Two models of mechanisms have been proposed to account for resource partitioning between species with overlapping niches, i.e., niche differentiation and competitive hierarchy. I examined branch-side uses by color-marked individuals of eight canopy-foraging species (Long-tailed Tit Aegithalos caudatus, Marsh Tit Parus palustris, Great Tit P. major, Eurasian Nuthatch Sitta europaea, Japanese Pygmy Woodpecker Dendrocopos kizuki, Great-spotted Woodpecker D. major, White-backed Woodpecker D. leucotos and Grey-headed Woodpecker Picus canus) in mixed-species flocks on artificial feeding trees for three years. The three tit species and the Nuthatch used the upper sides of branches most frequently whereas the four woodpecker species used the lateral sides of the branches most frequently, as expected from the niche differentiation model. Among the three tit species and among the four woodpecker species, small-subordinate species foraged on a wider range of branches than large-dominant species, as expected from the competitive hierarchy model. In addition, branch-side uses were compared between individuals, years, social situations and food abundance conditions for five species (Marsh and Great tits, Nuthatch, Japanese Pygmy and Great-spotted woodpeckers) that were abundant over one year. The Marsh Tit and the Japanese Pygmy Woodpecker changed branch-side uses when feeding with the Great Tit and the Great-spotted Woodpecker, respectively. Annual differences in branch-side use by the Marsh Tit was also related to frequency of hostile encounters with the Great Tit. Intraspecific differences in branch-side use were shown in subordinate species (Marsh Tit and Japanese Pygmy Woodpecker) but not in dominant species (Great Tit and Great-spotted Woodpecker). Such individual variability may be important in allowing subordinate species to reduce intraspecific competition when foraging in the same flock. The effect of food abundance on branch-side use was not demonstrated in this study.
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