What bird tells Bard about Smaug's weak spot?

The thrush tells Bard about Smaug’s weak spot.

While Bilbo explores the dragon’s lair beneath the Lonely Mountain, he notices a small bare patch on Smaug’s otherwise jewel-covered underside. A thrush overhears Bilbo describing this vulnerability and later communicates the information to Bard the Bowman near Lake-town.

Smaug attacks Lake-town after leaving Erebor, and ordinary weapons fail against his armoured body. Guided by the thrush’s message, Bard learns that an unprotected place lies on the dragon’s belly, near the heart. He fires his final arrow into that spot and kills Smaug, causing the dragon to crash into Lake-town.

The bird is sometimes confused with the Eagles, who also aid the heroes elsewhere in the story. The Eagles rescue Thorin’s company from the goblins and later participate in the Battle of Five Armies, but they do not reveal Smaug’s vulnerability. The thrush’s role is specifically tied to the discovery and communication of the dragon’s weak spot.