Discovery of a high-potential new breeding site for Gray's Grasshopper Warbler Locustella fasciolata amnicola in the Shimokita Peninsula, Honshu, Japan

Abstract

We examined the status of the Gray's Grasshopper Warbler Locustella fasciolata in the area around Shiriyazaki on the Shimokita Peninsula in the northern part of Honshu Island, Japan, which was previously known to be outside of its breeding distribution. We conducted line transects and confirmed that 19 individuals were singing. Using digital voice recorders, we confirmed that they sang from June to August in 2021 and 2022. Considering the oviposition period and when this species leaves its breeding grounds, multiple individuals are likely breeding in the area. Additionally, we conducted song analysis, morphological analysis, and molecular phylogenetic analysis. These analyses confirmed that the captured individuals were Gray's Grasshopper Warblers, specifically the subspecies L.f. amnicola, previously known to breed only in Hokkaido and further north.