Abstract
he Arctic warbler (Phylloscopus borealis) species complex is commonly present in the Palearctic region. By 2014, the three bird subspecies were split into three species, Arctic warbler (P. borealis), Japanese leaf warbler (P. xanthodryas), and Kamchatka leaf warbler (P. examinandus), based on different breeding areas and distinct vocalizations. However, their similar coloration and body size make it difficult to distinguish these species in the nonbreeding season. Taiwan is located in the potential migration routes of the Arctic warbler species complex; however, no confirmed record of P. xanthodryas and P. examinandus exists. In this study, we compared the mitochondrial cytochrome c oxidase subunit 1 (CO1) sequences of samples from breeding sites during the breeding season and confirmed that the three species could be distinguished based on CO1 gene sequences. We also examined the species of the Arctic warbler species complex samples collected from eastern Taiwan. For the first time, we confirmed that all three species visited Taiwan during migration season. In the Taiwanese samples, no clear distinction could be made between species based on plumage coloration and size, indicating that these traits are not reliable for species identification. Reassessment of the CO1 gene sequences of the three species deposited in the Barcode of Life Data System revealed that the taxonomic status needs to be updated for 31.8% of the samples.