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.