Answer: Stones.
She kept careful notes. The crows learned faster than expected. Older birds taught juveniles; mothers nudged beaks to show technique; subgroups of crows specialized — one became expert at tasks involving ropes, another at manipulating small objects. Once, when Mei hid the peanuts in a transparent box with a lid, a crow named Slate pushed another juvenile toward a stick propped against the lid. The juvenile used the stick to lift one edge and free the food. Mei observed what she later wrote in her notebook: "Not imitation alone — guidance."