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
Song Perceptions Depend on Sex
Humans may listen to birdsongs for solace and entertainment, but
among songbirds, including black-capped chickadees, males listen to
one another to identify potential rivals for territory, and females
listen to find potential mates who possess territories. During the
breeding season, male chickadees defend territories of, on average,
about 5 hectares from rival males and attract females with whom they
reproduce and rear young. If a rival male sings in another's
territory, the male already established there drives out the
intruder by singing vigorously, approaching the trespasser and, if
the rival persists, physically attacking him.
For more than four decades, to discover how songbirds communicate,
biologists have conducted experiments playing back songs. Recordings
can simulate the intrusion of a rival male on or near another's
established territory. Skillfully recorded and reproduced songs of
the species elicit forceful territorial defense, whereas playing
back songs with altered acoustic features may provoke a weaker
territorial response, depending on the importance of the altered
features to the potency of the song.
We wondered whether accuracy in relative pitch is important to the
potency of chickadee songs. In several studies, we played back
normal fee-bee songs and a variety of fee-bee songs with altered
frequency ratios to male chickadees on their territories. One of us
(Ratcliffe), along with Scott MacDougall-Shackleton and Dan Weary,
now at the University of British Columbia, looked at the roles of
both the glissando and the internote frequency ratio in eliciting
territorial defense. Territorial male chickadees in their native
environment heard a normal song and altered songs lacking the
glissando element. One altered song included a fee note flattened to
the start frequency of a normal fee note, which increased the
internote ratio relative to the bee note. A second altered song
presented a fee note flattened to the end frequency of a normal fee
note, which therefore left the fee-bee internote ratio unchanged
compared with a normal song. Male chickadees flew closer to the
speaker on more occasions and called more often to a normal song
than to the playback of either altered song. The two altered songs
provoked about equal territorial defense. The first altered song had
abnormal glissando and abnormal internote frequency ratios, yet the
male chickadees responded no less to it than to the second altered
song, which had only an abnormal glissando frequency ratio.
Our results puzzled us. We could not understand why male chickadees
produce accurate discrete pitch changes in their songs only to
ignore the vastly inaccurate internote pitch changes they heard in
playback experiments. We were skeptical of this conclusion in part
because other songbirds—in particular male white-throated
sparrows and veeries—are known to produce precise, predictable
pitch changes in their songs, and playback experiments that altered
these ratios reduced territorial defense in both species. We
continued to seek reduced territorial responses in black-capped
chickadees to songs with altered internote frequency ratios. In one
series of experiments, we digitally copied and spliced together fee
and bee notes from several chickadees to form songs with normal
internote ratios and songs with ratios as disparate from normal as
1.00 (equivalent notes) and 1.29 (between four and five semitones
apart). Judging by several behavioral measures, male chickadees
persisted in defending their territories equally strongly against
all of these songs, whether the internote ratio was normal or
altered. Taken together, these field experiments have led us,
finally, to conclude that the glissando frequency ratio is far more
important to territorial defense by male black-capped chickadees
than is the internote ratio.
But what about female responses? Among most songbirds, chickadees
included, females respond weakly or not at all to simulated
territorial intrusions. But female choice is a powerful evolutionary
force, and we decided to address whether female chickadees care
about the pitch changes sung by potential mates. One of us
(Ratcliffe) and Ken Otter, now at the University of Northern British
Columbia, brought female black-capped chickadees into breeding
condition in the laboratory by artificially extending their day
length and administering the hormone estradiol. When female
songbirds are in breeding condition, the songs of males from their
species elicit a distinctive copulation-solicitation display. We
played the receptive females two normal songs, a song with an
increased internote ratio and a song with an altered glissando.
Female chickadees displayed longer to both normal songs than to the
song with an altered internote ratio, and they displayed more often
to the normal songs than to the one with an altered glissando. In
other words, the females chickadees appeared to discriminate
sexually in favor of normal songs over songs with either glissandos
or internote ratios altered. The surprising implication of our
playback experiments is that although males need to produce a
precise internote pitch change to attract and stimulate females, the
males themselves seem not to care at all about whether their
territorial rivals produce wildly inaccurate internote ratios.
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