from the hoobit book
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When Bilbo came up against Smaug in his lair, he saw a small bare patch on the dragon's left breast. Bilbo then went back and told the Dwarves about Smaug's weakness. A small thrush roosting near the mountain's secret door then became an angel in disguise as it flew to tell Bard the Bowman of Esgaroth about Bilbo's discovery. When Smaug attacked the town he was met by Bard's, Black Arrow which hit the weak spot and killed him.