Justification for the use of a migratory route as a proxy for the
past colonisation pathway
Our data on the migratory route of the Brown Shrike across the ECS is
far too limited for population-level inference, thus the universality of
the route for the entire L. c. superciliosus remains unclear.
Careful interpretation of our tracking data is required because
individual variation in migratory routes is recognized (Stanley,
MacPherson, Fraser, McKinnon, & Stutchbury, 2012). Migratory routes
inferred from the analysis also included large uncertainties (Figure 4)
because of the methodological limitations of light-level geolocators
(Lisovski et al., 2019). However, Lefranc & Worfolk (1997) stated that,
based on direct observation during migration, many L. c.
superciliosus fly at least 700 km across the sea from Japan to the
coast of China’s Jiangsu Province or the Zhoushan Islands. Our
geolocator data concur with their statement. Although further deployment
of geolocators is needed, their statement guarantees that the focal sea
crossing route is at least one of the major routes for the archipelagic
population.
We have taken the recent view that idiosyncratic routes of obligate
long-distance migratory species are highly persistent traits throughout
their evolutionary histories (Winger et al., 2019) in order to interpret
part of our tracking data as the route of past colonisation. However,
the dominant view is to consider
migratory routes as highly labile traits that are easily subjected to
natural selection (Pulido, 2007; Winger et al., 2019). Thus, one could
claim that the observed migratory route across the ECS is a result of
optimization to the present environmental conditions and does not
reflect past colonisation (Alerstam, 2011; Berthold, Helbig, Mohr, &
Querner, 1992; Sutherland, 1998; Zink & Gardner, 2017).
For the following two reasons, we believe Winger et al.’s (2019) and our
interpretation to be more plausible than the counterview.
First, our system does not include
any region that experienced severe glaciation. Previous studies have
used species that inhabit regions previously covered by ice sheets
during the LGM (Haché et al., 2017; Milá, Smith, & Wayne, 2006; Ruegg
et al., 2006; Sokolovskis et al., 2018). Reverting from being migratory
to becoming sedentary residents was probably a major response to
glaciation, which was inferred by SDM analyses (Zink & Gardner, 2017).
This implied that migration needed to be labile throughout a species’
evolutionary history and migratory routes should trace the post-LGM
expansion to the present breeding range. By contrast, we have shown that
suitable breeding habitat for the archipelagic population remained in
the Japanese archipelago even during the LGM (Figure 3b-d). This implies
that the focal migratory route traces a distribution change before the
LGM, supporting our view that idiosyncratic routes over a large barrier
have been conserved. Second, similarity between autumn and spring
migration across the ECS (Figure 4) may support the conservatism of the
focal route. If a migratory route
is optimized with the present environment, different selection pressures
upon passages in different seasons should independently shape the route
(Stanley et al., 2012; Tøttrup et al., 2012). Generally, loop migration,
in which spring and autumn migration routes are divergent, is thought to
have evolved by retaining one ancestral route retracing colonisation
while deriving the other to adjust to current environmental conditions
(Newton, 2008). Wind is an important determinant (Alerstam 2001). This
loop migration phenomenon has been shown for a population of the
Oriental Honey Buzzard Pernis ptilorhynchus breeding in Japan.
Its autumn migration involves crossing the ECS whereas its spring
migration route detours around the Korean Peninsula to cross a narrow
sea channel to enter the Japanese archipelago (Nourani, Yamaguchi,
Manda, & Higuchi, 2016). It has been shown that a route over the
Tsushima Strait was specifically suitable for spring wind conditions,
when wind directions and strengths over the ECS were unstable (Nourani
et al., 2016; Yamaguchi, Arisawa, Shimada, & Higuchi, 2012). These
conditions would be unfavourable also for passerines such as shrikes
(Alerstam 2001; Tøttrup, Pederson, Onrubia, Klassen, & Thorup, 2017).