Sex-specific selective effect of winter weather on morphological traits in a small passerine bird

Harsh environmental conditions often impose strong selection on the phenotype of natural populations through impacts on their fitness. For overwintering birds, winter is an important period for survival because the weather conditions in winter is usually severer than in other seasons and birds often suffer more thermoregulation costs while food is in short supply. Thus, the selective effect of weather conditions on phenotype in winter is expected to be strong. In this study, we examined the relationship of overwinter survival of Silver-throated Tits (Aegithalos glaucogularis) with their morphological traits under different levels of winter conditions (winters with and without snowstorms) to explore the differential selective effect of winter weather on their morphology. We found that regardless of whether the winter experienced a snowstorm, the female Silver-throated Tits with a smaller bill surface area or smaller bill depth and a smaller surface area:volume ratio were more likely to survive during the winter, which supported the hypothesis of selection for heat retention. Furthermore, the females with a smaller body length survived better than the larger females, indicating that the lesser food requirements for smaller body sizes may confer advantages during the winter when food availability was reduced. In addition, in agreement with the finding in many short-lived birds that survival rate increases with the increase of age, older (≥2-year-old) female Silver-throated Tits had higher overwinter survival than 1-year-old females. However, we did not find any correlation of morphological traits and age with survival in male Silver-throated Tits. These results illustrate differential selective effects of winter weather on female and male Silver-throated Tits and contribute to the knowledge of sex-specific selection on the phenotype of natural populations.