Ornithol Sci 3: 23–32 (2004)

Exotic Red-billed Leiothrix Leiothrix lutea numbers have increased in southern, western and central part of Japan since the early 1980’s. Unlike most naturalized birds, this species mainly breeds in natural deciduous forests. We researched the breeding bird density in a deciduous forest on Mt. Tsukuba (877 m), a major breeding area of leiothrix in central Japan, in 1994 and 1995. Leiothrix bred at very high density (350–400 pairs per 100 ha) and was the most dominant species in both years. In 1995, six complete nests were found in a 1 ha area. In spite of the high breeding density of leiothrix, native avifauna showed species diversity, total breeding density and total biomass, similar to other Japanese deciduous forests. Therefore, we suggest that leiothrix have invaded Mt. Tsukuba without severe competition with native bird species.