The Origins of Music: Preface - Preface

rate copy of the memorized template.This process requires hearing

oneself sing;birds are unable to reproduce memorized songs if they are

deafened after memorization but before the practice period (Konishi

1965).

Sensitive Periods for Song Learning

In many species,referred to as closed-ended learners,song memoriza-

tion occurs during a restricted period of development,often within the

first few months after hatching.This sensitive period has been demon-

strated in the laboratory by presenting young birds with a series of

taperecorded songs drawn from the dialects of their own species.By

replacing each song with a new variant after a limited period of time (on

the order of ten days or two weeks),it is possible,with time and patience,

to determine the age at which memorization occurred by matching the

bird’s adult song to the library of songs presented (Marler and Peters

1987;Nelson,Marler,and Palleroni 1995).In other species,referred to

as open-ended learners,song learning reoccurs each year.For example,

the repertoires of starlings (Sturnus vulgaris;Eens,Pinxten,and Ver-

heyen 1992) and canaries (Serinus canaria;Nottebohm and Nottebohm

1978) increase or change from year to year.

Variation across species with respect to when songs are learned raises

several interesting questions:what determines the length of the sensitive

period for song learning? why do some species learn songs during a short

period in development whereas others continue to expand their reper-

toires in adulthood? do changes take place in the brains of closed-ended

learners to prevent further learning?

The length of the sensitive period appears to be regulated by both

external and internal factors.Young birds deprived of an opportunity to

hear song will memorize at a later age than those that were tutored as

fledglings,indicating delayed closure of the sensitive period (Kroodsma

and Pickert 1980;Slater,Jones,and TenCate 1993).However,closure

cannot be delayed indefinitely.Birds prevented from hearing the songs

of their own species throughout this period will produce abnormal songs,

as shown in figure 5.1.Once the bird begins to practice an abnormal song,

subsequent tutoring will not improve it,indicating that irreversible

changes have occurred in the brain.

The Neurobiology of Sensitive Periods

If a bird is able to learn to sing at one age but not another,we must con-

clude that some change occurs in the brain between these two time

67 The Neural Basis of Song Learning in Birds

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