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.