INTRODUCTION
Environmental conditions are not equal on Earth. In many cases, a gradient of environmental conditions exists over various geographical ranges. For example, altitudinal (Roy et al. 1998; Hodkinson 2005) or latitudinal gradients (Lawson & Weir 2014) or the distance from the ocean (Makarieva et al. 2009) may cause a gradient of environmental conditions due to changes in solar radiation and the intensity of oceanic moisture flux. These environmental gradients shape ecosystems and biodiversity at various levels, and hence, many studies have examined their effects across different ranges (Gaston 1996; Petrůet al. 2006; Alexander et al. 2011; Pellissier et al. 2018). In addition, various ecological patterns and biological rules have been applied when describing the impact of changing environmental conditions on organisms and biological systems across different gradients. Such patterns represent one of the key research themes to macro-ecologists and biogeographers (Gaston 2000; Ricklefs 2004; Steudel et al. 2012) and include the latitudinal diversity gradient (Mittelbach et al. 2007; Condamine et al. 2012), Bergmann’s rule (Olson et al. 2009; Osorio‐Canadas et al.2016) and Allen’s rule (Allen 1877).
Due to the seasonal variation in environmental conditions, constraints imposed by the environment on animal ecology and physiology are released during specific time windows throughout the year. This allows organisms to engage in crucial season-dependent processes such as breeding at the most appropriate time. Consequently, scheduling of different annual processes and life-history traits are expected to vary over spatial gradients of environmental conditions. This is because the time windows in which season-dependent processes can occur change in relation to local environmental conditions. Additionally, differences in seasonality between regions may influence the scheduling of extended and demanding events and processes along the annual cycle, such as reproduction, growth, moult and migration, which usually do not overlap (Hemborget al. 2001). The ecological and biological impacts of various constraints arising from environmental conditions that vary across spatial gradients have been widely described for altitudinal (across the earth surface’s height axis; e.g. , Colwell & Lees 2000; Altshuler & Dudley 2006) and latitudinal (across the equator-poles axis; e.g. , Mittelbach et al. 2007; McKinnon et al.2010; Lawson & Weir 2014) rather than longitudinal (across the east-west axis; but see Murray et al. 2004; Han et al.2011) gradients. The overlooked change in environmental conditions across longitudinal gradients is surprising due to the extreme change in climatic conditions over this axis in many parts of the world, for example, in the Eurasian Palearctic biogeographic zone (Seebohm 1901; Walter et al. 1975). The spatial climatic gradient of the Palearctic region is characterized mainly by the duration of the cold season and its intensity (longer duration and colder temperatures in the east than in the west for a given latitude; Figure 1). This gradient may affect the annual cycle scheduling of fundamental seasonal processes, the time available for growth and development and the nature of various constraints on organisms, populations, species and ecosystems. Yet, to the best of our knowledge, no empirical data have demonstrated this effect, to date.
In this study, we test a continental longitudinal effect across the Palearctic biogeographic zone, which stretches over Eurasia, on wing-feather moult, an important process in the yearly cycle of passerine birds (Jenni & Winkler 1994). Among passerine species, moult strategies are shaped mainly by time constraints during the annual cycle, in which long periods of breeding, migration and feather moults take place (Barta et al. 2006, 2008; Kiat et al. 2019a). The scheduling of these extended and energetically demanding activities is determined by environmental conditions and the timing of the other annual cycle events during the year, as these activities usually do not overlap. For example, in migratory birds, breeding takes place after spring migration is completed, and feather moult starts typically only after breeding (Jenni & Winkler 1994). Nevertheless, not only is the start time of each activity influenced by scheduling constraints, but so is the overall duration. Specifically, variation in the duration of feather moulting is expected to affect additional properties of the moult process (Kiat et al. 2019a).
Among passerine species, one of the common strategies for dealing with time constraints is a reduction in moult extent (number of moulted feathers) during the first year of the life (Bojarinova et al.1999; Kiat & Sapir 2017). First-year birds are known to be particularly sensitive to time constraints (Marchetti & Price 1989; Hanson & Coss 1997), and consequently, partial moult is common among first-year individuals (Jenni & Winkler 1994; Kiat & Sapir 2017). Generally in passerines, the moult during the first year of life is intended to replace poor-quality nest-grown feathers with higher-quality feathers. Further, in many species, feather moulting changes the plumage of the bird such that it gains an adult-like appearance (Jenni & Winkler 1994; Kiat & Izhaki 2016; but see Kiat & Sapir 2018). Therefore, reduced moult extent at this stage of the bird’s life could affect plumage functionality, including its appearance (Kiat et al. 2019b), and consequently bird attractiveness, as well as the bird’s flight capacity (Bridge 2008). All of these are expected to have implications on future survival and reproduction (Senar et al. 1998; Minias & Iciek 2013; Crates et al. 2015).
We hypothesize that the time available for moulting is longer in the Western Palearctic compared to the Eastern Palearctic due to east-west differences in the duration and intensity of the cold season (longer and colder in the east than in the west, for a given latitude) and that the longitudinal difference in the time available for moult may be a direct consequence of the difference in climate across the Palearctic region along the east-west axis. In addition, longer migration distances in species and populations that breed in the Eastern Palearctic may indirectly reduce the time available for moulting by further shortening the time available for moult in the breeding areas (Kiat et al.2019a; Briedis et al. 2020). We thus predict that feather moult duration and consequently its extent are affected by climatic conditions across a longitudinal axis and that species-specific moult extent will be greater among western populations than eastern ones. Finally, we propose that additional factors that affect the moulting process, such as latitude, which was found to negatively affect moult extent (Kiat & Sapir 2017), or body mass, which increases moulting speed (de la Heraet al. 2009; Rohwer et al. 2009; Kiat & Izhaki 2016), may modulate this longitudinal effect. These factors are predicted to reduce the difference in moult extent across the east-west axis because of their own effect on moult extent.