Inter- and intraspecific variation in flight muscle fibers is associated with migratory timing

The life-history strategies of organisms are shaped by trade-offs among traits that influence survival, development, reproduction, and ultimately fitness. However, the specific physiological and environmental mechanisms driving population-level variation in phenotypic traits, particularly in relation to migratory adaptations, remain poorly understood. Elevational migration represents an adaptive behavior for mountain birds that offers a unique model to investigate adaptive phenotypes because of the diverse migratory strategies that have been observed within and between species. Flight muscles, as multifunctional organs, exhibit phenotypic variation that aligns with locomotor and thermoregulatory demands. Linking flight muscle phenotypes to migratory strategies can provide insights into inter- and intraspecific variation in migratory adaptations. In this study, we examined spring arrival dates at high-altitude breeding sites for three closely related high-altitude breeding birds in the mountains of Asia: Blue-fronted Redstart (Phoenicurus frontalis), Himalayan Bluetail (Tarsiger rufilatus), and Rufous-gorgeted Flycatcher (Ficedula strophiata). We analyzed relationships between arrival dates and phenotypic traits, including flight muscle physiology, morphology, and internal organ mass, alongside environmental effects on these traits. Our findings indicate that species arriving earlier encountered lower temperatures and exhibited reduced fiber cross-sectional areas coupled with higher fast oxidative glycolytic (FOG) fibers densities. Males displayed pronounced protandry and higher fast glycolytic (FG) fibers densities. Ambient temperature significantly influenced flight muscle phenotypes, highlighting intraspecific variation and the potential plasticity of flight muscle fibers as crucial mechanisms for adapting to migration strategies and environmental conditions. Additionally, this study underscores the potential role of sexual selection in shaping functional phenotypic variation, further advancing our understanding of adaptive strategies in high-altitude migratory birds.