again showing the importance of acoustic input. Too little work has been
done to directly test the auditory template model; however, these data
suggest that sensorimotor processes are important in budgerigars, chickadees, and hummingbirds. There is no evidence of a delay between memorization and production of calls in budgerigars or chickadees as has been
found for song learning in many songbirds.
5.2.4.2. The Process of Call Matching for Birds:
Delayed Critical Period or Selective Attrition?
Budgerigars (Farabaugh et al. 1994; Brittan-Powell et al. 1997), chickadees
(Mammen and Nowicki 1981; Nowicki 1989), and Australian magpies
(Table 4.1) retain the ability to match contact calls of social partners past
fledging and into adulthood. This could result from a delayed closure to the
critical period for call memorization, the absence of a critical period, or
because of selective attrition of previously memorized calls. Taxa for which
acoustic isolation experiments have been done supported extended critical
periods.
When acoustically isolated budgerigars were subsequently housed with
individuals who gave normal calls, they modified their calls to closely resemble species-specific calls (Brittan-Powell et al. 1997). An extended critical
period is more likely to explain this result because juveniles neither heard
nor produced normal calls early in development. Adult birds also changed
calls to match group mates’ calls (Farabaugh et al. 1994), again suggesting
the absence of a critical period, although in adults this pattern could result
from selective attrition. In budgerigars, group signatures clearly rely on
social interaction.
Critical periods differ for each note in the chick-a-dee call. A notes
develop normally in isolated birds, as do B notes, but Hughes et al. (1998)
suggest that selective attrition may be involved in fine-tuning A note structure. A critical period for C notes is implied by the finding that juvenile
chickadees exposed to adult chick-a-dee calls only after the age of 38 days
developed C notes as abnormal as total isolates (Hughes et al. 1998). Vocal
plasticity of D notes is retained into adulthood (Nowicki 1989), and selective attrition may be involved in D note convergence within flocks (Hughes
et al. 1998). However, there is no direct test of the selective attrition hypothesis for group-signature development.
Hand-raised juvenile Anna’s hummingbirds that were housed together
from 64 days to adulthood sang remarkably similar, although abnormal,
song (Baptista and Schuchmann 1990), suggesting that vocal plasticity was
retained to 64 days or beyond. A single isolate showed no evidence of learning from an adult with which it was housed after one year, suggesting that
this species does have a critical period that ended before this time. The data
from this one study are somewhat contradictory with regard to critical
period, and strong conclusions are not possible with such small sample
sizes.
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J.W. Boughman and C.F. Moss
done to directly test the auditory template model; however, these data
suggest that sensorimotor processes are important in budgerigars, chickadees, and hummingbirds. There is no evidence of a delay between memorization and production of calls in budgerigars or chickadees as has been
found for song learning in many songbirds.
5.2.4.2. The Process of Call Matching for Birds:
Delayed Critical Period or Selective Attrition?
Budgerigars (Farabaugh et al. 1994; Brittan-Powell et al. 1997), chickadees
(Mammen and Nowicki 1981; Nowicki 1989), and Australian magpies
(Table 4.1) retain the ability to match contact calls of social partners past
fledging and into adulthood. This could result from a delayed closure to the
critical period for call memorization, the absence of a critical period, or
because of selective attrition of previously memorized calls. Taxa for which
acoustic isolation experiments have been done supported extended critical
periods.
When acoustically isolated budgerigars were subsequently housed with
individuals who gave normal calls, they modified their calls to closely resemble species-specific calls (Brittan-Powell et al. 1997). An extended critical
period is more likely to explain this result because juveniles neither heard
nor produced normal calls early in development. Adult birds also changed
calls to match group mates’ calls (Farabaugh et al. 1994), again suggesting
the absence of a critical period, although in adults this pattern could result
from selective attrition. In budgerigars, group signatures clearly rely on
social interaction.
Critical periods differ for each note in the chick-a-dee call. A notes
develop normally in isolated birds, as do B notes, but Hughes et al. (1998)
suggest that selective attrition may be involved in fine-tuning A note structure. A critical period for C notes is implied by the finding that juvenile
chickadees exposed to adult chick-a-dee calls only after the age of 38 days
developed C notes as abnormal as total isolates (Hughes et al. 1998). Vocal
plasticity of D notes is retained into adulthood (Nowicki 1989), and selective attrition may be involved in D note convergence within flocks (Hughes
et al. 1998). However, there is no direct test of the selective attrition hypothesis for group-signature development.
Hand-raised juvenile Anna’s hummingbirds that were housed together
from 64 days to adulthood sang remarkably similar, although abnormal,
song (Baptista and Schuchmann 1990), suggesting that vocal plasticity was
retained to 64 days or beyond. A single isolate showed no evidence of learning from an adult with which it was housed after one year, suggesting that
this species does have a critical period that ended before this time. The data
from this one study are somewhat contradictory with regard to critical
period, and strong conclusions are not possible with such small sample
sizes.
204
J.W. Boughman and C.F. Moss