Large-scale spatial gradients of environmental conditions shape
organisms, populations and ecosystem. Even though environmental
gradients are a key research theme in macro-ecology and biogeography,
the effects of large-scale, east-west, environmental gradients are
largely overlooked compared with north-south gradients. Our study
focused on feather moult, an important and energy demanding process in
birds. By comparing Eastern and Western Palearctic populations of 21
species, we found that passerines in the Eastern and Western Palearctic
differ in the number of feathers moulted. This difference is most likely
the result of a large-scale climatic gradient in cold season duration
and differences in time available for moulting. Eastern populations were
characterized by a limited extent of feather moult that was additionally
affected by migration distance and body mass. These results highlight
the importance of linking annual cycle processes at the organismal level
to the specific environmental conditions in the distribution range of
each species.