Acoustic similarity to parental calls promotes response to unfamiliar calls in zebra finch fledglings

Individual recognition, using acoustic, visual or olfactory individual signatures, is crucial for the coordination of social interactions and its occurrence has been demonstrated experimentally in many taxa. In this context, mistaking one individual for another is expected to be costly, for example through misdirected parental care or social punishment. To minimize the occurrence of such false responses, individual signatures should be distinct and selection should act on receivers to perceive these differences. However, it is largely unknown how precise signal perception is and whether similarity between individual signatures influences the occurrence of false responses. We used acoustic parent-offspring recognition in zebra finches, Taeniopygia guttata, to test how acoustic similarity between individuals affects a chick's response behaviour. In a playback experiment, fledglings were presented with calls of parents as well as calls of nonparents that varied systematically in their similarity to the parents' calls. The acoustic similarity between calls of parents and unrelated adults partly explained the response pattern of fledglings. Offspring preferentially responded to adult calls that were similar to their parents' call. The response pattern further seemed to incorporate a baseline responsiveness to conspecifics since most fledglings responded to very dissimilar adults. These findings demonstrate that the strength of response is related to signal similarity, which is potentially an important underlying mechanism shaping distinctiveness in signal design.