Conclusions
Environmental conditions on Earth have specific spatial and temporal attributes that can have immense influence on biological systems. Understanding how environmental gradients shape various biological processes on Earth can improve our understanding of organismal life-history and annual-routine scheduling of different activities such as migration, feather moult and breeding, as well as their evolutionary development. Furthermore, this improved understanding can also help us predict future responses to changes that may occur on the planet. In this study, we focused on moult, an important process in the avian yearly cycle, and how this process may be shaped by an environmental longitudinal gradient, a seldom studied, large-scale environmental factor. Our findings highlight the importance of linking the evolution of annual cycle processes at the organismal level to the environmental conditions to which the organism is exposed. We propose that moult extent decreases as a response to the more extensive and colder winter in the Eastern Palearctic compared with the Western Palearctic. Consequently, this work exemplifies the importance of integrating biogeography and climatology to enhance our understanding of biological patterns and their causes over large spatial scales.