Vocal complexity in a socially complex corvid: gradation, diversity and lack of common call repertoire in male rooks

Abstract

Vocal communication is widespread in animals, with vocal repertoires of varying complexity. The social complexity hypothesis predicts that species may need high vocal complexity to deal with complex social organization (e.g. have a variety of different interindividual relations). We quantified the vocal complexity of two geographically distant captive colonies of rooks, a corvid species with complex social organization and cognitive performances, but understudied vocal abilities. We quantified the diversity and gradation of their repertoire, as well as the inter-individual similarity at the vocal unit level. We found that males produced call units with lower diversity and gradation than females, while song units did not differ between sexes. Surprisingly, while females produced highly similar call repertoires, even between colonies, each individual male produced almost completely different call repertoires from any other individual. These findings question the way male rooks communicate with their social partners. We suggest that each male may actively seek to remain vocally distinct, which could be an asset in their frequently changing social environment. We conclude that inter-individual similarity, an understudied aspect of vocal repertoires, should also be considered as a measure of vocal complexity.

Can we trace the social affiliation of rooks (Corvus frugilegus) through their vocal signature?

“…Calls from different individuals from the same colony or nest cluster are not more acoustically similar than calls from individuals in different colonies or different nest clusters, and accordingly do not cluster together, as shown by the UMAP projection. An individual signature had already been detected in the most common “caw” calls of male rooks (Benti et al, 2019), although subsequent analysis showed that this could be due to a lack of common call types in the repertoires of male rooks (Martin et al, 2024). Females do however share common call types regardless of their origin, including the nest call.…”

“…The acoustic distance between calls was estimated using the Dynamic Frequency-Time Warping (DFTW) distance (see Martin et al, 2024 for details). This distance finds the optimal alignment between two spectrograms, first along the time axis, then along the frequency axis, and finally along the time axis again.…”

“…We used a data-driven approach to assess whether the nest call carries elements of individual, group and colonial signatures. Given prior evidence (Benti et al, 2019; Martin et al, 2024), we expected to find signatures at the individual level. Additionally, given that captive birds had lived together for several years, we predicted that birds of both captive colonies would exhibit a stronger colonial signature than the wild birds.…”