Astrocyte complexity and cell density in HVC shape repertoire size and song innovation in a tropical songbird

Brain characteristics play an important role for vocal learning in birds. Although some characteristics, such as cell density, have always been considered important for song memorization, other traits, such as astrocyte morphology, have been neglected or poorly studied. We studied the relationship between cell density in the High Vocal Centre (HVC), astrocyte morphology (intersections and length), and vocal learning capabilities in a tropical songbird. Individual birds were recorded, and different brain characteristics were analysed using whole-brain imaging, cellular density analysis, and confocal microscopy. Our findings suggest that song innovation relies on both a higher HVC cell density and more complex astrocytic structures, likely reflecting enhanced song complexity and innovation. In contrast, repertoire size appears to be primarily constrained by astrocyte morphology, emphasizing its role in memory and motor control. This study highlights the neural trade-offs required for song complexity, linking vocal behaviours to developmental conditions and sexual selection. By elucidating the interplay between astrocyte architecture and birdsong in tropical songbirds, we provide insights into the evolution of vocal learning and innovation. These results underscore the importance of astrocytes in shaping behavioural complexity and offer a framework for understanding the neural basis of acoustic communication.