Feeding and Digestive Conditions of the Seeds of Mallotus japonicus Based on Analysis of Excretions from Crows (Corvus macrorhynchos & C. corone)

Abstract

We collected excretions from two crow species (Corvus macrorhynchos & C. corone), that were dropped on the road around tea bushes or citrus plantations at Udo hill, Shizuoka Prefecture from August to October 2003. The excretions often contained many Mallotus japonicus seeds. In order to clarify the diet of the crows and the digestive condition of the seeds, the excretions were collected periodically and their contents examined. Within the 283 excreta samples collected, 16 plant species were identified, of which M. japonicus seeds were by far the numerically dominant component. However, the crows also fed frequently on other plant fruits and insects. M. japonicus seeds have a thin outer seed coat that contains lipid. Presumably, because the edible parts per seed are small, crows need to consume many M. japonicus seeds to obtain significant amounts of lipid and nutrients. Depending on the combinations of three types of outer seed coat, the degree of digestion was different among the excretions. The seeds in those excretions containing insect fragments were well digested, whereas the seeds mixed with parts of other fruits were less digested. In the process of digestion, the hard chitinous insect parts would abrade or fracture the outer seed coat and promote digestion. Indigested seeds had the edible parts remaining, and these seeds were eaten or transferred by ants from several excretions. The indigested seeds in the crow excretions might be important for secondary dispersal of M japonicus.