Variability and individuality in the contact calls of jackdaws (Corvus monedula).

In socially complex species, vocal signals often convey individual identity, enabling recognition and coordination of individualized groups. Jackdaws (Corvus monedula) are highly social corvids that form life-long monogamous pair bonds. They frequently produce 'tchak' contact calls while foraging in loose ground-based heterospecific flocks where maintaining cohesion without visual contact may be advantageous. We recorded tchak contact calls from nine adult males during natural foraging and analysed their acoustic structure using both conventional acoustic methods and machine learning approaches. Unsupervised classification revealed two structurally distinct call variants, a harmonic and a noisy form, which differed in spectral and temporal properties. While the harmonic variant occurred in eight out of the nine males, the noisy form was only recorded in five individuals. Both variants encoded individual identity, and spectrogram-based analyses yielded higher classification accuracy. These results suggest that tchak calls convey stable identity cues which facilitate individual recognition. The coexistence of two structurally and individually distinctive call forms within a single call type highlights the flexibility of jackdaw vocal communication.