Mixed-species flocking is a phenomenon of two or more species moving together in same direction while foraging and might benefit participants by improving foraging efficiency or reducing risk of predation. We explored the effects of environmental factors on mixed-species bird flock structure on high mountains of Taiwan. We found that flock structure and flocking propensity of species varied seasonally, increasing significantly with lower air temperature. Moreover, vertical structure of vegetation had strong relationship with species composition of mixed-species flocks. We conclude the formation of mixed-species flocks was strongly correlated with environmental condition and might be attributed to improving foraging efficiency.