THE STRUCTURAL SIGNIFICANCE OF A VOCALIZATION THAT STIMULATES GAPING IN BLACK-CAPPED CHICKADEES

In a variety of avian species, vocalizations used to stimulate gaping in nestlings during feeding share similar features such as broad frequ ency range, multiple spectral components, and sudden onset. It is curr ently not understood what functional significance, if any, these acous tic features have for close-range communication. One example of such a vocalization is the ''squawk'' of the black-capped chickadees (Parus atricapillus). The present study analyzes the significant features of the squawk by observing the response of chickadee nestlings to field p laybacks of natural and computer-modified squawks. Of the features tes ted, frequency range appears most critical, but responses to frequency ranges change with age, such that low frequencies are most stimulatin g around the time of hatching but cease to elicit gaping after 5 days posthatch. Aperiodic stimuli and computer-modified squawks with gradua l onsets tend to be less stimulating than natural squawks at some ages , but these differences are not significant at any age. The functional significance of the broad frequency range of the squawk is discussed, including the possible role of accommodating shifting frequency prefe rences related to ontogenetic changes in auditory sensitivity and elab oration of the vocal repertoire.