What does bard have that make it possible for him to defend his city from smaug

In Chapter 14, Bard, the grim-voiced, grim-faced man, is willing to challenge Snaug and he has help from a messenger bird, called a thrush. The thrush relays information that Bilbo discovered while in Smaug's lair: the hollow of Smaug's left breast is not plated with his red-gold armor. When Bard strikes this spot, Smaug falls dead, his massive body crushing the city of Esgaroth.