Island-mainland song divergence is decoupled from genetic and geographic distance in silvereyes.

Population connectivity shapes both genetic and cultural variation, including in learned vocal behaviors such as birdsong. Silvereyes (Zosterops lateralis) are well-known island colonizers, providing a useful model for examining how cultural evolution unfolds across populations with differing geographic isolation. We compared song dialects from 4 populations in South East Queensland: 2 mainland (Z. l. cornwalli) and 2 island populations (Z. l. chlorocephalus), for which patterns of genetic connectivity had previously been described. From field recordings, we constructed syllable repertoires and measured spectral features to quantify dialect structure and acoustic divergence. We found no correlation between acoustic distance and either genetic or geographic distance among populations, indicating that dialect divergence is not strongly constrained by neutral genetic structure or spatial separation at this scale. Island populations showed greater similarity to one another, as well as slightly higher syllable diversity, than mainland populations. These patterns are consistent with cultural divergence shaped by local, social, or ecological conditions rather than by geographic or genetic distance. Our findings refine current understanding of island dialect evolution in silvereyes by showing that cultural differentiation can occur despite limited genetic divergence and relatively small geographic distances. Because song is socially transmitted, cultural traits may respond rapidly to environmental change, suggesting that conservation strategies for island passerines may benefit from integrating the cultural dimensions of avian behavior.