Drought-Induced Beak Changes Make Finch Songs Unfamiliar
By Maja Mielke
Darwin’s finches no longer recognize the birdsong of conspecifics whose beak shape has evolved due to ecological adaptation. Researchers tested this by playing simulated calls of future finches to individuals in the wild. Their findings demonstrate how functional adaptations can lead to species divergence and eventually the evolution of new bird species.
The beaks of Darwin’s finches evolve rapidly when natural selection favors certain shapes that are best suited to the most abundant food source. For example, during particularly dry seasons, birds with large and strong beaks, capable of cracking hard and dry seeds, have a survival advantage. These birds are more likely to reproduce, meaning the next generation will have, on average, larger beaks than the previous generation.
However, having a larger beak doesn’t just affect what birds can eat, it also affects how they sing. Previous research has shown that finches with thicker, stronger beaks tend to sing at slower trill rates and with lower frequency ranges compared to finches with smaller, more delicate beaks. Since songbirds rely on their calls to recognize conspecifics and select mates, changes in song can cause individuals to no longer recognize others as members of the same species.
A real-world example of this is the Medium Ground Finch, one of Darwin’s finches from the Galápagos Islands. Although still considered a single species, there are large-beaked and small-beaked morphs within this species. These morphs sing differently and mostly mate within their own group, a process called assortative mating.
However, until now, it wasn’t clear how much change in beak shape and birdsong is necessary before one population of Medium Ground Finches would fail to recognize the song of another population entirely. Using knowledge about how droughts affect beaks and birdsong, researchers from the University of Massachusetts simulated the beak shapes and calls of hypothetical future finches after varying numbers of drought events. They then played the calls of these future birds to male individuals in the wild to measure their response behavior. The results of this study were recently published in Science.
The researchers observed that, despite changes in song characteristics of the calls being played, the finches still responded with defensive behavior to the simulated birdsong after three drought events. They frequently left their perch early to search for the ‘intruder’. But after six droughts, the birdsong had changed so much that the birds’ response behavior decreased significantly.
These results indicate that functional adaptations, like those driven by extreme climate events, can alter bird song enough to drive speciation, albeit in the later stages of species divergence.
References:
Jeffrey Podos, Katie M. Schroeder, Ecological speciation in Darwin’s finches: Ghosts of finches future. Science 386,211-217(2024).DOI: 10.1126/science.adj4478