FEATURE ARTICLE
Relative Pitch and the Song of Black-Capped Chickadees
Chickadees, like people, have a strong sense of relative pitch. These birds use skillful, precise pitch changes to advertise their quality and attract mates
Ron Weisman, Laurene Ratcliffe
Music and Relative Pitch
Relative pitch is the ability to produce or recognize a note based
on a reference note. The ability to hum a simple tune when given a
set note with which to start, as most people can, shows relative
pitch in action. A sense of relative pitch is common among
non-musicians even if they never use the term, but musical training
develops this knack. Trained singers have an advanced appreciation
of pitch change: They can produce precise relative frequency changes
from one note to the next in a melody.
To compare the appreciation for relative pitch among humans and
songbirds, it is necessary to understand how musicians and
bioacousticians measure pitch change. Musicians describe the change
between any two notes on the musical scale as an interval.
The interval between any two adjacent notes is defined as a
semitone. The ratio between the frequencies (measured in
cycles per second, or hertz [Hz]) of the higher and lower of two
notes separated by a semitone is always 1.059, regardless of where
the two notes are located on the scale. Musicians speak of semitones
and intervals; bioacousticians use the terms frequencies and
frequency ratios.
Pianos and keyboards easily illustrate the discrete intervals
between notes. For example, hitting the E note in the fourth octave
(E4, 330 Hz), then counting down four keys (where both black and
white keys are counted) and hitting the C note in the same octave
(C4, 262 Hz) produces a relative pitch change of four semitones, as
measured by musicians, and a frequency ratio of 1.26, as measured by
bioacousticians. One can easily transpose a four-semitone pitch
change on a piano. For example, moving up the keyboard and hitting
the E note in the fifth octave (E5, 659 Hz) and the C note in the
same octave (C5, 523 Hz) produces the same relative pitch change as
in the fourth octave: four semitones or a frequency ratio of 1.26.
Although they use different notations, musicians and bioacousticians
agree on the constancy of pitch change across the keys. If one
measured the frequencies produced by hitting E4 and then C4 on each
of 20 pianos, the average ratio should be 1.26 with a very small
standard deviation among the pianos. The small standard deviation
helps illustrate that pitch changes in music are discrete.
Keyed instruments, such as pianos, are constrained to produce
discrete pitches, whereas keyless instruments, such as violins and
human voices, can produce pitches that vary continuously. In
practice, however, trained violinists and singers produce discrete
pitch changes analogous to a piano's keys. Put another way, although
the human voice can vary in pitch continuously from one frequency to
another, that is not the way humans use their voices or their
musical instruments. People produce pitch changes with low
variability about their frequency ratios.
Humans recognize pitch changes in discrete intervals and are good at
identifying notes relative to an external reference pitch—in
other words, relative pitch. Most people easily recognize a melody
transposed to a different key, or starting frequency, because the
song maintains the same intervals, or frequency ratios, among the
notes. Trained musicians, of course, are better than others at
identifying intervals and transposed melodies, but practically
everyone has some sense of relative pitch in simple melodies. For
example, people instantly recognize the song "Happy
Birthday" regardless of the key in which it's sung.
Human musicians have not artificially adopted common scales to
coordinate their musical performances; people, whether musicians or
not, produce and perceive discrete, constant frequency ratios across
different renditions of the same melody. In comparing relative pitch
perceptions across species, we use low variability about mean
frequency ratios in the same song across singers to provide evidence
that songbirds, like people, have a strong sense of relative pitch.
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