind if someone goes out and gets a larger sample size showing a more profound difference!
It’s not every day that you photograph and audio record a bird that has never been photographed or audio recorded before.
But that’s what we managed to do Thursday night, on the trails above Rancho La Noria in the state of Nayarit, Mexico. At the end of a madcap 12-day birding trip, we finally connected with our primary target, one of the most elusive birds in Mexico – Cinereous Owl (Strixsartorii, also known as Mexican Barred Owl).
Known from about 30 specimens taken between 1873 and 1948, this bird has eluded almost all detection for the past half century, and its taxonomic status has been a major mystery. Is it a subspecies of the Barred Owl of North America, as the American Ornithologists’ Union currently classifies it? Is it a species in its own right, as the International Ornithological Congress decided to call it following recommendations from a 2011 genetic study? Or is it a subspecies of the Fulvous Owl, as some have speculated following recent records of Fulvous Owl from within the supposed range of Cinereous ?
To answer these questions, good photos and (especially) audio recordings of Cinereous Owl were sorely needed. And here are the first ones.
When we first began considering a trip to West Mexico with our friend Carlos Sanchez earlier this spring, the Cinereous Owl was a major motivator, and we planned a good portion of the trip around it. We contacted Steve N. G. Howell, the author of the field guide to Mexican birds, one of the only people to have encountered Cinereous Owl in the wild. He recommended the trails above Rancho La Noria (a.k.a. Cerro San Juan), where he had found the bird in the past, but had been unable to get audio recordings despite four trips specifically for that purpose. So we knew the bird wasn’t going to be easy.
Rather than head straight for Rancho La Noria, we started our search at Cerro La Bufa in Jalisco, on the recommendation that the habitat was very similar to that at Rancho La Noria, but more extensive, higher in quality, and easier to access by road. Nevertheless, in a night of owling that included playback of Fulvous, Barred, and Spotted Owls (on the assumption that Cinereous probably sounded enough like one of these to respond to a tape), we only managed to find Spotted Owl, Mottled Owl, and Whiskered Screech-Owl.
We tried again a few nights later on Volcan Nieve (a.k.a Nevado de Colima), the source of at least one of the specimens of Cinereous Owl. Again, no luck. However, in our limited time we were only able to survey a small portion of the habitat along one of several roads on the peak, so Cinereous Owl could easily be hiding up there somewhere.
We probably should have tried owling during our night at the Reserva Chara Pinta (Tufted Jay Preserve) near El Palmito, Sinaloa, but by that point in the trip we were too tired. So our last chance for the mythical bird came down to our final hours in Mexico, at the spot Steve Howell had originally recommended.
Carlos opted for hard-earned sleep instead of an exhausting wild goose chase, so we headed up the Rancho La Noria trails at dusk without him. Our process was the same as at the other sites: stop, listen for a couple minutes, and then try speculative playback of all three Strix owls, to see if we could get some kind of response. At our third stop, we didn’t even get to the playback, because we heard the bird we were listening for singing spontaneously far up the trail! Just as Steve had described, it sounded a lot like Fulvous Owl, but slightly different.
Elated, we quickly moved up the trail to get closer to the still barely audible owl. After several more stops it became apparent that we weren’t going to be able to get closer to it on the trail, so we tried some playback. This did not succeed (at least initially) at bringing the singing bird closer, but we did start to hear what sounded like the wail call of a female Strix owl from the same area. So we bit the bullet and started bushwhacking down a steep slope. We didn’t make it far when the apparent male of our target pair called from right over our heads, twice! Quick reflexes and the wonders of pre-record buffering, and we had the first ever recording of Strix sartorii (at least the first good ones, after our far more distant cuts a few minutes before).
And that was it. After several minutes of silence we played back what we had just recorded, we played back Fulvous Owl, and we played back the female wail call of Spotted Owl – no response. But we were still hearing a consistent wail call in the distance, and we reasoned that we would stand a better chance of getting more audio if we tried to get closer to the source of that call. Two steep ridges later and we were far closer to the calling owl, and also on a trail that, had we but known of it, would have saved significant effort. A bit more walking, and we were soon staring right at the source of the wail call – not a female as we had assumed, but a begging fledgling Cinereous Owl(see the photo above).
We spent the next hour or so with the young bird, documenting what we could, but the adult birds never showed. So we decided to get a precious few hours of sleep and return before dawn, when we hoped the adults would vocalize more and possibly return to feed the juvenile.
A few hours later, this time with Carlos, we were standing on the same stretch of trail. It didn’t take us long to find the same shrieking fledgling, but there was no sign of its parents. As dawn approached and the adults never showed we got more and more nervous, but then finally the young one made a rather more urgent sounding call, flew off out of sight, and the sounds of a feeding event floated through the woods.
After two apparent feedings (both recorded; see above), an adult bird finally sang a hundred meters or so down the trail. Some quick repositioning, a few audio cuts later, and a bit of playback, and we were looking right at it – an adult Cinereous Owl! Over the next half-hour we obtained all the evidence we could have hoped for, audio and visual. A few other residents of the forest took some exception to the singing owl (including a White-throated Thrush and a Sharp-shinned Hawk), but in the end we had to tear ourselves away from it all. It was time to return to Guadalajara, and the US, tired but thrilled at our success.
While seeing such a rare animal was an awesome experience, the real interest for us was to see how these birds compared to both Barred and Fulvous Owls, and try to figure out where Cinereous Owl actually belongs. All of the below is with the caveat that our sample size is small (just one adult, likely a male).
Most Strix owls, including Barred and Spotted, have two song types, one with notes in a species-specific rhythm, and another with notes in fairly steady series. We recorded both song types from Cinereous.
At first glance, the rhythmic song of Cinereous Owl is far more like that of Fulvous Owl than the familiar “who cooks for you” of Barred Owl. In fact, it is so similar that some might argue it is evidence that Cinereous and Fulvous could be considered conspecific. But a closer look shows some differences. The song of Cinereous is significantly lower pitched than that of Fulvous (which is to be expected since Cinereous is a larger bird). And it consistently had two extra notes appended to the end, even when singing naturally before playback. (This Fulvous Owl in Chiapas gave one extra note at the end of its song in response to playback.) The note shape and pace also differed, with Fulvous having more disyllabic notes given slightly faster, imparting a different cadence to the whole song.
The difference in the series song may actually be more striking, because it’s possible that Fulvous Owl lacks this vocalization type altogether. We have not been able to find a single recording of the series call from Fulvous Owl, even in their pair duets or in response to playback, when other species are more likely to give them. Instead, excited Fulvous Owls seem to stick with the rhythmic song, often in duets, tossing in occasional “caterwauling” barks. Although it’s hard to prove a negative, it’s tempting to conclude that Fulvous Owl lacks a series song, given that good online recordings are available of over 15 different individuals/pairs calling, many in response to playback.
Since we weren’t able to find a recording of the series song from Fulvous Owl, we can only compare what we got from Cinereous with Barred Owl. Compared to the series song from Barred, what we recorded from Cinereous was similar in pitch, but quite different in pace and overall pattern. In Barred, the series song rises slightly but consistently at an even pace before ending in a slightly modulated, falling note. In our sample of Cinereous, the series song started and ended at a lower pitch, with a few higher pitched notes in the middle, and a more stuttered pace throughout.
Plumage-wise, Cinereous Owl clearly looks more like Barred. It isn’t hard to see why they were lumped for so long, especially considering that the vocal differences were unknown. But that’s not to say they’re identical, and Cinereous Owl can be told from both Barred and Fulvous Owls with a good view.
The first thing that immediately struck all three of us when we saw the adult Cinereous Owl was how pale its face was. Compared to both Barred and Fulvous Owls, the facial disk of Cinereous Owl is paler (without obvious darker concentric rings), and compared to Fulvous, grayer. Indeed, the overall colder gray coloration is a good way to distinguish Cinereous from Fulvous Owl. This is most obvious in the differences of the underparts pattern (dark sepia streaks on an off-white background for Cinereous vs. warm brown streaks on a light buffy background for Fulvous), and in the color of the collar (cold gray for Cinereous vs. golden brown for Fulvous). Compared to Barred, which is far more similar to Cinereous in these features, the main difference is that the streaks below on Cinereous are slightly darker, not a medium brown. See the photo below for a comparison of all three species (and click to see a larger version without the labels). Here also are a few more good photos of Fulvous Owl from Oaxaca  and from farther south .
A final, if far more subtle visual clue, deals with the feathering on the toes. In both Barred and Cinereous Owls, the feathers extend all the way down the toe to the base of the claw. In Fulvous, the toes are partly bare of feathering. This can be difficult to discern in the field, but with good photographs it can be a useful feature (and it is visible in the large version of the comparison photo, which you can see by clicking on the photo above).
Conclusions and Questions
1) While our sample size is limited, what we were able to audio record clearly supports considering Cinereous Owl separate from Barred Owl. Vocally, the two are not even close. Add that to the genetics in the 2011 paper, and there’s no reason to continue considering sartorii a subspecies of Barred Owl. (This is also why we support the name “Cinereous Owl” instead of “Mexican Barred Owl”).
2) Is sartorii perhaps a subspecies of Fulvous Owl? This situation is a bit more murky. Vocally, the two are close, at least in the rhythmic song. But with subtle differences in that song, and more marked differences in plumage and size, we think the case can be made that both be considered separate. If it can be determined definitively that Fulvous Owls lack a series-type song, that would be a strong argument in favor of separate species status for Cinereous.
3) Several people have wondered whether the apparent Fulvous Owls recently found in Oaxaca are really Fulvous Owls. We think our photos (in conjunction with the photos of Fulvous Owl linked above, and the many available audio recordings) demonstrate that the Oaxaca birds can be definitively identified as Fulvous Owls, and ruled out as Cinereous. But putting this question to rest raises a host of others. How far north and west are Fulvous Owls found? Do they currently overlap with Cinereous Owls in Oaxaca (which has specimen records for both)? Is one population expanding? Are the two interbreeding? Do intergrades exist?
What is needed now are more recordings and photos, especially from the other parts of Cinereous Owl’s range, in Guerrero and (even more importantly) Veracruz and Oaxaca. Hopefully the recordings we were able to obtain from Nayarit will help anyone out there trying!
For more information on Cinereous and Fulvous Owls, check out the detailed bibliography at the end of Michael Retter’s blog post from 2012. At this writing, most of the links are broken if you try to click on them, but will work if you copy and paste the URLs into your browser.
It’s hard to deny – among many birders there is a preconception that sparrows are brown and boring looking. And while I would argue that point for many of the species in the group, it isn’t very easy to defend Brewer’s Sparrow (Spizella breweri). It’s a small, brown, and subtly marked bird that doesn’t stand out much when you see it. But just wait until you are standing on a sage flat in the pre-dawn and hear a Brewer’s Sparrow! There are few auditory experiences in the west as sublime as being out in the crisp, cool air, surrounded by the smell of sagebrush, and hearing the tinkling buzzes of Brewer’s Sparrow all around you.
In addition to having one of the best voices in the west, Brewer’s Sparrow also has an interesting taxonomic facet to its name. The nominate breweri subspecies is familiar to most anyone who birds in the appropriate habitat. Less well known is that there is another subspecies – taverneri – that was first described as a separate species from far to the north of most Brewer’s Sparrows (Swarth and Brooks 1925). Unlike the nominate subspecies, it is restricted to stunted, willow-dominate thickets right below treeline – hence the common name of “Timberline” Sparrow. It was later lumped with Brewer’s Sparrow, but recent morphometric and genetic studies have again raised interest in the form (see Klicka et al 1999). For a long while conventional wisdom said that Timberline Sparrow occurred only well north of Brewer’s, in an isolated range centered around southern Yukon. But as more people studied the bird it was found to also occur near treeline further south, first in the Canadian Rockies, and then in Montana. Birds have been found in similar habitats even further south, to at least southern Colorado, but it is less well known what to make of these birds (more on this later in the post).
Before I dive into the vocals of these birds, a bit on the visual identification. As Brewer’s Sparrow doesn’t have much in the way of obvious field marks under the best of circumstances, it shouldn’t come as any surprise that telling Timberline from the nominate isn’t easy by any means. The most often cited field marks are usually bill size (Timberline has a longer but thinner bill), strength of the face pattern and crown streaking (noticeably stronger in Timberline), overall color (noticeably grayer in Timberline), and sometimes some faint streaking on the flanks or sides of the chest in Timberline. All of these marks are hard to use in isolation, at least for someone without a lot of experience with both forms. Personally, I believe that if I spent time closely looking at migrant or wintering Brewer’s Sparrows I might be able to find birds that I have a strong suspicion are Timberlines, but that I wouldn’t be able to make a claim with certainty.
Enter vocals. Differences in the song between Brewer’s and Timberline are the most often cited difference between the two. Before getting into the nitty-gritty of it all, though, we need a bit of a primer in the sounds that Brewer’s Sparrow (sensu lato) make – it’s more complicated than just song and call. That wonderful dawn chorus I mentioned in the intro to this post is what is known as the “long song” – a successive series of different trills, typically starting out very high-pitched and then descending into a long string that can sometimes last 20 seconds or more. Mostly given very early in the morning and late in the evening, it can sometimes be heard at other times of the day but generally less often. Below is an spectrograph of long song from Mesa County, Colorado, to demonstrate the structure of the voc.
The other song of the species is appropriately known as the “short song”. This vocalization, much shorter than long song, is typically comprised of one to three different trilled phrases, each phrase most often (but not always) even in pitch, but different in pitch from the previous one. It is also one of the most variable songs in the west – some birds can sound startlingly like Chipping Sparrow, while others incorporate elements almost reminiscent of Prairie or Blue-winged Warbler songs. When you hear a Brewer’s Sparrow during daylight hours it is much more likely to be giving this vocalization than the long song, though again there are exceptions to this. Most Brewer’s Sparrows only have one short song that they give, whereas long song can be variable in terms of which trilled elements they include. If you want to read more on the different song types, Arch McCallum has a page dedicated to the topic here.
Like most other sparrows, Brewer’s also has various calls – flight call, chip call, twitter, etc. I don’t really go into those here, in part due to very small sample size available from Timberline Sparrow of these calls, and also since I don’t personally believe that they can be identified to form.
Of the two song types, it is long song that I believe is most readily distinguishable between the two forms. The “classic” long song of breweri typically sounds very buzzy throughout, descending from very high-pitched, insect-like buzzes to lower pitched buzzes, but rarely having anything that one would describe as “musical” about them. This is due to the broadband nature of all of the elements of the song. Most phrases in the long song are made up of two alternating notes that comprise the buzz, and in nominate breweri both of these elements are broadband, or if one is less so then the broadband one dominates. Below are several examples of long song from breweri from around their range.
The long song of Timberline Sparrow, on the other hand, gives a much less buzzy impression. The overall structure of the song is the same – it typically starts out with very high-pitched, insect-like phrases that quickly descend into the variable series of trills. The difference comes in the fact that elements of the majority of the phrases in the song are noticeably less broadband than in breweri, so that they sound more musical and tinkling. This difference isn’t huge, and there is considerable variation, so it is important to listen to enough of the song to get a general impression rather than make the call on just one or two elements. In general, though, I think that once you’ve gotten a handle on how each subspecies sounds when long singing you can tell the different most if not all of the time. Listen to several examples of Timberline Sparrow below (from four different birds; the first three are from the same individual in the Yukon, the next five from different birds in Alberta) and see if you can tell the difference:
When it comes to short songs, these differences can also apply. Unfortunately, short songs seem to be a less reliable way to distinguish Timberline from Brewer’s. The main reason for this is probably that in long song there are lots of difference elements and the impression the entire vocalization gives is a composite of how they sound, whereas in short song you only have a few to draw on. So say you get a Brewer’s on the musical end of the range of variation and a Timberline on the buzzy end there could be some overlap. That said, I do believe that on average the short songs of Timberline are less broadband and more musical. First, here are a variety of breweri short song, both on the buzzy and musical ends of variation:
And here are two examples of Timberline short songs, one on the musical end and one on the buzzier end:
So by now you’re thinking “great! Now I can easily tell the difference between Timberline and Brewer’s, at least by the long song”). Unfortunately there’s reason to be cautious; the differences between these two forms aren’t entirely black and white. There are some recordings out there of breweri that are less buzzy than average, approaching Timberline. And when you listen to the long songs of Brewer’s that I link above you can hear that the examples from California seem to be a bit less buzzy than those from Colorado. When you combine this with a relatively small sample size of available Timberline Sparrow recordings and it’s hard to be completely certain how reliable the vocal differences are. That said, I personally have never heard a Brewer’s Sparrow that sang a long song entirely like that of the Timberline Sparrows I’ve heard. But if you’re investigating a suspect bird on territory out of the known range of one form or the other, I would want more than just one line of evidence before making the call.
This is especially relevant to birders in states where Brewer’s Sparrows have been found breeding near or at treeline but south of the known range of Timberline Sparrow. A few years ago there was a lot of buzz about this issue in Colorado, where I am from. Most birders in the state accepted that these birds breeding in highland stunted willow patches were a southern range extension of Timberline Sparrow, and that they were different than the more widespread Brewer’s Sparrow they were all familiar with. And I’ll admit I was also in that camp, at least to start.
But then back in 2007 I actively tracked down two different populations of these highland birds. I was rather surprised, at the time, at how un-different they were. The first group I recorded all sang songs that sounded unremarkable to me. In the second group, though, I did find a bird that was noticeably more musical and stood out, but it was surrounded by birds that were “normal”. Here’s the recording I made of that more musical sounding individual. Sounds a lot like Timberline Sparrow, doesn’t it?
What was going on? To this day I am not entirely sure. At the time my theory was that lowland breeding breweri moved upslope after their first brood to breed in dry stunted willow habitat superficially similar to the sage flats they used lower down. But now I am unsure. I didn’t record any long songs from either of those populations, and knowing what I do now after having more experience with definite Timberline Sparrows I am unwilling to make a call until I record some long songs from these southern highland birds. It remains one of those exciting areas that needs to be looked into, a place where anyone with a recorder willing to make the effort to be standing on a high-mountain hillside waiting for the dawn can help unravel a mystery.
Here in Colorado, we have lots of ravens. Conventional wisdom says the ones on the southeastern plains are Chihuahuans, and the rest are Commons. But not everyone accepts the conventional wisdom. Some believe that Chihuahuan Ravens are not only the default raven in the southeast, but also regular wanderers north to the Wyoming border. Others maintain that to find a real Chihuahuan Raven, you have to go south of Colorado altogether.
For a long time, I haven’t been sure what to believe. But I’ve been browsing my own audio collections and those online, and I’m starting to think that voice is actually a very good character — maybe the best field character — for separating these two species. I’m willing to be convinced otherwise, and I’m interested in hearing from anybody who has experience with these two birds. But at the moment, here’s what I think I know about the sounds.
It seems to me that Chihuahuan Raven may actually have one of the most predictable (least variable) voices of any North American corvid. Its classic call is a harsh “caw” that is deeper and much more coarsely burry than most “caws” of American Crow. It is of medium pitch, slightly nasal, and barely upslurred. It actually has something of a quacking quality — reminiscent of the harsh-sounding quacks that female Mallards do when they’re agitated.
Once you start comparing multiple recordings, this call of Chihuahuan Raven starts to seem remarkably consistent rangewide and from bird to bird:
Occasionally a couple of the birds in these recordings switch to some similar but slightly higher-pitched calls. And females will also do the clucking gurgles that many female corvids do (you can hear some on my recording above from Chihuahua). Other than that, though, these six examples are all so similar that they could almost be from the same individual bird.
From one of the least variable North American corvids, we transition to one of the most variable.
I’ve seen a number of sources argue that Chihuahuan Raven averages higher-pitched than Common Raven. This is a really problematic statement. It’s true that if you hear any really low croaks, you can be sure they’re from a Common:
But if you hear any really high croaks, you know they’re also from a Common:
(The ravens in that Red Top Ranch recording, by the way, were flying in a loose flock of 5 over featureless shortgrass prairie in southeast Colorado — location, habitat, and behavior all suggesting Chihuahuan Raven. But the voice doesn’t seem to match.)
Common Raven runs the gamut of possible pitches and tone qualities, from far below Chihuahuan’s pitch range to far above it. Chihuahuan occupies a limited space in the middle. Not only are Commons variable, but they often change sounds while you’re listening to them, unlike Chihuahuans. I might go so far as to say that if you hear a group of ravens giving a whole bunch of different-sounding calls, you’re almost certainly listening to Commons:
Some recordings of Common Raven come pretty close to matching the typical call of Chihuahuan Raven, but exact matches seem to be rare. Interestingly, the most Chihuahuan-sounding Commons I’ve found are mostly from California:
You may recall that over a decade ago, Common Ravens in California were found to be genetically distinct from other Common Raven populations — more closely related to Chihuahuan Ravens than to other Commons. So should we split Common Raven? Or reclassify California’s ravens as Chihuahuan?
Well, maybe not. The study found that Chihuahuan Raven and “California” Common Raven were still pretty distinct genetically. And more recent followup research has showed that “northern” Common Ravens and “California” Common Ravens have apparently been interbreeding extensively in western North America for a very long time, and the genetic differences between them may be decreasing over time. You could make a case for lumping ALL of North America’s ravens into one species, but I’m not sure that’s warranted, especially if Chihuahuans and Commons are not regularly interbreeding. More studies will need to be done on that.
On the whole, I could be underestimating the vocal variation in Chihuahuan Raven. And maybe there’s a lot of interbreeding going on in places like southeast Colorado. But at the moment, it really looks to me like Chihuahuan is a very predictable-sounding bird. And if it is, then vocalizations may be the single best way to identify one in the field.
Last winter, in the Great Redpoll Invasion of 2012-2013, huge numbers of these little Arctic birds pushed farther south than anytime in recent memory, prompting a surge of interest in redpoll identification and taxonomy. My good friend Andy Boyce even got people talking about whether Hoary and Common Redpolls are separate species, or identifiable forms, and the many resulting discussions taught me (and others, I’m sure) a lot about how we define and detect species boundaries. And the conversation hasn’t quit: Tom Johnson and Luke Seitz are contributing with a redpoll identification article in the July/August issue of Birding.
During the course of these discussions, a couple people argued that Common and Hoary Redpolls have been shown to differ in vocalizations. To verify this claim, I set out to track down the relevant peer-reviewed literature, including a paper in Swedish, two key publications in Russian, and a book in German. It took some six months, but thanks to the help of a terrific interlibrary loan crew (and some assistance from Albert Lastukhin in Russia), I finally managed to get copies of all the sources. My dusty old fluency in Russian came in handy, as did my one year of college German. (Oh, and Google Translate.)
The original claims of differences in the literature
In 1981, Maria Zablotskaya provided a detailed spectrographic and behavioral analysis of the vocalizations of Common Redpoll; the following year, she teamed up with Boris Veprintsev to do the same for Hoary Redpoll. These authors are often cited as having found differences in the sounds of the two species, but it’s important to note that they did not explicitly address the issue of species differences (or species boundaries) at all. The first paper treated Common Redpoll only; the second paper discussed Hoary Redpoll only, with only a few passing comparisons to Common. They named several more calls for Common than they did for Hoary, but this is likely due to the fact that they spent less time with Hoaries and had fewer recordings available for analysis. A careful reading of both papers (along with Ernst 1998) shows that there probably aren’t any sounds in the repertoire of one species that don’t have an equivalent in the other species. This doesn’t necessarily mean that the species sound the same, but it does mean that they give similar types of calls.
In the face of doubt about whether the two redpoll species should be lumped, a couple of authors stepped forward in the mid- to late 1980s to defend the two-species concept. Ulf Molau’s (1985) “The redpoll complex in Sweden” focused on plumage and measurements, but also made the following important claims about vocalizations (translated from the Swedish by me, with help from Google Translate):
Common and Hoary Redpoll differ not only in size and plumage characters, but also in vocalizations. The low-key contact sounds uttered on the ground or in a tree crown (or in a cage) may in Common Redpoll be described as a catchy “chit-chit-chit.” Hoary Redpoll’s contact vocalization is a rougher, almost House Sparrow-like “chirp-chirp.” Similar differences also exist in the familiar “flight song” which is rougher and more metallic in Arctic Redpoll. Both Common Redpoll races flammea and cabaret are, however, completely identical as far as vocalizations are concerned.
These conclusions are based on Molau’s own observations of wild, banded, and long-term captive birds. He did not provide spectrograms.
Marc Herremans (1989), on the other hand, does provide spectrograms in support of the notion that the redpoll species differ in at least two vocalization types. He says of the contact calls:
Common [gives a] characteristic “machine gun”-like low and pure chatter, che-che, che-che-che, che-che-che-che-che. Arctic [=Hoary] sounded clearly slower, higher-pitched and less pure, djeet, djeet-djeet, djeet-djeet-djeet.
He illustrates these with spectrograms from two individuals of each species:
The two Commons (D and E) resemble each other on the spectrogram, and the two Hoaries (A and B) more or less resemble one another, but there are clear differences between the species, according to this graphic.
Case closed? No, not quite.
You see, redpolls are cardueline finches, and Paul Mundinger showed way back in the 1970s that in this subfamily, many calls are learned, not innate. Members of pairs change their call types to match one another, as a bonding mechanism. Mundinger (1979) paired a captive female Common Redpoll with a male Pine Siskin, and a male Common Redpoll with a female Eurasian Siskin. In both pairs the two members ended up with nearly identical flight (contact) calls. In these cases Mundinger wasn’t certain which individuals had changed their calls to match the other’s, but he thought it likely that the male Pine Siskin had switched calls to match his redpoll mate, and that the male redpoll had matched calls with his Eurasian Siskin partner.
Herremans certainly knew of Mundinger’s work. In fact, he replicated it: he demonstrated that three individual redpolls (a male Common, a female Common, and a female Hoary) all adopted identical “breeding calls” when paired with one another in captivity. But he didn’t seem to appreciate the implications of this work, which called his earlier claims of species differences into serious question. Herremans seemed to believe that call matching only happened with “breeding calls” among mated pairs, but Mundinger’s work was with the flight/contact calls, and at least in one species (Pine Siskin), he showed that even two males matched calls when caged together. (Two females, however, did not.)
So redpoll calls are learned. Does that mean there can’t be consistent differences between the species? Not at all. But if the calls are learned AND flockmates match one another’s calls, we might expect to see flock-specific call dialects.
And that’s exactly what we see when we compare online recordings. There’s tremendous variation from one Common Redpoll to the next:
Within flocks, redpolls tend to sound the same. Between flocks, they tend to sound different. These differences can be pretty large in geographic scope: I got recordings of three different redpoll flocks last winter from three different Colorado counties, and they are all pretty similar to one another. But they are quite different from the redpolls in Washington and Alberta.
This is the pattern we would expect to see if flockmates match one another’s calls. We would expect birds in one flock to converge on a common pattern, while birds in another flock converge on a different common pattern. If redpolls respond preferentially to birds of a similar call type (like crossbills, their closest relatives within the Carduelinae), that would explain why Herremans’ captive Hoary Redpolls showed little response to the calls of wild Commons flying by. Maybe it was because they weren’t conspecifics — but maybe it was just because they weren’t flockmates.
There may be consistent differences between the vocalizations of the redpoll species. Perhaps they use vocal differences to reject mates of the wrong species. Perhaps we can use differences to tell the species apart. But we can’t do that on current knowledge. There simply aren’t enough available recordings to document the limits of variation within species, in order to hash out the differences between species, if they exist.
There’s also the question of whether Hoaries and Commons in the same winter flocks may match each other’s calls. We don’t know whether they do, but given the willingness of these species to match calls of even Pine and Eurasian Siskins in captivity, it’s at least possible.
So it’s premature, I think, to try identifying redpolls by voice. But that’s no excuse to ignore it. We need to learn more about the calls of these birds, and for that we need more recordings. The next time the redpolls come south, I hope they meet a barrage of microphones.
“Russet-backed” vs. “Olive-backed” Swainson’s Thrushes
The April 2013 issue of Colorado Birds recently hit my mailbox. It’s an excellent issue of a top-flight regional birding journal, and I’m particularly excited about the article about identifying the two distinctive subspecies groups of Swainson’s Thrushes: “Russet-backed” birds from the Pacific Northwest, and “Olive-backed” birds from elsewhere in the range.
(In case you’re wondering about the source of my enthusiasm, I should mention that I not only edit the journal, but also co-authored the article, with the eminent Steve Mlodinow and Tony Leukering.)
I grew up seeing hundreds of Swainson’s Thrushes in South Dakota each spring — all birds of the “Olive-backed” persuasion, though I had no idea of that at the time. When I encountered my first “Russet-backed” Thrush in northern California in 2000, I was certain I was looking at a Veery. The uniform, rich chestnut-brown upperparts ruled out all other thrush species, or so I thought at the time, and it wasn’t until many years later that I realized my mistake.
Over the past decade, the work of Kristen Ruegg and her colleagues has shown that the two forms of Swainson’s Thrush not only look different, but migrate on different schedules to markedly different wintering grounds. They hybridize in a contact zone in British Columbia, but that contact zone is quite narrow, prompting occasional rumors and rumblings of a potential future species split.
One of the proposed lines of evidence concerns differences in vocalizations. And, with some cribbing from the Colorado Birds article, that’s what I’ll be writing about today.
Differences in song?
A 2006 study by Ruegg et al. claimed that the songs of “Russet-backed” and “Olive-backed” Thrushes differ in statistically significant ways. The spectrograms in the study seem to indicate some nice obvious differences:
When I first laid eyes on this figure, I thought this ID should be a cinch. The Russet-backed songs in the figure are much longer on average (song 1.1 is just under 2.5 seconds long, while song 4.2 is under a second), and they finish with high, fluting, polyphonic phrases like those in a Veery song, while the Olive-backed songs stay relatively low and simple.
But when I looked at a scattering of songs from across the country, it quickly became clear that these differences did NOT hold up across the species’ entire range.
Obviously the Olive-backed songs from elsewhere on the continent seem just as likely to go on longer and end in high, fluting flourishes. I’ve listened to a large number of songs of both species, and I can’t find any way to distinguish them by ear.
To be fair, Ruegg et al. weren’t just looking for differences obvious to the eye and ear — they were performing quantitative analyses based on spectrographic measurements. Even so, they recorded songs from just two coastal populations and two inland populations (plus a fifth population in the hybrid zone). Their results may be due to the differences between the local dialects of these few regions, rather than any larger or more consistent difference between the subspecies groups.
If anybody knows of a way to identify the songs of these two groups by ear, I’d love to hear about it.
Differences in calls
I am unaware of any consistent differences between the “wee?” flight calls of the two subspecies groups, which is a mellow rising whistle rather like the call of the Spring Peeper frog. The “quit”-type contact call, however, does seem to differ slightly on average. Olive-backed tends to give a quick, sharp “quit” that may recall the sound of a dripping faucet or the “whit” of an Empidonax flycatcher. In Russet-backed, this call is often slightly longer and more musical, an obviously upslurred whistle that might be transliterated as “wee” or “pwee,” like a shorter, sharper version of the flight call.
These examples are illustrative:
Even this slight difference, however, cannot always be trusted. One recording I found documents a “Russet-backed” from northern coastal California who gives, in between his songs,
a series of “typical” contact calls of Olive-backed Thrush (1:25 – 1:50);
a series of “typical” contact calls of Russet-backed Thrush (2:20 – 2:35); and
a wide variety of “flight calls” (0:37 – 0:43, 2:49, and 3:43 – 3:48).
At least some individual Olive-backed Thrushes give equally variable calls, so caution is in order.
Perhaps the most distinctive vocalization is the alarm call, which is a two-part sound ending in a low, loud, semi-musical purr or chatter. Olive-backed tends to introduce the chatter with a sound like the contact call, “quit-BRRR,” while Russet-backed tends to begin with a much longer and more musical note that is actually more like the flight call: “weee-BRRR.”
On the whole, the differences in voice between these two subspecies groups are pretty subtle, and almost never diagnostic, due to extensive variation within each group. I recommend using voice as a supporting character in the field, to be used in conjunction with visual field marks.
The taxonomy of the Golden-fronted Woodpecker and its relatives has been causing headaches for over a century. Not only does the species display complex geographic variation in plumage, but where ranges meet, it hybridizes with the closely related Gila Woodpecker to the northwest, the Red-bellied Woodpecker to the northeast, and the Hoffman’s Woodpecker to the south, which in turn sometimes hybridizes with the Red-crowned Woodpecker in Costa Rica. The result is a mind-bending mosaic of similar-looking woodpeckers stretching from Ontario to Venezuela.
Current taxonomy draws species boundaries despite the hybridization, in part due to differences in vocalizations. For example, Red-bellied and Hoffmann’s Woodpeckers both give a long, churring rattle call that Golden-fronted lacks. Vocal differences probably help the woodpeckers keep each other straight in areas where ranges meet, making them more likely than not to choose a mate of their own taxon, keeping the level of gene flow low and maintaining the boundary between the species in the long term.
As I was going through the Golden-fronted Woodpecker recordings in my collection, some of which are from Texas and some of which are from Mexico, I realized that at least one call in the Golden-fronted repertoire varies geographically. Birds in southern Texas and northern Mexico give a rough, single-syllabled “gaf” call rather like that of the Red-bellied Woodpecker:
But birds from Veracruz south give a distinctive, two-syllabled “CHUCK-a”:
These differences hold up in every recording I’ve managed to find of this call type. The single-note “gaf” can be heard at least as far north as the Texas Panhandle and as far south as San Luis Potosi. The double-noted “CHUCK-a” has been recorded not only in Veracruz but also in Guatemala and El Salvador.
It turns out that these differences almost exactly mirror the geographic boundaries between two major subspecies groups. The “aurifrons group” is the “classic” Golden-fronted Woodpecker of Texas and northern Mexico. The double-noted birds appear to correspond to the “santacruzi” group, which is found from Veracruz all the way south to Honduras (including the “polygrammus” group of southern Pacific-slope Mexico). It’s worth noting that the subspecies dubius from the Yucatan Peninsula apparently gives a slightly different version of the two-syllabled call, more of a “chuck-trrr” , sometimes shortened to a single-noted “chuck” rather like the call of aurifrons .
A 2009 genetic analysis by Erick García-Trejo and colleagues found that the “aurifrons” group and the “santacruzi” group were genetically distinct, with northern aurifrons more closely related to the Red-bellied Woodpecker, Melanerpes carolinus, than to southern birds. As announced in Birding magazine and elsewhere, this set the stage for a formal split of the species by the AOU into Golden-fronted Woodpecker (M. aurifrons) and Velasquez’s Woodpecker (M. santacruzi). However, nobody has apparently submitted a formal proposal to the North American Checklist Committee yet — so I took it upon myself to do that. Sometime later this year, we’ll find out what they say!
There are few species in North America as ambiguous as Northwestern Crow (Corvus caurinus). Even in a group of birds that are exceedingly similar the differences between American Crow (Corvus brachyrhynchos) and Northwestern Crow are minuscule at best. The only “surefire” way to tell them apart is by range; however a number of sources also cite vocal differences as a distinguishing characteristic (e.g., the Sibley Guide, some versions of the National Geographic Guide, Birds of North America).
American Crow is a very familiar bird to most people in the US – it occurs throughout the lower 48, except for certain areas near the Mexican border. Northwestern Crow is typically described as being found from northwestern-most Washington state along the Pacific coast to Alaska, as far west as Kodiak Island and the adjacent mainland. Outside of a narrow overlap zone in Washington state and southern British Colombia you can typically call the species by range alone.
But the situation is more complex than just being able to tick a bird for your list based on where you are. The American Crows found along the Pacific coast (ssp. hesperis) are at the small end of the spectrum for the species, sometimes look slightly “shinier”, and have subtly but noticeably different voices than American Crows further east (as an aside, the official designation of what constitutes hesperis is murky at best, and birds from as far away as Utah or Colorado could be this subspecies depending on some readings of the literature).
For a long time I’ve had a theory that the real “Northwestern” Crow was actually a “Pacific” Crow, since the American Crows along the northern California and Oregon coasts sounded and looked more like “Northwestern” Crows in Washington and further north, and different from the American Crows further east. If this was true, I theorized, then the crows west of the Cascades in the more humid areas of Washington, Oregon, and California, were a different species than those east of the Cascades in the drier rainshadow of the mountains.
So on a few recent trips I’ve taken to Washington I’ve made an effort to record crows wherever I found them, including going out to what were supposed to be the most “pure” Northwestern Crows left in the state, on the San Juan Islands and along the outer coast of the Olympic Peninsula. I also recorded some crows to the east of the Cascades, from the Okanogan Valley and further east. What I was hoping to find was either that the supposedly pure “Northwestern” Crows had a notably different voice than the taxonomically ambiguous ones around Seattle and along the outer coast in southern Washington, OR that all the crows west of the Cascades had similar voices that were different than crows to the east.
Unfortunately, as often happens with research of this type, what I found was different than either ideal situation. I’ve been able to hear what I feel are distinguishable differences between birds in Colorado and the ones in western Washington. But when I compared birds I recorded to the east of the Cascades in Washington I begin to hear what sounds like the start of a cline to the birds in Colorado. Recordings from further afield, in Oregon, California, and British Colombia seem to continue this cline. On the other side of the spectrum, though, when comparing the crows at the northwestern end of the range of American Crow to Northwestern Crows from Washington and further north into Alaska I don’t hear any vocal differences. This would seem to show that the end of the cline in crow vocalizations is not where the species boundaries are currently drawn.
When comparing the sounds from various crows it’s important to have an idea of what vocalizations each species makes. Anyone who has ever listened to a crow knows they have a remarkably large repertoire, everything from caws to screeches to rattles to song-like warbles. The most common calls, though, are variations on the “caw” call. These can typically be lumped into two groups, long and short, but there is a nearly complete continuum between those. Often within any individual bird the long calls sound lower and more nasal, so care has to be taken when comparing different individual crows and making assumptions about differences in pitch. Here are two cuts from the same individual bird in Washington showing a long and a short “caw”.
Ok so now to start the comparisons – listen here to what I consider a “typical” American Crow call from most of the continent, first longer calls then shorter:
And here is what I consider a typical American Crow from the Seattle area:
And a few from further east in Washington:
And here are some American Crows from further south along the Pacific coast:
Also listen to ML#13108 from the Willamette Valley of Oregon, ML#118854 from Tulare County, California, and ML#58191 from Hedley, British Colombia, all inland locations.
The ones around Seattle to my ears sound a bit more nasal, and on average slightly lower pitched than the birds from east of Washington state. It isn’t a huge difference, and there is some overlap, but whenever I arrive in Seattle after spending time in Colorado or further east I hear the difference. The birds from Okanogan County to the east of the Cascades sound similar to the Seattle birds to my ear, but are starting to having a hint of an eastern “accent”, as are the birds from coastal California. The recording from Oregon, though, sounds more eastern to my ears, and the ones from Tulare County, California, and British Colombia sound eastern to me.
Now listen to the calls of putative “Northwestern” Crows, also from Washington:
Not very different, right? And here are links to some Northwestern Crows in the Macaulay Library from further north, far enough away from American Crow to be pure, supposedly: ML#132185, ML#136466, ML#58198.
Also not very different from the American Crows in Washington!
There are still some potential wrinkles to work out. Without knowing the sex of the crow recorded it’s hard to determine what potential differences in male and female voices there are and how that affects the variation. And it’s hard to get a real handle on the complete variation in each population without more recordings.
Now I can’t comment on the distinctiveness of Northwestern Crow based on genetics, structure or plumage, or physiological differences. But I would contend that despite what many field guides, and BNA say, there are no compelling vocal differences between that species and American Crows in the Pacific Northwest. With this lack of a vocal dividing line there seems to be little support for drawing the species boundaries in their current location. Add to that the apparent evidence of a cline between the crows in the Pacific Northwest and those east of the mountains to the birds even further east, and the evidence for having two species at all starts to look weak indeed.
Antpittas are one of those quintessential groups of tropical birds. They may not be colorful, like tanagers, coatings, or manakins, nor are they large and noisy birds that are out in the open and in your face, like toucans or parrots. But there’s just something about them and their weird, nearly tailless bodies on top of stilt like legs, and their way of moving through the dense growth in bounding leaps that makes them irresistible. Add that to the fact they’re rarely seen and they are usually among the most wanted birds on a visit to the southlands.
Because antpittas are shy and rarely seen, their vocalizations tend to get a lot of attention. After all, it’s an exceedingly rare event that you see an antpitta before you hear it! In addition to being a good way to find them, it’s also a good way to learn something about their taxonomy. Since antpittas are suboscines, they don’t learn their songs, and systematic differences in their vocalizations indicates some underlying genetic difference as well. It was vocal variation, in part, that resulted in the recent split of Sucre Antpitta (Grallaricula cumanensis) from Slate-crowned Antpitta (Grallaricula nana; SACC 421). And there are other groups that are just begging to be studied: Tawny Antpittas (Grallaria quitensis) sound different between Colombia, Ecuador, and Peru; Chestnut Antpitta (Grallaria blakei) has a little known and unnamed southern population that sounds different from the more common northern one; Rusty-breasted Antpitta (Grallaricula ferrugineipectus) is already split by many authorities (though not the South American Checklist Committee) into northern and southern forms because of their different songs and calls. And beating them all, Rufous Antpitta has no less than SEVEN vocal types, all very different from the others, that surely represent a number of separate species.
And it’s that species that I’ve just written a Xeno-Canto feature on. Given its exceptional complexity, even for a tropical species group, I also made a map (shown below) of where the various subspecies occur, and added map points for the specific recordings I discuss in the feature. From north to south the (sub)species shown are: spatiator (dark blue), saltuensis (brown, no recordings), unnamed ssp (green), nominate rufula (purple), cajamarcae (yellow), obscura (light blue), and occabambae and cochabambae (red). If you want to read more about Rufous Antpittas and their remarkable vocal variation, follow this link and read on!
Warbling Vireo is among the many widespread North American species with east/west vocal forms that meet on the Great Plains. Along with other examples of this type of song diversity (Marsh Wren, Blue-gray Gnatcatcher), eastern (gilvus group) and western (swainsonii group) Warbling Vireos may represent two species, and if they are ever split, song would be the best way to identify them.
Look at a range map of Warbling Vireo and you’ll see that it continues in a pretty much unbroken swathe across North America. The eastern and western song types meet at the Great Plains/Rocky Mountains interface, and in at least some of those both occur in a fairly close proximity but remain identifiable to type. For example, Warbling Vireos singing on the eastern plains of Colorado not far from the foothills are typically of the eastern type, while those from the foothills west are of the western type. In the Black Hills of South Dakota eastern types occur in the lower elevations, and are replaced by westerns in the higher ones (Birds of South Dakota). What is not very well studied, however, is whether there is a cline in song types anywhere in their range, and how the birds react to each others’ song types in areas in/near potential overlap.
The plumage of the two forms is extremely similar. Western is often cited as having a darker crown and more defined eyeline, giving it less of the “blank-eyed” look associated with Warbling Vireo. It is also a bit whiter on the underparts and darker on the back, and the bill is slightly thicker (Birds of North America). However, these differences are very subtle, not well studied, and hard to see in the field. A much more reliable way to separate the types is by their primary song, which has consistent (if not always obvious) difference. I’ll first talk about the eastern song type, which many readers should be familiar with it, and then contrast the western form.
The song of the eastern Warbling Vireos is what gave the bird its name. It is a pleasant caroling song that rolls along, often ending in an emphatic higher note, transcribed at times as “if I could see it I would seize it and squeeze it til it squirts” or some variation thereof. The song phrases are typically around 2.5 to 3.5s long, and are made up of a series rich whistles that are slightly modulated. In the song of eastern Warbling Vireo, most of the initial notes are near the same pitch, with a few higher notes thrown in towards the end of the song. Below are a couple of examples of eastern song:
Western Warbling Vireo songs differ from that of eastern mostly in terms of pitch. Most western songs tend to have more high pitched notes, and these are placed more evenly throughout the song, breaking up the rhythm so that the whole strophe sounds less sing-songy than the song of Eastern. While the song of eastern gives the impression of a series of low, caroling notes, the song of western gives a jumbled and less structured feel, with an overall higher pitch. This difference in sound takes a little bit of practice to pick out, and there are some birds (especially in the contact zone?) that are harder to place as one or the other. A couple of samples of the western song type:
Where from here?
Despite the fact that the songs of these two types of Warbling Vireo are fairly well differentiated, very little is known about what goes on in the contact zones. This is where just about any birder visiting the western Great Plains and eastern Rocky Mountains can make a difference. Pay attention to the Warbling Vireos! Are they eastern, western, undefinable? There are very few recordings available from these areas, so anything you find will be interesting and useful.