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.