40 Scientific American, May 2022
Adam Fishbein (waveform and spectrogram)exact physiological mechanism underlying the birds’ superhuman
sensitivity remains unknown, but it may be related to features of
their inner ears, which differ from our inner ears in having a rela-
tively shorter cochlea that is slightly curved rather than coiled.
When I began studying how birdsong compares with human
language in 2015, when I was in graduate school at the University
of Maryland, I wasn’t thinking much about fine structure. Instead
I was looking to uncover languagelike grammatical abilities in
birds. But as I dug deeper into this question and conducted many
experiments with birds, I came to realize that the key to under-
standing what they are communicating in song may lie in these
fine acoustic details rather than the sequences in which they occur.
The grand champion of the birds tested in Dooling’s 2002 study
was the Zebra Finch. This small, lively songbird native to Austra-
lia is the most popular species for lab-based modern birdsong
research, largely because it both sings and breeds prolifically in
captivity. Its song, produced only by males, is also relatively sim-
ple, consisting of a single motif of three to eight syllables repeated
over and over again, usually in the same order. The simplicity of
the song makes it more straightforward to study than others.
Because the males learn both the syllables and the sequence in
which they occur from a tutor, typically their father, one might
think that both levels of the song are important in perception.
We tested that notion in a 2018 study that examined how well
Zebra Finches hear the difference between natural song motifs
and motifs where syllables are either temporally reversed or shuf-
fled in sequence. We trained birds to report whether they could
hear the difference between sounds. They listened to a repeated
sound and then pushed a button to initiate a trial where the
sound either changed or remained the same. If a bird pecked a
certain button when the sound was different, it counted as a cor-
rect hit, and the bird got a food reward. If it pecked that button
when the sound was the same, the lights in the chamber went off,
and it counted as a guess. Using this method, we evaluated the
birds’ ability to discriminate between the repeating sound (the
natural song motif ) and novel sounds (motifs whose syllables we
had temporally reversed or shuffled). From the birds’ perspec-
tive, they were simply trying to earn tasty food.
Interestingly, the Zebra Finches performed nearly perfectly at
discriminating reversed syllables, which can be difficult for our
human ears to detect, but they did
poorly at discriminating shuffled sylla-
bles, which are more salient to us.
When you reverse a syllable, one of the
main things that changes is fine struc-
ture, so it is not surprising that the
birds knocked it out of the park on that
exercise. Yet their difficulty with se -
quence differences is unexpected, not
only because those changes are easy for
humans to hear but also because the
males learn to produce song syllables
in particular sequences. Their difficulty
in perceiving shuffled syllables may
mean that for these songbirds, se -
quence matters in the learning process
but does not carry much information
for communication.
Given the results of these experi-
ments with artificially modified songs,
my colleagues and I began to wonder
how fine-structure perception is rele-
vant for natural song communication.
Hearing reversed syllables is impres-
sive, but the birds never actually pro-
duce such sounds. So the next question
we asked was how well birds hear sub-
tle natural acoustic changes in song.
My colleagues had already shown in
another 2018 paper that Zebra Finches
can hear tiny differences in the fine
structure of one another’s calls, which
can carry information about sex and
individual identity. To examine their
perception of fine structure in song, we
took advantage of the fact that Zebra
Finch song bouts consist of a single
motif repeated over and over with the
same syllables in the same order—or atIndigo Bunting SongTime (seconds) 0.3 0.6 0.9Time (seconds) 0.01 0.02 0.03Fine Structure
A closer look at the Indigo Bunting’s song reveals finer details that the bird may be
listening to rather than the paired syllables. Here we see the sound waveform of the
beginning of the song depicted in the first infographic ( top ). Zooming in on the high-
lighted portion of the second syllable reveals the rapid fluctuations in frequency and
amplitude that occur at the millisecond time scale within a song syllable (bottom).