COS 216-2 - Insights on the contribution of demography, plasticity and microevolution in changing the body shape and size of a highly aerial migratory bird over decades

Background/Question/Methods

One of the best-known responses to global warming is the change in size and shape of vertebrates. However, the contribution of demography, individual plasticity and microevolution in driving these changes remains little understood, although this knowledge is essential for a thorough understanding of the evolutionary responses of natural populations. Here, we investigated decades-long changes in size and shape of the highly aerial migratory Alpine swift (Tachymarptis melba). This bird flies without landing up to 6 months per year and thus its body size and shape are expected to be under strong stabilizing selection. Using a within individual centring approach, we studied effects of demography and individual plasticity on adult wing, tail, sternum and body mass in a Swiss population monitored for over 20 years. Using quantitative genetic models, we then explore heritability and genetic correlations between traits to better understand their evolvability and test for microevolution.

Results/Conclusions

Analyses at the population levels show an increase in adult wing and tail length, but not in body mass or sternum, over ~2 decades. By dissecting among and within individual effects, we show that changes in size observed at the population level were driven by changes in size of newly recruited individuals, especially in females. All four traits show moderate to strong heritability (h²=0.2–0.7), and we found evidence of genetic correlation only between wing and tail length (r=0.7) and between sternum and mass (r=0.4). These genetic correlations between some traits but not others can account for how and why changes in wing and tail length were not matched by changes in body mass or sternum length, therefore determining an overall change in shape in this bird species. Our quantitative genetic models also revealed no change in genetic values over time, indicating that changes in adult body size and shape were primarily driven by phenotypic plasticity and not by micro-evolution in our study population. Overall, our study highlights the importance of demography and plasticity in driving changes in size and shape in adult Alpine swifts. Moreover, results on trait genetic architecture provide an insightful explanation on trait evolutionary constraints.