A study on the influences by introduced birds on native birds.

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Naturalisation of introduced birds has been a serious problem in the conservation of biodiversity of native species throughout the World, In Japan, although not a few species of exotic bird species have established naturalised populations, there have been few ecological studies on this problem. In this study, I clarified the ecology of the Red-billed Leiothrix, one of the most successful introduced species, and its influence on the breeding of sympatric native bird species, particularly the Japanese Bush Warbler.The Leiothrix was the most dominant species among the forest living bird community and outnumbered the Bush Warbler. Only these two species bred in the bush of dwarf bamboo. While most nests of the Bush Warbler were located in the habitats of dense bamboo bush, the Leiothrix nested in bushes of much wider range of density. The generalistic habit of habitat selection is one of the reasons for the Leiothrix to have been increasing the number rapidly.The breeding season of the Leiothrix was much longer than those of other sympatric species. The breeding success was very low in both species, a main cause being the predation by the Jay and snakes.The Leiothrix foraged mainly on leaves and twigs of tall trees in the lower layers of forests, segregating from the Bush Warbler foraging mainly in the bush. The overlap of vertical range of foraging space between the Leiothrix and paid species was large, but there was the horizontal segregation between them.The Leiothrix has been increasing because they occupied a vacant niche, i.e. bush of dwarf bamboo in the floor of the deciduous broadleaved forests, foraging in the lower layers of the forests and long breeding season. The habitats of the Leiothrix and Bush Warbler has been fragmented by the Sika deer. It is necessary to monitor the population dynamics of these two species with the progress of the habitat fragmentation.