Song as a behavioural pre-mating barrier in early population divergence: Insights from the Canary Islands Chaffinch

Acoustic signaling is key to individual and species recognition, playing a major role in sexual and social communication. Since reproductive isolation is often maintained through pre-mating mechanisms, song can be an early isolating trait leading to assortative mating, promoting reproductive divergence, and potentially contributing to speciation. However, whether song differences alone are sufficient to prevent interbreeding or if other traits also contribute, remains a matter of debate. Playback experiments provide a more direct way to test the role of song as a reproductive barrier. Here, we use playback experiments to test the hypothesis that song acts as a pre-mating barrier in two recently diverged populations of an island passerine, the Canary Islands Chaffinch ( Fringilla canariensis palmae ), which inhabit ecologically distinct laurel and pine forests within the island of La Palma. Assuming that male song has diverged in the two habitats, we tested if territorial males from a given habitat responded differently to songs from intruding males from their own habitat or from the other habitat type, using a closely related mainland species as a control. We found that probability of response was weaker to songs of the closely related species and to the different-habitat birds than to songs of the same-habitat birds, but differences for the latter were weak. The intensity of response followed the same pattern. Overall, song divergence between laurel and pine forest chaffinches does not appear strong enough to cause clear behavioural discrimination against individuals from the alternative habitat. Other factors such as morphological and ecological divergence associated with adaptation to local resources might better explain population differentiation. However, testing female responses will be essential to determine whether songs convey lineage-specific information that may elicit assortative mating.