Migratory route analysis
Under the Bayesian framework, autumn and spring migration routes were
inferred for three individual L. c. superciliosus (Figure 4,
Figure S1.4). Full annual data were retrieved from two tags, and
similarity was observed between their autumn and spring routes. One tag
stopped recording on the continent due to cell failure three months
after deployment. Despite uncertainties involved with location
estimates, higher mean probabilities for migratory locations were
observed among the three individuals both in the western part of the
Japanese archipelago and along the CRECS to northern Taiwan. The
probability map and the median migratory tracks suggest that birds
migrated across the ECS between these two regions and did not detour
over the Korean Peninsula. The data from the two successfully retrieved
geolocators revealed that both shrikes had wintered in the Sunda
Islands, specifically on Java and Flores.
Discussion:
The results from the three approaches supported the LDD hypothesis for
genetic divergence of the Brown Shrike in the Japanese archipelago
(Figure 1a, 5a-b). A phylogenetic split between the southern and
archipelagic clades was inferred in the gene networks (Figure 2),
suitable glacial breeding habitat for L. c. superciliosus was
estimated to have existed around the CRECS by the SDM (Figure 3), and
its current migratory route was observed between the CRECS and the
Japanese archipelago across the ECS (Figure 4). Our results cannot
reflect gradual expansion of a population via normal dispersal over a
terrestrial landscape. This is because, during the glacial periods
around which the divergence was estimated, the gulf separating the
Japanese archipelago, the Korean Peninsula and the CRECS was relatively
wide (Figure 5; Matsuzaki et al., 2019; Ota, 1998), compared to the LGM
to which we projected our SDM (Figure 3). LDD colonisation in a
stepping-stone manner along the Ryukyu islands from the CRECS or Taiwan
may also be less likely. This is supported by our results showing that
most of the Ryukyu islands were predicted to have been unsuitable during
a glacial period (Figure 3b-d), and the migratory route was not inferred
along them (Figure 4), unlike the routes reported for other species
(Shiu et al., 2006; Koike, Hijikata, Higuchi, 2016). Meanwhile, since
northern Taiwan was connected to the CRECS during this period (Figure
5), the inferred migration aligns with our prediction. Therefore, within
our hypothesis-testing framework, we imply that our results reflect the
LDD-driven divergence of a Japanese migratory subspecies across the ECS.