about two weeks later (Brockway 1964; Wyndham 1980b). Individual birds
have a repertoire of contact calls, but use one or two most of the time
(Farabaugh et al. 1994). Flock mates share call types (Farabaugh et al. 1994),
resulting in flock-specific repertoires of contact calls. Budgerigars can
discriminate between very similar contact calls given by their flock mates,
suggesting that calls can be used to identify individuals within flocks. This
perceptual discrimination appears to be learned because birds can only
make these finely detailed discriminations when they are familiar with the
individuals from whom the calls are recorded (Brown et al. 1988).
During normal vocal development, juveniles increase the extent and
complexity of FM, decrease peak frequency, decrease bandwidth, and
increase duration (Brittan-Powell et al. 1997). Contact calls appear to
develop from a call given late in the nesting phase, the food-begging
call. Broods show few differences and sexes develop similarly early on.
However, evidence of call sharing emerges within a month of fledging
(Brittan-Powell et al. 1997).
Direct evidence demonstrated that contact calls are learned and that both
acquisition and social modification occur. Manipulations of auditory experience demonstrated its fundamental importance. Birds deafened at birth
developed abnormal calls (Dooling et al. 1987; Heaton and Brauth 1999).
Contact calls of deafened birds retained some features of the species’ calls,
including maximum and minimum frequency, bandwidth, and duration,
but differed in tonality, number of tonal elements, and extent of frequency
modulation (Dooling et al. 1987). Deafened birds produced normal foodbegging calls but abnormal contact calls (Heaton and Brauth 1999), so vocal
learning was confined to calls whose function was enhanced by similarity
to social partners. Hand-reared birds gave contact calls that were simpler
than typical budgerigar calls and that differed in bandwidth and frequency
modulation (Brittan-Powell et al. 1997). Hand-reared birds had more
acoustic experience than deafened birds, and their calls resembled typical
budgerigar contact calls far more than those of the deafened birds. Contact
calls and the song of deafened adults became abnormal within 6 months,
indicating that auditory input is required to maintain normal call structure,
and demonstrating the absence of a critical period. These studies showed
that acoustic experience was essential for normal vocal development in
budgerigars and revealed a continuum of effect.
Social experience had a powerful influence on which call types were
acquired and how existing calls were modified. Learned acquisition and
social modification occurred throughout development and into adulthood.
Young birds learned from adults with which they were housed even
when those adults produced abnormal contact calls or were heterospecific
(Brittan-Powell et al. 1997). Abnormal features included bandwidth, degree
of frequency modulation, and duration. Juveniles housed with zebra finches
gave contact calls that resembled zebra finch calls but that showed less
harmonics and more frequency modulation than the heterospecific call.
4. Comparative Vocal Learning
173
have a repertoire of contact calls, but use one or two most of the time
(Farabaugh et al. 1994). Flock mates share call types (Farabaugh et al. 1994),
resulting in flock-specific repertoires of contact calls. Budgerigars can
discriminate between very similar contact calls given by their flock mates,
suggesting that calls can be used to identify individuals within flocks. This
perceptual discrimination appears to be learned because birds can only
make these finely detailed discriminations when they are familiar with the
individuals from whom the calls are recorded (Brown et al. 1988).
During normal vocal development, juveniles increase the extent and
complexity of FM, decrease peak frequency, decrease bandwidth, and
increase duration (Brittan-Powell et al. 1997). Contact calls appear to
develop from a call given late in the nesting phase, the food-begging
call. Broods show few differences and sexes develop similarly early on.
However, evidence of call sharing emerges within a month of fledging
(Brittan-Powell et al. 1997).
Direct evidence demonstrated that contact calls are learned and that both
acquisition and social modification occur. Manipulations of auditory experience demonstrated its fundamental importance. Birds deafened at birth
developed abnormal calls (Dooling et al. 1987; Heaton and Brauth 1999).
Contact calls of deafened birds retained some features of the species’ calls,
including maximum and minimum frequency, bandwidth, and duration,
but differed in tonality, number of tonal elements, and extent of frequency
modulation (Dooling et al. 1987). Deafened birds produced normal foodbegging calls but abnormal contact calls (Heaton and Brauth 1999), so vocal
learning was confined to calls whose function was enhanced by similarity
to social partners. Hand-reared birds gave contact calls that were simpler
than typical budgerigar calls and that differed in bandwidth and frequency
modulation (Brittan-Powell et al. 1997). Hand-reared birds had more
acoustic experience than deafened birds, and their calls resembled typical
budgerigar contact calls far more than those of the deafened birds. Contact
calls and the song of deafened adults became abnormal within 6 months,
indicating that auditory input is required to maintain normal call structure,
and demonstrating the absence of a critical period. These studies showed
that acoustic experience was essential for normal vocal development in
budgerigars and revealed a continuum of effect.
Social experience had a powerful influence on which call types were
acquired and how existing calls were modified. Learned acquisition and
social modification occurred throughout development and into adulthood.
Young birds learned from adults with which they were housed even
when those adults produced abnormal contact calls or were heterospecific
(Brittan-Powell et al. 1997). Abnormal features included bandwidth, degree
of frequency modulation, and duration. Juveniles housed with zebra finches
gave contact calls that resembled zebra finch calls but that showed less
harmonics and more frequency modulation than the heterospecific call.
4. Comparative Vocal Learning
173