One of these alterations is the discovery of Smaug's vulnerable spot by Bilbo. In the book, this is communicated to Bard by a thrush, emphasizing Bilbo's role in the dragon's demise. In the movies ...

Armed with the Black Arrow made for killing dragons, Bard notices the weak spot in Smaug's scales on his own and eliminates the dragon. These changes add intensity to the scene, singling out Bard ...