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.