A thrush is a small to medium sized slender bird known for its harmonious song. It plays an important role in the termination of Smaug in The Hobbit. It is first referred to in chapter 3 when Elrond translates the moon letters written on Thror's map. It appears again much later in the story (ch.11) when Thorin and Co. first arrive at the back door of the Lonely Mountain, and again in ch.14 when it tells Bard of Smaug's weakness.
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.
A thrush appears at the end of the film, The Hobbit: An Unexpected Journey, flying towards 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.