Specific interactions in seed dispersal by the Japanese white-eye Zosterops japonicus: Factors influencing its preference for two plant species, Aralia elata and Zanthoxylum ailanthoides

Many frugivorous birds act as seed dispersers for freshly fruited plants. Typically, one bird species has multiple plant partners, functioning as a generalist disperser. However, specific interactions also can be formed, depending on the traits of birds and plants. Our field observation from 2017 to 2019 suggested that Zosterops japonicus as frugivore prefers the fruits of Aralia elata and Zanthoxylum ailanthoides, thus dispersing the seeds locally in central Japan. In this study, we examined the factors of the preference by comparing the morphological traits among plants and observing foraging efficiency on each plant. The tree traits of the two plants were characterized by large crop size and high number of fruits that Z. japonicus can forage on a certain point of a branch (NFZ). Their fruits were also characterized by small size and high share of lipids in the edible parts. The traits of edible parts differed between the two plants; A. elata fruits had succulent pulps, whereas Z. ailanthoides fruits only contained oil composed of oleic aids. Additionally, the number of fruits per second consumed by Z. japonicus was significantly higher in the two plants than those in other plants. The foraging efficiency was enhanced by large crop size, large NFZ, small fruit size, and high lipid content of fruits. The high foraging efficiency may increase the preference of Z. japonicus for the two plants, leading to the specific interactions.