Introduced black rats (Rattus rattus) have been reported to damage endemic plants of the Ogasawara Islands by gnawing. This study used seasonal field observations of plants together with analysis of rat stomach contents and age structure to help understand the cause or mechanism of black rat twigcutting activities. Twigs of Ochrosia nakaiana and Hibiscus glaber were found to be cut by black rats on the islands of Nishijima, Anijima, and Mukojima in March and April 2006 and 2007. Fragments of twig tissues in rat feces proved that the rats ate twigs, rather than only gnawing or cutting them. Age compositions of trapped black rats showed that the season of plant damage corresponded with that of low breeding activities of the rats and scarcity of preferred foods (January–March). We assume a link between low breeding activities of the black rats and food shortage, which motivated the rats to consume twig tissues.