A Song for August 07

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I could not find a song recorded on this date, so August 7 will have to settle for a contribution from August 2, produced by a redpoll sitting on top of a tall spruce in Alaska. The conspicuous perch and the repetitive delivery make this an example of singing, but the zree he is singing with is a call. The song of the Common Redpoll is more complex and variable, like the songs of other finches. Several of these finches have a zree call. Perhaps the Pine Siskin's is best known, but all the goldfinches also have one, so be wary of relying on this call to identify siskins.

Pause the sonogram and take a look at one of these calls. If it looks a little disheveled, you have a good eye for visual mathematics. The sound is produced by oscillations of membranes in the bird's syrinx. The rate of these oscillations is called the fundamental frequency. It can vary with time. It is usually the lowest band on the sonogram. Most of the bands above it are "harmonics," and they are constrainted to be multiples of the fundamental. They should therefore rise when the fundamental rises and fall when it falls. Because they are integer multiples of the fundamental each higher harmonic should rise and fall more steeply than all the ones below. If you see a band that does not obey these rules, it cannot be a harmonic. These finch calls are rich in such bands. What are they?

Birds have the extraordinary ability to produce two fundamentals at once, because the two sides of the syrinx are controlled independently and can oscillate independently. Each fundamental can have multiple harmonics. Further, any of these bands can interact, producing bands that are sums of two others as well as bands that are differences of two others. All of this is going on in the zree calls above. Pieplow refers to such sounds as "polyphonic."

The sound that comes out of the bird's mouth is not a multi-channel affair, each with a separate band. All the bands have been added together to make a single wave form. A numerical representation of that wave can be parsed, using the Fourier Transform, to unpack all the different bands. That's how we get sonograms. The pressure wave that is the sound can also be parsed into all its component frequencies by the cochlea of the vertebrate ear. The cochlea is an analogue device, really rather simple in design, but elegant in its capability. I don't know which is more impressive, the cochlea or the Fourier Transform. For some reason, the vertebrate brain has decided not to use all the information harvested by the cochlea, but presents all the frequencies to consciousness as, well, chords. Think of a zree as a time-varying chord.

Some day, birders will have chips in their brains that redirect all the info from the cochlea to the visual cortex, where it will appear as a sonogram. They can then page through Pieplow's field guides looking for a match. Actually, I suspect the crack ear-birders already have a capability akin to that, it's just that they don't see the sonogram. Part of the analysis is done outside consciousness, such that a bird says "zree" and the birder voices the words, "Lawrence's Goldfinch." It's all so easy, if you know how.

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Email: web at archmcallum.com

Nathan D. Pieplow. 2017. Peterson Field Guide to Bird Sounds of Eastern North America. Houghton Mifflin Harcourt.
Nathan D. Pieplow. 2019. Peterson Field Guide to Bird Sounds of Western North America. Houghton Mifflin Harcourt.
Donald Kroodsma. 2005. The Singing Life of Birds. Houghton Mifflin.