DISCUSSION
Large-scale spatial gradients in environmental conditions may affect organisms and ecosystems at different levels (Gaston 1996, 2000; Mittelbach et al. 2007; Steudel et al. 2012). In this study, we examined the effects of changes in environmental conditions, especially climatic conditions including the duration of the breeding and the post-fledging moulting period at high latitudes (Figure 1), on feather moult, a critically important process in the avian yearly cycle, with implications for plumage performance (Jenni & Winkler 1994; Bartaet al. 2008). Our results suggest a reduced moult extent (number of moulted feathers) by juveniles of most of the tested species in the Eastern Palearctic compared to the Western Palearctic (Figure 3, Supplementary Table 2 and Supplementary Figure 1). In addition, we found that the change in moult extent across the Palearctic’s longitudinal axis was modified by migration distance (between east and west) and by body mass (Figure 4 and Table 1). For a given latitude, the cold season is longer and colder in the east than in the west (Seebohm 1901; Walteret al. 1975; Figure 1). As a result, the migration distance in the Eastern Palearctic is longer (Supplementary Table 1) and breeding occurs later. Consequently, the time that is available for moulting in this region is shorter than in the Western Palearctic (Figure 5). Among juvenile passerines, the extent of wing feather moult is known to be largely affected by time availability (Kiat & Izhaki 2016; Kiat & Sapir 2017), explaining why the number of moulted wing feathers in the Eastern Palearctic was lower. We note that a spatial difference in feather moult timing between east and west populations was previously documented among Neotropical migrant passerines (Rohwer et al.2005). This difference was attributed to differences in precipitation and climate over a spatial scale of ~3500 km. To the best of our knowledge, our work is the first to document a difference in a life-history process across the entire Palearctic biogeographic zone (~7500 km).