RESULTS
We measured the extent of feather moult in 21 passerine bird species that breed in the Eastern and Western Palearctic regions (n = 3126 individuals; Figure 2 and Supplementary Table 1) using skin specimens stored in nine natural history museum collections as well as live individuals examined in the field. We found that partial post-juvenile moult is more extensive in the Western Palearctic than in the Eastern Palearctic region (Figure 3 and Supplementary Figure 1). In 13 of the 21 species examined, the selected models showed a longitudinal effect on moult extent (∆AICc > 2.00); in the remaining eight species, a model that included the longitudinal effect was not selected over the other models (∆AICc < 2.00; Supplementary Table 2).
By applying Phylogenetic Generalized Least Square (PGLS) regression using global bird phylogeny (Jetz et al. 2012), we found that the difference between moult extent in the Eastern and Western Palearctic regions is affected by species-specific differences in migration distance (between Eastern and Western Palearctic populations of each species) and body mass, but not by moult latitude (Figure 4 and Table 1). The longer migration distance in the Eastern Palearctic, probably as a result of the longer and colder winter in this region (Figure 1), caused a generally less extensive moult among passerines. In addition, high body mass was correlated with a low difference in moult extent between the Eastern and Western Palearctic regions. We conclude that these two factors differently affect the extent of moult in the Eastern and Western Palearctic; while the difference in migration distance increases the longitudinal difference in post-juvenile moult extent, body mass decreases it.