Autumnal use of fruits by bird vectors in a coastal forest near the city of Niigata
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概要
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We examined the use of fruits by birds by means of fecal analysis of specimens captured during banding research. Investigations were conducted over two autumns in a coastal forest near the city of Niigata, Japan. Japanese White-eye Zosterops japonicus fed on 12 species of relatively small fruits. Among bird species analyzed in this study, both the number of seeds obtained from feces and the percentage of individuals containing seeds were highest in the Japanese White-eye. Although the percentage of Japanese Bush Warbler Cettia diphone containing seeds was low, the number of individuals captured was the highest and nine species of seeds growing in the forest understory occurred in its feces. Most fruits eaten by birds were either significantly smaller than their bill gape widths, or not significantly different in size compared with their gape widths. In cases where fruit size was significantly larger than gape width, the ranges of fruit sizes and gape widths partly overlapped. The gape widths of Brown-eared Bulbul Hypsipetes amaurotis and several species of large thrushes Turdus spp. were significantly larger than fruit sizes in all plant species identified. These birds fed on relatively large fruits of evergreen broad-leaved trees such as Japanese Machilus Persea thunbergii, Neolitsea Neolitsea sericea, and Nepal Holly Ilex integra. However, bird species having large gape widths showed no tendencies to prefer large fruits. We considered that there were sufficient resources (plant species and fruits) for bird species in each habitat or foraging position in this coastal forest in autumn. Most of the species of seeds in bird feces were present as fruits in the forest stand studied. Plant species growing abundantly in or near this forest and producing relatively small fruits (e.g., Chinese Hackberry Celtis sinensis) were often food sources for many bird species. We suggest that in the future, immigration of new plant species from gardens of neighboring residential areas may occur as a result of birds feeding on them and distributing their seeds in their droppings.
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