DEVELOPMENT OF A SELECTIVE RESPONSE TO AN ADULT VOCALIZATION IN NESTLING BLACK-CAPPED CHICKADEES

There are many studies on how songbirds develop song production, but f ew on how songbirds develop appropriate responses to conspecific vocal izations. The black-capped chickadee, Parus atricapillus, produces a v ocalizations, the 'squawk', that stimulates gaping in nestlings during feeding. To determine whether nestlings gape selectively at the squaw k, playbacks of several conspecific vocalizations plus a heterospecifi c vocalization were presented to nestlings within natural nests. A pre ference for the squawk did not appear until day 2-3 and then steadily increased, until by day 11-13, nestlings gaped only at the squawk. To determine whether there are constraints on which vocalization can deve lop as the gaping stimulus, newly-hatched nestlings were reinforced wi th food for gaping either at the squawk or the faint feebee, the two m ost common adult vocalizations at the nesting site. Regardless of rein forcement, nestlings gaped most frequently at the squawk, In addition, after the first few days posthatch, nestlings became as responsive to a third, unreinforced, heterospecific vocalization as to the squawk. The responsiveness to the heterospecific vocalization coincided with t he expanding range of auditory sensitivity that occurs at the same age during passerine development. Thus, while field observations show tha t nestlings gape mostly to the squawk relative to other. parental voca lizations, experimental evidence indicates that there is not an exclus ive link between the signal (squawk) and its response (gaping), especi ally during the first week posthatch when parents use the signal most frequently. Rather, an effectively selective response may be achieved redundantly by a variety of factors. Possible factors that are discuss ed include matching acoustic structure to nestling perceptual biases a nd, the behavior of the parents, such as through the selective use of the squawk, use of structural contrast among signals closely associate d with feeding, and controlling distance from and orientation toward t he nestlings.