The thrush is said to have belonged to an ancient breed known by Thorin's ancestors. The Men of Dale were able to understand the language of the bird, and they served the men as messengers. Bilbo is skeptical that any of the Lake-men still speak the thrush language, but is proven wrong when the bird tells Bard the Bowman of Smaug's only weak spot, thus enabling Bard to kill the dragon. The thrush in "The Hobbit" (book) is slightly bigger than a normal thrush.
A thrush appears at the end of the film, The Hobbit: An Unexpected Journey, flying towards the Lonely Mountain. It begins tapping a snail shell against the side of the mountain and the echoing of the tapping is heard all the way into the treasure room, causing Smaug to awaken.
The thrush is seen again outside of the Lonely Mountain in The Hobbit: The Desolation of Smaug during the secret passages opening.