AbstractThere are many studies on how songbirds develop song production, but few on how songbirds develop appropriate responses to conspecific vocalizations. The black-capped chickadee, Parus atricapillus, produces a vocalization, the 'squawk', that stimulates gaping in nestlings during feeding. To determine whether nestlings gape selectively at the squawk, playbacks of several conspecific vocalizations plus a heterospecific vocalization were presented to nestlings within natural nests. A preference 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 develop as the gaping stimulus, newly-hatched nestlings were reinforced with food for gaping either at the squawk or the faint feebee, the two most common adult vocalizations at the nesting site. Regardless of reinforcement, 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 the expanding range of auditory sensitivity that occurs at the same age during passerine development. Thus, while field observations show that nestlings gape mostly to the squawk relative to other parental vocalizations, experimental evidence indicates that there is not an exclusive link between the signal (squawk) and its response (gaping), especially 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 discussed include matching acoustic structure to nestling perceptual biases and the behavior