May 2022, ScientificAmerican.com 39means that, despite its salience to us, the
pattern of paired notes is not significant
for the birds in terms of recognizing fel-
low species members. If the Indigo Bun-
ting were to write a field guide descrip-
tion of its own song, it would differ con-
siderably from our assessment.
Testing how birds perceive song in
the wild is important, but it has its lim-
its. A bird could be out of earshot look-
ing for food when you want to start your
experiment, for instance. In the labora-
tory, researchers can test hearing in
birds with more precision and control.
When you go to the doctor’s office and
have your hearing checked, you are
instructed to raise your hand or push a
button to indicate that you’ve heard a
sound. Researchers use a similar ap -
proach to probe auditory perception in
birds. Because we can’t explicitly ask
the birds, “Did you hear that?,” we train
them to peck a button on the side of
their cage if they detect a sound or if the
sound they hear fits into a particular
category or differs from another sound.
Lab studies have found many simi-
larities in auditory sensitivities be -
tween songbirds and humans, includ-
ing the thresholds for hearing differ-
ences in pitch or detecting gaps between
sounds. But they have also re vealed sur-
prising differences between the abilities
of birds and humans to hear sequences
of sounds and acoustic details.
One pivotal finding from such work
is that birds perform surprisingly
poorly on recognizing a melody shifted
up or down in pitch. This is something
humans do naturally: we still recognize
the tune of “Happy Birthday to You” if
it is played in higher or lower registers
on a piano. Classic lab experiments in the 1980s and 1990s by
Stewart H. Hulse and his colleagues at Johns Hopkins University
showed that for birds, when the pitch of a sequence changes, the
tune sounds different, even though the underlying pattern remains
the same. Thus, the melodies we hear when we listen to birdsong
may be very different from the birds’ perceptual experiences.
Subsequent studies have bolstered that hypothesis. In 2016 a
team led by Micah Bregman, then at the University of California,
San Diego, reported that European Starlings can recognize trans-
posed sequences but only when all the fine details in the sounds
are removed. That work highlights the importance of those fine
details to birds when listening to song.AN EAR FOR DETAIL
you can Break a sound waveform down into two levels of descrip-
tion: envelope and fine structure. The envelope is made up of slow
fluctuations in the amplitude of the waveform, whereas the finestructure consists of the rapid fluctuations in frequency and
amplitude within the waveform. In other words, a sound’s fine
structure is how it changes at the millisecond level. Historically,
many birdsong researchers overlooked fine structure, in part
because it is not readily visible in sonograms or spectrograms,
which have been useful in helping people visualize song. But
zooming in on the waveform of an individual song syllable can
reveal these fine acoustic details.
Robert Dooling of the University of Maryland helped to pioneer
the study of fine structure in birdsong. For decades he and his col-
leagues have been working to assess birds’ ability to detect it. In a
pivotal study published in 2002, they tested birds and humans on
distinguishing sounds that differed only in fine structure. All the
bird species they tested—Zebra Finches, Domestic Canaries and
Budgerigars—performed much better than the humans did. The
birds were able to hear differences in fine structure two to three
times smaller than those the human subjects could detect. TheSound WaveformSpectrogramIndigo Bunting SongTime (seconds) 0.5 1.0 1.5Paired syllables Paired syllables Paired syllablesAmplitude:
Small (quiet) Large (loud)Frequency (pitch)HighLowAdam Fishbein (
waveform and spectrogram
)
Illustration by Liz Wahid (Indigo Bunting)Paired Syllables
A sound waveform ( top ) and a spectrogram ( below )
show the Indigo Bunting’s song. The spectrogram
depicts time on the x-axis, the frequency or “pitch”
of the sound on the y-axis, and the amplitude or “loud-
ness” of the signal in the redness of the lines (the redder
they are, the greater the amplitude). The brackets mark paired
syllables in the song. To human ears, the paired syllables are defin-
ing characteristics of the bunting’s song. But the birds respond simi-
larly to modified versions of their song in which the syllables are
unpaired, which suggests that they are focusing on different fea-
tures of the song than humans are.Indigo
Bunting