data (Clemmons and Howitz 1990) and one experiment (Hughes et al. 1998)
support the hypothesis that vocal learning is involved in the acquisition
of normal chick-a-dee calls. Clemmons and Howitz (1990) described the
normal process of call development and found that approximate renditions
of adult-like chick-a-dee calls are first given by fledglings at 28 days. These
calls were highly variable, especially in the introductory A, B, and C notes,
and did not acquire fully adult-like characteristics for another week. Acquisition of normal chick-a-dee calls required auditory and social experience
(Hughes et al. 1998). The A note developed normally in isolated individuals, but birds who did not hear normal adult vocalizations produced abnormal B and C notes. Hughes et al. (1998) did not study development of the
170
J.W. Boughman and C.F. Moss
A)
B)
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Figure 4.4. Chickadee calls. (A) Sonogram showing the four note types, marked
A, B, C, D. The study focused on convergence among group mates in D notes. (B)
Changes in frequency structure of D notes during the three-week experiment.
Means are indicated by a horizontal line, SD by the vertical bars, and sample size
by the numbers above bars. (From Nowicki 1989. Reprinted with permission from
Academic Press.)