Jem's initial reaction to the gifts in the tree is

When Jem is first told by Scout that there was gum in the tree, he immediately assumed the gum was poisoned, just because it was near the Radley's house. However, after walking (running) past the house multiple times on their way home from school he came to the conclusion that the tree was someone's hiding place. He figured there was probably some scrawny kid, who hid his stuff there on the way home, that way the bullies wouldn't take it. He thought this because of the multiple items they found in the tree, which ranged from gum, to a watch, and even coins.