What Made the Crow Lay Stones on the Railroad Tracks?

抄録

　In Yokohama, central Japan, a carrion crow laid stones on railroad tracks when hoarding bread or retrieving it to eat. The crow picked up stones on either occasion. When the stones were heavy or awkward for the crow to hold with its beak, the crow laid the stones on the tracks, perhaps intending then to take a more comfortable grip on the stone. The crow also put stones on the tracks to remove breadcrumbs from its beak. The suitable height and flat surface of the tracks probably induced the crow to lay the stones on the tracks. When the crow laid stones on the tracks, it often picked them up again and carried them away. On rare occasions, however, the crow left stones on the tracks, causing the trains to make emergency stops.