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The old thrush cracked against the door of Smaug's cave, exposing it, and allowing Bilbo to unlock the entrance just as "the gleam went out, the sun sank, the moon was gone, and evening sprang into the sky." The old thrush is an example of the aid of the natural world in the he heroe's quest.
The thrush is also a 'voice,' it's symbolic of the story of Noah. The thrush will be back before long!