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
Black-Capped Chickadees
Songbirds belong to the class Aves, which includes all birds, the
order Passeriformes, consisting of perching birds, and the suborder
Oscines, the true songbirds. The true songbirds are distinguished
from other perching birds by the anatomy of their vocal organ, the
syrinx, and by the fact that they need to learn their songs
while juveniles from adult members of the same species. Oscines are
an evolutionary success, comprising about half of the more than
9,000 species of birds and filling niches in almost every
environment on the planet. Because songbirds are so diverse in
appearance and behavior, no single species can represent the whole
group. Our studies ask not whether all species of songbirds use
relative pitch to produce and perceive songs but whether at least
some Oscines do so in order to identify the songs of their own
species. We have concentrated our studies in particular on
forest-dwelling songbirds such as the black-capped chickadee,
Poecile atricapillus (formerly Parus
atricapillus), because the songs of forest birds contain pure
tones—that is, tones of narrow frequencies—which are
relatively easy to measure and reproduce in perception experiments,
both in the laboratory and in the field.
Black-capped chickadees are familiar and abundant year-round in the
northern two-thirds of the United States and in much of Canada. In
his 100-year-old field book of songbird music, Mathews included
black-capped chickadees. True to his description, these chickadees
are acrobatic, noisy and highly social birds. Their name derives
from their call, which sounds like "chick-a-dee."
This call is given by male and female, young and old, to keep in
touch with members of the flock. The chick-a-dee call is complex,
and some of the notes include "buzzy" harmonics. During
the breeding season and less frequently throughout the year, males
sing the "fee-bee" song, used to hold territory and to
attract and arouse females. We focus on the fee-bee song in this article.
Mathews observed that the male chickadee's fee-bee song consists of
two or three clearly whistled notes. The first note,
"fee," is sung at a higher pitch than the second one,
"bee." Sometimes, he noted, the bee note appears to be
sung as two separate notes at the same pitch. Mathews suggested that
chickadees separate the fee and bee notes in their songs by a
clearly perceptible pitch interval. Recall that musical intervals
are simply precise discrete frequency ratios. About 70 years later,
Steward Hulse of The Johns Hopkins University restated Mathews's
hypothesis in modern bioacoustic terms when he suggested that
chickadees might hold the ratio between the frequencies of their fee
and bee notes constant across birds.
To test Mathews's and Hulse's hypothesis, we, along with Ingrid
Johnsrude, now at Queen's University, and Andy Hurley, now at
the University of Lethbridge, conducted a large bioacoustic analysis
of several songs from each of 154 male black-capped chickadees
singing on their territories in eastern Ontario. We found that,
indeed, the internote ratio between the frequencies of an individual
chickadee's fee and bee notes varied by less than 2 percent about a
mean value of 1.134 (just slightly more than two semitones). Also,
the ratio between the fee and bee notes varied less than 2 percent
across our sample of chickadees. Our spectrograms showed that the
fee note is not constant in pitch but instead sweeps downward in
frequency in what musicians call a glissando. The frequency ratio of
the decline in the pitch from the start of the fee note until its
end was 1.056 (just less than a semitone) with approximately 1
percent variability among the renditions either by an individual
bird or among birds.
The spectrograms show that male black-capped chickadees' songs
contain not one but two relative pitch cues: the decline in
frequency from the start to the end of the fee note—the
glissando ratio—and the decline from the end of the fee note
to the beginning of the bee note—the internote ratio. Quite
unexpectedly, we found that individual males that were brought into
the laboratory, isolated and recorded briefly occasionally sang
their fee-bees on different starting pitches. Additional field
observations by Brad Hill, then a graduate student, and Ross Lein,
at the University of Calgary, confirmed these findings. With our
collaborators, Andy Horn and Marty Leonard, now both at Dalhousie
University, and Scott MacDougall-Shackleton, now at the University
of Western Ontario, we measured many more pitch-shifted songs from
individually tagged males during their extended and persistent dawn
chorus. The chickadees surprised us. In the first dawn recording
session, each bird sang his fee-bee song at a wide range of
frequencies, changing frequency on average every 40 songs. Over
subsequent sessions, individual males filled in the gaps between
frequencies within the range we heard.
These bioacoustical measurements taught us some astonishing facts
about song production in black-capped chickadees. Individual
chickadees do not use discrete keys to place the starting
frequencies of their song but instead vary their songs continuously
across the range of frequencies used by members of the species. In
other words, when chickadees change frequencies from the start to
the end of the fee note and from the end of the fee note to the
start of the bee note, their pitch changes are very precise, but
their starting pitch varies. In musical terms, black-capped
chickadees transpose the sequence of relative pitch changes typical
of the song of their species across a wide range of pitches. This
combination of musical abilities is rare in birds. Recordings of
other songbirds—in particular, white-throated sparrows
(Zonotrichia albicollis) and veeries (Catharus
fuscescens)—show that individual birds do not change the
starting frequencies of their songs from rendition to rendition.
Constant note frequencies give other members of the species a way to
identify individual birds. At the same time, a constant frequency
ratio in the song of the species helps identify the species.
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