Every spring, male House Wrens choose a territory and start singing their little hearts out to proclaim ownership and to attract a mate. As the incredibly loud, long, bubbling songs pour forth, these little brown wrens transform into Olympic athletes, at least acoustically. Each song races through quiet introductory notes to a series of bouncing trills, and males can sing 600 songs an hour for hours on end.
Just the mechanics of how a half-ounce bird can make this much sound fascinate me. But as a graduate student at the Cornell Lab of Ornithology, I also wonder why they do it. If a Swamp Sparrow can claim its turf with a simple trill; if an Eastern Phoebe can woo mates with a scratchy “fee-bee,” then what is the extravagant fuss of a male House Wren all about? After five years of research I’m still not sure I know—the wrens’ songs are just so complex—but I’ve learned a lot about sound and song in the process.
One of the tools of my trade is the spectrogram, a graph that allows me to look at fine details in songs that my ear alone would miss (see sidebar). With a spectrogram, I can pick apart a wren song one millisecond at a time. I can see points where both sides of the bird’s syrinx, or voicebox, are singing different notes simultaneously. And I can begin to grasp the complexity of the House Wren’s ending trills, where he sings precise copies of several different types of syllables, all in the space of a couple of seconds. Now that I can see this complexity, I can start to study which aspects of a song are important to other wrens.
As you might imagine, some songs are harder to sing than others, and many scientists think birds respond to degree of difficulty in a rival’s or potential mate’s song. Perhaps it’s crucial, even for an expert songster like the House Wren, to push their abilities to the absolute maximum, like jazz musicians trading solos.
The problem is figuring out how to measure what makes a song difficult. The simple measures scientists have used for other species, such as song length or the absolute lowest or highest frequencies produced, may not be enough for a House Wren’s jumbled song.
I tried something more complicated, a measure called “trill performance” that seems to be important for Swamp Sparrows. This measurement examines the ability of a bird to sweep through a wide range of pitches while quickly repeating each note—like a slalom skier veering from gate to gate on the way down a mountain. I worked from spectrograms to measure the range of sound frequencies and the speed of repetition.
Then, to test the effect of trill performance, I played songs with different performance levels to male House Wrens in the wild. I expected males to be less impressed with lower-performance trills and therefore to act more aggressively toward them. I also noted the trill performance of males that females chose as mates, expecting that females would respond more favorably to higher-performance trills.
But unlike in Swamp Sparrows, House Wrens didn’t seem to notice trill performance at all. Perhaps these songs are too complex to assess even with this relatively sophisticated measurement. One thing’s for sure, there are plenty of other difficult aspects to a House Wren’s song. But as we keep at it, I’m confident we’ll be better able to wring information out of the songs birds sing. And I’m sure we’ll be using spectrograms to do it.
Originally published in the April 2012 issue of BirdScope.
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