A few evenings ago my kids and I were exploring Walden Ponds, a bucolic birding spot in Boulder County, Colorado. We were in the buggy back end of this sprawling complex of marshes and woodlots, listening to the sounds of early summer in the foothills of the Rockies: twittering Violet-green Swallows and chattering Bullock’s Orioles; harsh Western Wood-Pewees and a querulous Black-billed Magpie; and a dispirited Warbling Vireo.
The Warbling Vireo sounded like a bird of the expected western, or swainsoni, subspecies-group. These western birds—represented by multiple subspecies—are thought by some ornithologists to be so different from eastern birds as to deserve separate-species status. The names proposed for these two “species” are, unsurprisingly, Western Warbling-Vireo and Eastern Warbling-Vireo.
There are certainly many good records in Colorado of Western Warbling-Vireo; it’s our default warbling-vireo. And in recent years, it has become clear that Eastern Warbling-Vireos make it at least into the eastern tier of counties along the Kansas border. Check out the xeno-canto website, and you’ll find several recordings from eastern Colorado of Eastern Warbling-Vireo. But how far west do Eastern Warbling-Vireos get? Do they come into contact with Western Warbling-Vireos? Such information could help resolve the matter of species limits within the warbling-vireo complex.
We walked down the trail a bit farther, and heard another warbling-vireo. This one was different. My initial impressions of the song were, well, impressionistic: Its song was “brighter” and “sweeter.” If the bird we’d heard earlier was “dispirited,” then the bird we were listening to right now was “declamatory.” Hm. “Bright,” “sweet,” and “declamatory.” You might as well describe a bird’s plumage features as “nice,” “handsome,” and “interesting.”
With a little bit of effort, we were able to get good views of the bird. I was struck by the extensive yellow on the bird’s flanks, and I was especially impressed by the bird’s overall heft and honking big bill. Those marks are consistent with Eastern Warbling-Vireo, although I hasten to point out that field identification of the warbling-vireos—especially of relatively drab birds in late spring—is a problematic matter.
The bird burst into song again. And again. It was appreciably different from the nearby presumptive Western Warbling-Vireo, audible from where we were standing. Did we have enough to “pull the trigger” and claim a sight record of Eastern Warbling-Vireo from Boulder County?
My first instinct: Call Bill Schmoker! Or drag Bryan Patrick up from Colorado Springs. Those guys are seemingly inseparable from their long lenses, and they do a wonderful job of photo-documenting practically every rarity they encounter in the field.
Then I had another idea. Why not come back and make a recording of the bird? The key differences—other than DNA—between Eastern and Western warbling-vireos are vocal, not visual. Well, I could think of one good reason not to do that: I’m a total klutz with gadgets, and my largely unused recording gear is decidedly low-budget. But then I remembered something Nathan Pieplow had posted (by way of Nate Swick) a few days earlier to The ABA Blog:
If you can, make an audio recording. Use your cell phone. Use your camera on the video setting. Use a cheap voice recorder. Use your laptop. Use any device that can possibly record sound.
So I came back, laptop and cheap voice recorder in hand. The birds were still there. First, here are three songs, recorded at five-minute intervals, from the bird I’m calling an Eastern Warbling-Vireo.
Now let’s take a look at sound spectrograms of the three. For whatever reason, I can’t upload them to The ABA Blog, but here are quick links to sound spectrograms of the first song, the second song, and the third song. On all three, note the sharp, high-pitched note at the end. That’s classic for Eastern Warbling-Vireo, the song of which I’ve seen rendered thus: I can see you, I will seize you, I will squeeze you, till you SQUIRT. (Sorry, Nathan. I’ve just violated your Rule #4.)
Now let’s consider the sound spectrograms of a different warbling-vireo out there. I’m not so sure it’s one of the presumed Western Warbling-Vireos my kids found two evenings before. Regardless, take a look at this sound spectrogram. The terminal SQUIRT (again, apologies to Nathan) is missing. Here’s another song from the same bird, again with no terminal SQUIRT. And it’s the same in this third song from the bird.
Something else. In all three songs from this second bird, the song appears to be “flatter” overall—less clearly rising and falling—than the songs from the first bird. That’s a good point of distinction between Western and Eastern warbling-vireos, with the latter often described as more “singsong” than the former. But let’s be careful on this apparent difference; the horizontal axis is scaled a bit differently for the two birds, in such a way as to “flatten out” the song of the second bird. Just as birders need to watch out for photographic artifacts, so they need to be wary in the matter of interpreting sound spectrograms.
Here are the sound recordings, by the way:
Okay, so what birds were out there? I’m open to suggestions, but I think the first bird is an Eastern Warbling-Vireo: Its rising and falling song consists of relatively pure-tone phrases, and that sharp, high-pitched note at the end is hard to explain away. The second bird is a bit of a mystery to me: It lacks the signature SQUIRT at the end, and its song is perhaps looser and flatter overall—but not by much.
It would have been “elegant,” as mathematicians and physicists say, for there to have been one of each out there: an unambiguous Eastern Warbling-Vireo singing alongside a slam-dunk Western Warbling-Vireo. And that’s what I might have reported, if it weren’t for the actual physical evidence. “Elegant,” yes, but also ignorant, and, when you think about it, ultimately dissatisfying. The most wondrous thing about birding, if you ask me, is the constant ability of birds to surprise us, to throw us for a loop, to mess up all our simplistic assumptions.
We start off by putting a name on a bird. Then we get serious. Our brains kick into high gear, and we start to question our assumptions. Our eyes and ears get in on the act. We pick up on nuances and subtleties we hadn’t been aware of. We deconstruct the bird. The process is exhilarating. The fun has begun.
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POSTSCRIPT. In a comment below, Alvaro Jaramillo asks about the “scolds” of these birds. As it turns out, I obtained a recording of the scolds of bird #2. Here it is: Bird #2, Scolds. As with the links to the soundfiles in the main text of this post, you’ll need to right-click on the link to download the .wav file.
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