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).