The Eurasian Skylark, Alauda arvensis, is a passerine bird that mainly breeds in open habitats, such as lowland grasslands and farmlands. However, recent studies have shown that they form breeding territories on volcanic and alpine slopes in central/northern Japan. During the 2018 breeding season, we investigated the density and habitat selection of Skylarks on the volcanic slopes above timber line, of Mt. Tarumae, Hokkaido. Our results showed that there were 27 Skylark territories in the study area, with a density of 7.26 territories per square kilometer. This spatial arrangement was dense around the eastern slope. Skylarks favored habitats with a low vegetation height. The quantity of short plant species, i.e. Carex oxyandra and Arcterica nana was larger in territories than in randomly selected points. C. oxyandra plays important roles not only in forming low vegetation, but also in available and limited nest material in their breeding territories on the volcanic slopes. These results suggest that Skylarks occupy areas of low vegetation with C. oxyandra, as there are limited resources available for nest materials within the breeding territories that are on the volcanic slope above a timber line.