42 Scientific American, May 2022 Illustration by Liz Wahid (Zebra Finch)
information about emotion, health,
age, individual identity, and more
in the fine structure of song beyond
what our ears can detect. It is reason-
able to expect that other birds with
songs that sound repetitive to human
ears share the Zebra Finch’s powers
of perception.
You might be wondering whether
these small acoustic fluctuations in
song are just accidental or random, like
variations in the trajectory of a pitch-
er’s curveball toward home plate. In
fact, the key to fine structure may be
the avian voice box. Humans produce
the sounds that we shape into speech
with our mouths and tongues using a
single source at the top of our neck, the
organ known as the larynx. Birds, in
contrast, produce sound using a unique
two-branched structure that sits atop
the lungs called the syrinx. It carries
two sources of sound, one from each
branch, that can be controlled inde-
pendently. On top of that, muscles in
the songbird syrinx contract faster
than any other vertebrate muscle,
enabling millisecond-level temporal
control. Thus, the birds aren’t produc-
ing fine acoustic variation by a slip of
the beak—they can control it in addi-
tion to perceiving it.DANCE OF THE SYRINX
together these studies show that birds
listen to song differently than we have
traditionally imagined. Melodies and
sentence structure are essential to us
when we listen to music and speech.
We can’t help but project them onto
birdsong when we hear it. But differ-
ences in se quence don’t seem to matter
much to birds. Some species have diffi-
culty hearing even simple changes. For
humans, when these kinds of manipu-
lations occur in speech or music, they
totally disrupt the message or melody.
But birds seem to be listening most
closely to the acoustic details of indi-
vidual song elements, independent of
the sequence in which they occur. And
they hear details beyond what our ears can discern.
In thinking about how birdsong sounds to the birds, a better
analogy than human language or music might be dance. When
we learn a dance routine, getting the sequence right is necessary
for getting the moves right—like when I learned to follow a Lindy
Circle with a Charleston in my swing dance class. Screwing up a
transition can cause the structure of an individual move to fall
apart. But someone watching a dance does not extract muchleast researchers think of them as the same. In truth, there are
small differences in how a given syllable is uttered in each rendi-
tion of the motif. We tested the finches’ ability to discriminate
between different renditions of motif syllables and found that the
birds can hear the differences easily.
This result means that although to us the Zebra Finch song
sounds like the same motif on repeat, to the birds it does not.
We suspect that instead they could be perceiving a rich trove of
Zebra Finch SongNatural MotifAmplitude:
Small (quiet) Large (loud)Frequency (pitch)HighLowSyllables: A B C D EShuffled SequenceC E D A BSyllable B ReversedA B C D ETime (seconds) 0.2 0.4 0.6 0.8Sound Discrimination
The Zebra Finch song consists of what sounds to
humans like a string of identical motifs, each containing
several distinct syllables. But the motifs actually differ con-
siderably in their fine structure, and the birds can discriminate
between different renditions of the same motif. In the spectro-
grams shown, the natural motif is repeated as the “background
sound” (top). The same motif but with the second syllable reversed
in structure was one of the “novel sounds” (middle) presented to the
birds. The same motif but with the syllables shuffled around in
order served as another novel sound. The white circles mark sylla-
bles that have either been reversed or shuffled.Zebra
FinchAdam Fishbein (waveform and spectrogram)