Contact calls are ubiquitous among birds and are important in mediating social interactions. Despite this, the structure and function of these vocalizations have received relatively little attention, and the ontogeny of all bird calls is poorly known. In co-operatively breeding Long-tailed Tits Aegithalos caudatus, helpers preferentially provide care at the nests of close relatives and contact calls act as cues for kin recognition. However, it is not known whether these calls act as individual-specific or family-specific cues, or if they differ between the sexes. Furthermore, the precise nature and timing of the development of these calls is not known. Here we use spectrographic cross-correlation to show that two different contact calls in the Long-tailed Tit exhibit some degree of family-specificity but no sex-specificity, and the implications for kin recognition are discussed. We also provide evidence that both call types develop in the nest, and we give one of the first descriptions of nestling vocalizations from hatching through to fledging in a passerine species.