Earbirding

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 [1 2] 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.