Final answer:
In the context of the question, which likely pertains to the novel 'To Kill a Mockingbird', the correct answer is squirrels. Squirrels use tree knotholes for nesting or food storage, not to leave items for others.
Step-by-step explanation:
The question relates to the items found in the knothole of an oak tree within a literary context. In the reference provided, which serves as an example from nature, various organisms such as birds and squirrels use materials like twigs, bark, vines, bits of leaves, and parts of lichens to build their nests. Moreover, they use holes in tree trunks for nesting. However, in the context of the question which seems to stem from the novel 'To Kill a Mockingbird', squirrels do not leave items in the knothole for others to find; they instead would use it for their home or to store their own food. Thus, the correct answer to 'Which of the following is NOT one of the items found in the knothole of the oak tree?' is squirrels. Their presence in the knothole would be for their own nesting or storage purposes, not leaving objects for others.