This past Sunday, which happened to be Easter, I went out to do a little birding near my house in Boulder, Colorado. I was hoping to cash in on the incredible wave of migrants that had swamped the city over the previous couple of days, generating sightings of rarities ranging from Yellow-throated Vireo and Yellow-throated Warbler to Hudsonian Godwit and Sedge Wren. After a fairly uneventful morning of recording Mallards, Downy Woodpeckers, and Black-capped Chickadees, I walked past a bridge over a small canal and suddenly heard what I was looking for — the unmistakable sound of a bird rare in the county — the song of an Eastern Phoebe.
Almost immediately, though, I could tell that something was wrong. The bird didn’t sound quite right — its phrases were clearly variable, not stereotyped like a spring phoebe song should be — and it was missing the really strong burr at the end of the “fee-BRRR” song phrase that is the hallmark (and probable namesake) of the Eastern Phoebe. I realized, with growing excitement, that it could well be a hybrid.
As I’ve reported earlier, hybrid Black × Eastern Phoebes are on the increase, and have shown up along the Front Range in Colorado before. There was also a hybrid Black × Say’s Phoebe in California a couple of years ago. Hybrid phoebes are among the most interesting of all birds, because they allow us a unique window into the genetic control of song.
While recording audio of its song, I got some decent binocular looks at the singing bird. It looked quite typical of Eastern Phoebes, with a dark brown head; lighter brown upperparts and wings with a slightly grayer tone; faint brownish wingbars; and underparts that were mostly pale, but slightly darker on the upper breast, fading gradually down to the belly, which had a slightly yellowish cream color. From the front the bird had the very vague “vested” appearance caused by the slight contrast between darker flanks and yellow central belly, but this is not unusual on pure Eastern Phoebes. Unfortunately, I can’t recall the precise color of the throat or the level of contrast between throat and face, which would have been a useful mark. Ultimately, if I had seen the bird and never heard it, I might never have suspected anything amiss.
I think there are two lessons to learn here: 1) hybrid phoebes are becoming more and more common in Colorado; and 2) not all of them may be visually identifiable, but the voice gives them away. All the more reason to start recording audio!
In 2007, an odd-looking flycatcher set up a territory above a diversion dam along the Big Thompson River in Loveland, Colorado. It was clearly a phoebe, but what kind of phoebe, the first few observers couldn’t seem to decide.
Eventually, after puzzling over some internet photos of the smudgy-breasted bird, Nick Komar suggested that it could be a hybrid — one parent a Black Phoebe, the other an Eastern Phoebe. As soon as Nick’s suggestion landed in my email inbox, I knew I had to get sound recordings. For one thing, if it was truly a hybrid, then it was well worth documenting, because hybridization had never been reported among phoebes before. For another thing, I knew that a microphone trained on a hybrid flycatcher would open a unique window directly into the bird’s genetic code.
Why? Because unlike most other passerines, flycatchers don’t learn their songs. In a series of classic experiments in the 1980s, Donald Kroodsma raised Willow and Alder Flycatchers and Eastern Phoebes in the laboratory and determined that even if they were raised hearing only the wrong songs, or even if raised in complete silence, the birds all grew up to sing almost exactly like their parents.
This does not happen with most passerine birds. If you raise a finch or a junco without exposure to finch or junco songs, the bird might learn the wrong song, or it might grow up to sing incoherent babbles, but it definitely won’t sing like “normal” wild members of its species. Most birds with complex songs need tutoring and time to learn them — but not flycatchers. Every part of their vocal repertoire is somehow coded in their genes. Which means that if the genes of two species were to mix, one would expect to hear some sort of hybrid song.
Here’s what a normal Eastern Phoebe sounds like:
And here’s a normal Black Phoebe:
Birds with innate songs don’t usually show much geographic variation; they don’t have regional “dialects” the way that, for example, White-crowned Sparrows do. Therefore, in theory, Eastern Phoebes and Black Phoebes from all across their North American range should produce spectrograms that closely match the ones above.
That’s why, when I saw the spectrograms of the bird from Loveland, I knew we were dealing with a hybrid:
But “Smudgy,” as some of us called him, didn’t just mix up the pieces of Black and Eastern Phoebe songs. His song behavior was even stranger than one might expect from a hybrid.
For one thing, while Black and Eastern Phoebes both usually sing by alternating two different songtypes, Smudgy had three to choose from — like a Say’s Phoebe, the only species in the genus that had not given him genes. In addition, the introductory notes in Smudgy’s song were poorly stereotyped — that is, they weren’t always carbon copies of one another, but instead varied slightly with each delivery. However, the terminal portions of his songs were stereotyped. Furthermore, the different parts of Smudgy’s songs seemed to show different levels of influence from the parent species.
Why in the world would a hybrid sing like this? To know for sure, we’ll need more recordings of hybrid phoebes.
Hybrids appear to be on the increase, particularly in southern Colorado and northern New Mexico, where the northeasterly range expansion of Black Phoebe has started bringing it into sympatry with the westward-surging Eastern — but an apparent Black × Say’s showed up in northern California, and it’s worth noting that “Smudgy” was found at least 150 miles outside the range of either of his parent species. Since hybridity may well scramble the migration genes as well as the song genes, a hybrid phoebe may be possible almost anywhere on the continent.
In Cañon City, Colorado, smack-dab in the center of the contact zone between Black and Eastern Phoebes, SeEtta Moss has begun finding her own hybrid phoebes. She recently posted photos  and a video clip of a definite hybrid, as well as photos of another possible hybrid (perhaps a backcross). Here’s the spectrogram of the definite hybrid (using the audio from her video clip):
As you can see, this bird sings differently than Smudgy, which raises even more questions. As our sample of hybrid recordings grows, we may be able to discover some really fascinating things — not just about how genes control song, but also about how song evolved in these birds. More on that coming soon.
I’m no expert in the visual identification of female hummingbirds, so I can’t make too many comments on his photographs, but the vocalizations of hybrid birds are of long-standing interest to me, so his sound files and spectrograms certainly got my attention. A morning’s investigation convinced me that the sound file he posted does indeed provide good evidence that the bird at his feeder could be a hybrid, probably the progeny of a Black-chinned Hummingbird and a Costa’s Hummingbird.
Male Costa's Hummingbird, 16 April 2009. Photo by Chris Fritz (Creative Commons 2.0).
According to the excellent Hybrid Hummingbirds page at Trochilids.com, the hybrid combination of Black-chinned x Costa’s (which seems most likely for Rich’s bird given his comments on its plumage) was documented by Short & Phillips (1966), so if the bird is confirmed to be of that mix, I believe it would be the third documented such hybrid and the second known female.
Rich has already posted some spectrograms over at his place, but I’m going to go ahead and supplement his with some of my own. Since the fine details of hummingbird chips are pretty fine indeed, I’ve zoomed these things in MUCH farther than usual and widened the filter bandwidth for better time resolution.
First, here’s the typical chip of a female Black-chinned Hummingbird:
Note the downslurred intonation and the strong partials, with the third one strongest. If this call lasted 20 times longer, we would hear its nasal tone quality. Since it’s so brief, we just hear a high-pitched, fairly clear downslurred “squeak.”
Here’s the standard call of the female Costa’s Hummingbird, first at the same zoom level as above, then zoomed out to a slightly more sane perspective:
(Click here to listen to the audio at the Borror Lab’s website. It takes a bit for the bird to call.)
Obviously, the calls of Costa’s and Black-chinned Hummingbirds are extremely different on the spectrogram, and they can even be distinguished easily by ear with experience. As Rich’s spectrograms show (and my research has so far corroborated), Costa’s is the only hummingbird in the Calypte/Archilochus group with an upslurred call note (actually an overslur, as you can see above) — and the upslurred beginning of the Costa’s call is often perceptible to the ear. Black-chinned, Ruby-throated, and Anna’s all have perceptibly downslurred calls.
The calls of Black-chinned and Ruby-throated are by far the lowest-pitched, with many harmonics visible; they are less sharply inflected and therefore more musical than the calls of Anna’s Hummingbird, which start at a far higher pitch (around 9 kHz) and descend much more quickly, giving them a less musical “ticking” quality.
The hybrid at Rich’s feeder has downslurred calls with many visible harmonics, much like a Black-chinned Hummingbird, except higher-pitched and more sharply inflected:
(again, click here for Rich’s audio at Xeno-Canto.)
By itself, the higher pitch and sharper inflection (noted by Rich’s keen ears as “too high and percussive for Black-chinned”) didn’t at first convince me the bird was a hybrid. A careful reading of Rusch et al. 1996 finds mention of a rather similar vocalization from Black-chinned Hummingbird, which the authors called the “E note,” but even this note is not high-pitched enough to match the hybrid, and it is apparently never given outside the complicated chatters that hummingbirds make when sparring with each other. The same apparently holds true for Ruby-throated Hummingbirds (Rusch et al. 2001). So Rich’s recording is at least suggestive of hybridity.
But here’s something even more interesting: not every note on Rich’s recording is identical. Most of the calls on the cut actually start with a slight but noticeable upslur:
And sometimes it is very pronounced:
Although I don’t think it constitutes proof, this definitely suggests to me that the bird has some Costa’s Hummingbird genes. Rich, I think it’s time to contact your friendly neighborhood hummingbird bander. And keep that microphone running!
While going through the Macaulay Library’s collection of Common Nighthawk vocalizations, I came upon something strange: a recording of what might be a hybrid Common x Antillean Nighthawk from south Florida.
Here’s some background. In the early sixties, Charles A. Sutherland studied the breeding biology of nighthawks on Key Largo, Florida. He made observations and audio recordings of both taxa breeding there: Chordeiles minor chapmani and the very different-sounding C. m. vicinus, which was not yet recognized as belonging to a separate species (and wouldn’t be until the early eighties, when the AOU would cite vocal differences and sympatric breeding in creating Chordeiles gundlachii, the Antillean Nighthawk).
Sutherland wrote up his findings in a 1963 article in Living Bird that remains perhaps the best source published to date on Common Nighthawk vocalizations. Unfortunately, he excluded the Antillean birds from his discussion. On the other hand, he did donate his recordings to Cornell, and now they can be heard online at the Macaulay Library’s website.
In the vocal notes on his recordings, Sutherland often referred to the Antillean Nighthawks as the “pi-ti-mi-dick” birds, after the distinctive rhythm of their calls. There is one individual, however, that he refers to as “the improper pit-i-mi-dick bird,” apparently because the thought its call didn’t sound quite right for an Antillean Nighthawk. You can hear this bird on Macaulay Library cut 5904. It used to be classified as a Common Nighthawk, but as of this writing Macaulay’s got it labeled as an Antillean, and it’s certainly a confusing bird. Here are a couple of spectrograms:
Note that the bird gives a mix of short “pik” notes (the vertical lines on the spectrogram) and Common Nighthawk-like “peent” notes (the thicker blotches). By themselves, both of these elements suggest Common Nighthawk, as that species gives both calls. However, what seems odd is that the “piks” and the “peents” usually seem to merge into one, “pik pik pikpeent,” so that the typical rhythm ends up echoing that of the stereotypical chicken vocalization: “buk buk bukKAW.” By going through every recording labeled “Common Nighthawk,” I located nine cuts that included the “pik” call , usually alongside the “peent,” but none of them contain anything like this combination rhythm.
Here’s a typical Antillean Nighthawk call for comparison. It’s giving a relatively stereotyped “pit-i-mi-dick” of 2-6 notes in slightly decelerating series, the first element slightly longer than the others:
As far as I know, Antillean Nighthawks do not give long series of individual “pik” notes like Common Nighthawks do, but I’d be interested to hear from anyone whose experience suggests otherwise.
As you can see, the “improper pit-i-mi-dick” bird is quite different from an Antillean. It may simply be a Common Nighthawk who is stumbling repeatedly over his words. But it sounds suspiciously odd to me, and given that it comes from the area of sympatry, a hybrid seems quite plausible. Interestingly, Sutherland mentions in his vocal notes that the call of this individual is not accompanied by the rapid flutter of wings that usually accompanies the primary call in both Common and Antillean Nighthawks. Who knows what significance that fact may have?