We were previously warned that the dwarves were not heroic and that they were greedy. Certainly, the Hobbit's litany of unpleasant tasks was evidence of Thorin's willingness to be a titular leader but not the leader who goes into the unknown cave. Full of historical claims of justice, it is easy for Thorin to find his hubris, and having found his hubris, he will fall. Birds and clouds are symbolic portents of wise advice, fate and prophecy. As the birds have already proven themselves (to us), their speech and warning produces the effect of dramatic irony, for we know precisely what will happen. Thorin may or may not die (cliffhanger) but there will definitely be an altercation. If the reader wants a foreshadowing of the future, it can be found in one of the final images, an inversion of a well-established one: When Thorin shoots his arrow at his enemy, it "smote his shield and stuck their quivering." We have been considering Thorin as the contrast to Bilbo, but here we can see him as a foil of Bard. Bard is fluent where Thorin is not perceptive, and Bard strikes the enemy's spot, while Thorin strikes the shield. Finally, if the arrow is a symbol of a key, a metonym of military power and a phallic symbol of power, virility and potency: we should focus on the fact that Thorin's arrow is stuck in the shield and it is "quivering." In this sense, the triple-pun explains that the arrow is shaking (or is it nervous?), and it does not return to its quiver. We find the exact opposite in "Fire and Water" when Bard blesses his arrow (a form of apostrophe) before shooting it: "Black arrow! I have saved you to the last. You have never failed me and always I have recovered you. I had you from my father and he from of old. If ever you came from the forges of the true king under the Mountain, go now and speed well!!"
Chapter Sixteen: A Thief in the Night

Thorin continues to speak of the Arkenstone because it means so much to him, as it is an heirloom and he threatens to take revenge on whoever has prevented him from getting it. In spite of this warning, Bilbo decides that he will leave the mountain and offer the Arkenstone to Bard; then, Bard can offer the Arkenstone to Thorin in exchange for a fair portion of the treasure. Of course, there is so much suspicion on both sides that Bard has no way of guaranteeing that Thorin would make good on his promise to offer repayment. At any rate, Bilbo establishes himself as a figure of incredible honor‹even though he may be a traitor of sorts.