The focus of acoustic communication studies differs between birds and primates due to differences between the avian and primate social structures. Contact calls exist in almost all species independent of social structure, indicating their importance for animals. Contact calls contain various types of information about the sender. They function to keep groups cohesive or to help reunite visually separated individuals. In this review, we first describe the type of information contained in the acoustic structure of contact calls, and then we consider the function of contact calls in animals' social lives, particularly their function as identity advertisements.

Recognizing other individuals by integrating different sensory modalities is a crucial ability of social animals, including humans. Although cross-modal individual recognition has been demonstrated in mammals, the extent of its use by birds remains unknown. Herein, we report the first evidence of cross-modal recognition of group members by a highly social bird, the large-billed crow (Corvus macrorhynchos). A cross-modal expectancy violation paradigm was used to test whether crows were sensitive to identity congruence between visual presentation of a group member and the subsequent playback of a contact call. Crows looked more rapidly and for a longer duration when the visual and auditory stimuli were incongruent than when congruent. Moreover, these responses were not observed with non-group member stimuli. These results indicate that crows spontaneously associate visual and auditory information of group members but not of non-group members, which is a demonstration of cross-modal audiovisual recognition of group members in birds.

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In long-range fission-fusion complex societies, individuals are often recognized by audiovocal signals because of long-range propagation. The signature voice system is a well-known mechanism involving both acoustic individuality of a certain call type and discrimination ability. Previous studies on vocal individual recognition of birds have emphasized its involvement in breeding contexts such as mate, parent and offspring, and territorial-neighbour recognition. However, there has been less focus on the recognition of non-breeding flock members despite the socio-ecological demand of such ability in the complex social lives of highly social birds including corvids. Here we report a signature voice system in jungle crows by showing both acoustic individuality of contact calls and discrimination ability. We first performed a discriminant functional analysis on contact ka calls of five crows to examine their discriminatory potential and demonstrate inter-individual distinctions. We next used an operant conditioning to verify the perceptual ability to discriminate non-breeding familiar conspecifics based on ka calls. Four of the five crows successfully transferred discrimination of individual calls to the novel ka calls. Our results provide the first evidence of a signature voice system as a perceptual mechanism for individual recognition of familiar individuals in non-breeding flocks of a highly social crow.