Why did the Thrush help Bard the Bowman of Laketown to defeat Smaug when the dragon attacked the settlement?
When Bilbo is frustrated he throws a stone at an old thrush (the same one that speaks to Bard) and Thorin admonishes him:
“Leave him alone!” said Thorin. “The thrushes are good and friendly-this is a very old bird indeed, and is maybe the last left of the ancient breed that used to live about here, tame to the hands of my father and grandfather. They were a long-lived and magical race, and this might even be one of those that were alive then, a couple of hundreds years or more ago. The Men of Dale used to have the trick of understanding their language, and used them for messengers to fly to the Men of the Lake and elsewhere.”
Second, it helps the reader believe that Bard really is the descendant of Girion of Dale because Bard is able to understand the thrush. Third, it shows that there are all sorts of magical creatures populating the landscape. http://middle-earth.xenite.org/2012/09/18/why-is-only-bard-able-to-understand-the-thrush/
Smaug himself gives us the answer as he brags to Bilbo:
"Revenge! Revenge! The King under the Mountain is dead and where are his kin that dare seek revenge? Girion Lord of Dale is dead, and I have eaten his people like wolf among sheep. And where are his sons' sons that dare approach me? I kill where I wish and none dare resist." —Smaug
The chronologies of the Westlands tell of how, in the twenty-eighth century of the Third Age, the mightiest Dragon of the Age came from the north to the great kingdom of the Dwarves in Erebor, the Lonely Mountain. The Fire-drake called Smaug the Golden was vast and bat-winged and a fearsome bane to Dwarves and Men. With consuming Dragon-flame, Smaug ruined the city of the Men of Dale and broke the door and wall of the Lonely Mountain.
For two centuries, Smaug ruled the Lonely Mountain uncontested, laying waste to the lands around the mountain, so that the blasted domain of the Dragon of Erebor became known as the Desolation of Smaug.
Smaug killed the thrush's friends and laid waste to his home. It's only natural that this vital bit of information would seem to provide him the perfect opportunity to help rid the area of the hated dragon and to 'dare seek revenge' on Smaug for all that he had destroyed. He would gladly pass that on to his friend the Bowman.