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
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.
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