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
More than 2,000 years ago, the acerbic philosopher Marcus Tullius
Cicero observed that Roman songbirds compose more excellent melodies
than any musician. He certainly doesn't stand alone in history on
that count; it is a nearly universal human experience to find joy
and wonder in birdsong—and to compare the songs to human
music. People have been transcribing melodies of birds into the
notation of music since at least the 18th century; Vivaldi's
Goldfinch concerto and Handel's Cuckoo and the
Nightingale organ concerto include musical notation for
birdsongs. In the early 1900s, the New England naturalist and
composer F. Schuyler Mathews presented the songs of many North
American birds in musical notation in his Field Book of Wild
Birds and Their Music in order to help readers identify
species common in the eastern United States.
Later biologists did not share Mathews's enthusiasm for musical
descriptions of birdsongs. Donald Borror, a master bioacoustician
and field biologist, found many of the song descriptions inadequate
by modern standards, as he wrote in his foreword to the 1967
reprinting of Mathews's book. Borror acknowledged that Mathews
lacked modern electronic equipment and that his primary interest was
in the musical content of birdsongs. Today, however, Mathews's
approach seems dated and quaint. Musical notation is simply unable
to provide the detailed, accurate and reproducible descriptions
required in modern bioacoustical analyses of vocal communication
among songbirds. Mathews's approach helps musically trained people
recognize birdsongs, but it fails to objectively describe birdsongs
and calls. For bioacousticians, accurate observations are a crucial
first step in analyzing the role of songs in the life of a species.
Modern ornithologists believe that songbirds first appeared some 40
million to 50 million years before human beings. That people derive
pleasure from birdsongs and recognize their musical features
suggests that despite the vast evolutionary gulf between birds and
mammals, songbirds and humans share some common auditory perceptual
abilities. In this article, we focus on relative pitch, the
ability to recognize relationships between acoustic frequencies. We
review studies suggesting that humans and songbirds share the
ability to perceive relative pitch changes and exploring the
evolutionary functions of birds' perception and use of relative pitch.
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.
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.
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.
The Role of Experience
Like human musicians, songbirds learn their songs; oscines do not
produce precise frequency ratios in their songs by some feat of gene
transcription. Most humans, of course, have some innate musical
ability and can sing and recognize at least some melodies, and
trained musicians clearly learn to perfect their relative-pitch
skills. Such observations led us to consider whether black-capped
chickadees learn to produce the precise, discrete pitch changes in
their fee-bee songs. We have reared male chickadees isolated from
adults on several occasions and have examined spectrographic records
from these birds as well as from isolated birds reared by Steve
Nowicki at Duke University, along with his students Melissa Hughes,
now at the College of Charleston, and Bernard Lohr, now at the
University of Maryland. Unlike chickadees reared normally, isolated
males sing quavering, low-volume notes and seldom produce consistent
pitch changes between notes.
Some songbirds can learn the song of their species from tape
recordings, but many species, including chickadees, learn best when
they interact with live adult males. Of more than a dozen male
black-capped chickadees reared with taped tutors by one of us
(Ratcliffe) with Scott MacDougall-Shackleton, and by Don Kroodsma
and his colleagues at the University of Massachusetts, only three
birds learned good approximations of taped fee-bee songs, and only
one bird shifted the pitch in his songs. The remaining birds sang
several whistled notes and sometimes changed pitch dramatically mid-note.
From these studies of isolated and tape-tutored males, we concluded
that young male chickadees acquire their precise control over the
production of relative pitches in their songs by listening to and
interacting with adult males who are themselves skilled singers.
This conclusion, in turn, led us to ask whether experience with
males and their songs is necessary for chickadees to develop
accurate relative-pitch perception. Isolated males sing only
aberrant notes, but, we wondered, is this simply an impairment in
relative-pitch production, or do the birds lack accurate
relative-pitch perception?
To address this question, one of us (Weisman) and Milan Njevovan,
now at the University of Alberta, housed both field-reared and
isolation-reared chickadees individually in sound-resistant chambers
and taught them to fly to a feeder to obtain food. Once a chickadee
was standing on a perch directly opposite the feeder, we played a
sequence of two pure-tone notes from a speaker beside the feeder. In
this discrimination task, flying to the feeder after some two-note
sequences was rewarded with food, whereas flying to the feeder after
other two-note sequences was not rewarded.
The logic of the experiment was that chickadees should be able to
discriminate rewarded note sequences faster when they can categorize
the rewarded note sequences by a common frequency ratio than when
they need to memorize a random collection of rewarded note sequences
one by one. Not surprisingly, when we played field-reared birds the
note sequences with randomly rewarded frequency ratios, they needed
twice as many sessions to acquire the discrimination as field-reared
birds that were in the constant-ratio group. Most important,
isolation-reared birds needed four times as many sessions as
field-reared birds to learn the constant-ratio discrimination. It
appears that field-reared male chickadees can perceive the constant
ratio in the rewarded note sequences, whereas isolation-reared males
are impaired in relative-pitch perception and instead need to
memorize the rewarded note sequences one by one. Extrapolating from
our results to the perception of fee-bee songs, we can conclude,
first, that free-living male chickadees perceive but seem to ignore
the constant internote ratio in rivals' songs during territorial
interactions and, second, that males reared in isolation have
difficulty in perceiving, as well as in producing, the internote
relative pitch change in fee-bee songs.
Evolutionary Benefits of Relative Pitch
We wondered about the possible evolutionary benefits of male
chickadees' ability to shift pitch and female chickadees' knack for
identifying pitch changes. In what follows, it may seem that we have
granted chickadees remarkable foresight and cognitive powers to
choose ways of behaving that will maximize their reproductive
success. In fact, we are only using a shorthand common among
biologists to describe the way selection has favored behaviors that
result in greater reproductive success.
Chickadees are only one of several songbirds that transpose their
songs over a wide range of frequencies. For species with a single
type of song, transposing a song to match a rival's pitch might be
an aggressive act, similar to singing the same song type as a rival
in a species with multiple song types. Over most of North America,
male black-capped chickadees sing a single song type, the fee-bee
song. One of us (Ratcliffe) in collaboration with Dan Mennill, now
at the University of Windsor, found that in duels over territory,
male chickadees indeed use pitch matching to escalate conflicts with
their rivals. Maintaining fixed frequency ratios in song might be
one way chickadees provide consistent cues to their identity across
song transpositions. But given that males do not seem to
discriminate particularly among songs with varying internote pitch
intervals, we concluded that the adaptive benefits of pitch matching
must lie elsewhere.
We turned to the dawn chorus, that critical time of day when, among
many songbirds, males advertise for mates, and females, even those
with partners, prospect for mating opportunities. (Mating outside
the pair bond is called extra-pair copulation.) Male chickadees
regularly shift their songs during dawn singing, when overt
territorial conflicts are rare. This suggests the motivation for
frequent pitch transpositions during dawn singing might be
communication with females.
This led Ratcliffe and former graduate students Peter Christie and
Dan Mennill to study singing during the dawn chorus in more detail.
We suspected that males might be shifting pitch to convey
information about their quality to females, information females
might value. Female chickadees prefer high-ranking males as social
mates, with rank referring to dominance position in the previous
winter's flocks. If females are relegated to a lower ranking social
mate, they choose high-ranking males as partners in extra-pair
copulations. Such matings account for the paternity of young in as
many as a third of their clutches, as revealed by genotype analysis
of the mother, the young and the social mate.
Male song can divulge the developmental conditions, genetic quality
and current vigor of the singer and is known to affect females'
choices in social mates and copulation partners. We measured the
social rank of chickadees during aggressive encounters at winter
feeders where high-quality males prevail, then compared these
dominance rankings with several fee-bee song features recorded
during dawn singing. The most important finding was that across
multiple pitch transpositions, high-ranking males maintain the
internote frequency ratio with much lower variability than do
lower-ranking males. In musical terms, when higher ranked males
transpose their songs upward in pitch during dawn singing, they hold
the interval—that is, keep the internote ratio
constant—whereas lower ranking males sing sharp—that is,
they reduce the frequency ratio.
Previously, we found that female chickadees appreciate the quality
of male songs in laboratory tests, but do female chickadees really
listen to song duels in the wild? Ratcliffe together with Peter
Boag, also of Queen's University, and Dan Mennill conducted a
striking test that showed females do indeed eavesdrop on their
mates' encounters with rivals. In 6-minute playback tests, we played
recordings of high-ranking males having simulated encounters with an
intruder. The mock-intruder either sang submissively, avoiding
matching the resident's pitch, or aggressively, pitch matching the
resident. The results showed that high-ranking males that lost in
the simulated encounter with an aggressive intruder also lost
paternity. Extra-pair paternity in their mates' clutches rose from
10 percent to 50 percent after submissive and aggressive playback
respectively. Evidently, the female social partners of the recorded
males engaged in more extra-pair copulations when they perceived a
change for the worse in the quality of their social mates. These
results suggest male chickadees have good reason to advertise their
quality by holding constant the internote frequency ratio in their
songs—female chickadees are listening carefully.
The Virtuosity of Chickadees
When we began studying songbirds almost twenty years ago, we chose
black-capped chickadees because they were abundant and sang simple
songs. Subsequently, our work has revealed that chickadees'
seemingly simple songs present precise, discrete relative pitch
cues. Males shift the starting pitch of their songs to match a
rival's song as a threat and to display their vocal virtuosity to
females during the dawn chorus. By preserving the relative-pitch
relations between notes during these shifts, males provide reliable
clues about their identity and quality.
Music has been part of the human experience since our inception as a
species. Like chickadees, young humans learn about relative pitch
from adult humans. Humans use precise, discrete pitch changes in
music and transpose melodies while keeping the intervals between the
notes constant. As among songbirds, humans tend to find musicians
sexually attractive. Under the circumstances, it seems remarkably
easy to relate to black-capped chickadees. It appears likely that at
least several, if not most or all, oscine species can perceive
relative pitch changes. As a resource, relative pitch may be ignored
by some songbirds and used by others, but it is certainly exploited
relentlessly by black-capped chickadees.
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