Individuality in the contact calls of cooperatively breeding long-tailed tits (Aegithalos caudatus)

The ability to discriminate between individuals or groups of individuals is important for the evolution of sociality. Individual vocal recognition is thought to be widespread in social birds, yet few studies have investigated its role in cooperatively breeding species. In long-tailed tits, helpers preferentially provide care to close kin, and individuals are able to discriminate between the vocalisations of kin and non-kin. However, the mechanism underlying this recognition system is unknown. Here we quantify the relative variation between and within individuals in three of the contact calls used by this species. Spectrographic cross-correlation revealed that two of these calls, the 'churr' and the 'triple', were individually distinct. We therefore analysed the variation in a series of acoustic parameters in each of these two vocalisations. For both the churr and the triple, discriminant function analysis was able to allocate calls to the correct individuals according to variation in several frequency parameters. We hypothesise that long-tailed tits are able to discriminate between the calls of conspecifics based on these parameters. This is the first quantitative description of potential recognition cues in a cooperatively breeding bird in which vocal discrimination is known to occur.