Breeding Distribution of the Isolated Iijima's Leaf Warbler Population in the Tokara Islands and its Ecological, Morphological, and Genetic Characteristics

Iijima's Leaf Warbler Phylloscopus ijimae is a summer visitor, endemic to the Izu and Tokara Islands, and its habitat range is especially restricted in Tokara. In this study, the detailed distribution and density of the Tokara population were investigated, together with its ecological, morphological, and genetic characteristics. The distribution of singing males was surveyed on five islands within the species' known distribution range, and the warbler was most abundant on the largest, Nakanoshima, but less abundant on Kuchinoshima, Suwanosejima, and Akusekijima, and undetected on Tairajima. During the period of 2001 – 2023, the abundance on Nakanoshima was judged stable from trim analysis based on the annual route census data (averaging 1.57 individuals /ha). The breeding ecology on Nakanoshima was similar to that reported in the Izu Islands, but the nest sites were more restricted to the branches of the bamboo Pleioblastus linearis, growing thickly under the evergreen forest. It is known to be difficult to identify the sex of this species from external characteristics, but the sex of the captured individuals could be clearly determined by DNA- based methods using universal primers. Mean external measurements of the Tokara males were generally smaller than those of the Izu males, and the differences in tarsus and total head lengths were significant (P < 0.05). The results of the probabilistic principal component analysis also supported the trend that the Tokara males are smaller. Female measurements could not be examined due to insufficient samples. Four haplotypes, two each from two regions, were identified from the mitochondrial cytochrome b region partial sequences (936 bp), and haplotypes from each region were placed in two different clusters of the neighbor- joining tree, respectively. The average genetic distance between regional populations was 0.064 in nucleotide substitution rates, suggesting that the two isolated populations are best placed in two independent conservation units.