The global warming analogy
Many studies have examined the influence of various environmental
factors over large-scale biogeographic gradients. These include the
influence of latitude (Mittelbach et al. 2007; Lawson & Weir
2014), altitude (Roy et al. 1998; Hodkinson 2005) or longitude
(Füreder et al. 2005; Dunn et al. 2006; Finlay et
al. 2011); however, large-scale longitudinal effects have only rarely
been investigated thus far. Our results demonstrate the importance of
considering longitudinal effects in studies of biological processes that
depend on environmental conditions across biogeographic zones.
Specifically, within the Palearctic, there are substantial differences
in the climatic conditions along a longitudinal axis which calls for
further studies of the implications of this spatial climatic pattern.
We note that there are several similarities between the longitudinal
effects that have been revealed in our work and those described in
studies of global climate change. These similarities include earlier
arrival to the warmer Western Palearctic breeding areas compared to the
Eastern Palearctic ones, similar to the earlier arrival to breeding
areas in recent, warmer years, compared to historic arrival dates
(Møller et al. 2010; Tomotani et al. 2018). In addition,
the larger moult extent during the first year of life in Western
Palearctic passerine populations compared to Eastern Palearctic
populations (current study) resembles the higher moult extent found in
recent years compared to the past (Kiat et al. 2019b). Moreover,
migration distance is shorter in the Western Palearctic compared to the
Eastern Palearctic, and this distance is even shorter as a result of the
global climate warming (Visser et al. 2009; Pulido & Berthold
2010). Therefore, understanding longitudinal and large-scale spatial
gradients of biological phenomena that result from climatic conditions
may improve our understanding of various impacts attributed to global
climate change on avian life-history patterns and the evolution of
annual-routine scheduling.