Smaug the terrible and magnificent is one of the most powerful foes in existence at the time of The Hobbit books. He single-handedly wiped out the entire city of Dale, as well as slaughtered most of Thorin’s people. Not only is he a particularly large and fearsome dragon, he is also greedy and intelligent beyond compare, he knows what every single piece of treasure in his gilded halls is, and notices instantly if something has been moved or stolen. But beyond the desolation and the fire that he brought, the people of Lake Town know very little about him, because he has been locked away in the mountains ever since.
When Thorin and company thus undertake their quest to reclaim Erebor, the kingdom of their forefathers, they awaken Smaug from a long and deep slumber. In a rage, he tries at first to weed them out of the mountain and fry them, but when this proves too difficult a task, he decides instead to take his vengeance upon the people of Lake Town for helping them. There is nothing that the dwarves can do but sit and watch as he flies towards the village to desecrate it. But unbeknownst to the dwarves, there is another who has already gone ahead to warn the people, and try to limit the damage of the mighty beast. This seemingly small and insignificant character (shown only briefly in the movie adaptations) actually has a crucial role in saving the day, and enabling the few people who survive the attack and make it out unscathed.
Earlier on, at the end of the first film, a small thrush is shown bashing a snails shell against the side of the mountain. The noise echoes down into the chambers below, and the large eye of the sleeping dragon reveals itself and opens. Although this moment seems like a fleeting fancy to increase the drama and keep the audience wanting to come back for the next film to find out what happens, it is actually far more crucial than that in the book.
When Bilbo goes down into the chambers below, and has his encounter with Smaug, Bilbo is able to distract the dragon long enough with all of his talk of barrel riding and flattery, to be able to spot the missing chink in Smaug’s armor. The loosed scale has left a hole where the think hide can be penetrated. Bilbo rushes to share this with the dwarves, as it is the only possible hope of being able to defeat the beast. However, as Bilbo is imparting this information, he is bothered by the thrush who is listening in.
He goes to shoo it off, and Thorin stops him, saying that “The thrushes are good and friendly” and that “they were a long-lived and magical race.” It turns out that the thrushes had an alliance with Thorin’s family and aided them in both politics and trade: “the men of Dale used to have the trick of understanding their language, and used them for messengers.” This turns out to make all the difference in saving both the dwarves' lives, and the vulnerable town on the lake below. For when the dragon goes to kill everyone, the thrush goes too, and rushes ahead to intervene.
Meanwhile, Bard, the only man in the village brave enough to do something about it, (rather than fleeing like the Master of Lake Town and his slimy assistant Alfrid) is preparing to fight the dragon with the black arrow that was left to him by his father - the only weapon that can kill the beast.
As he begins to aim his bow, “Suddenly out of the dark something fluttered to his shoulders. He started- but it was only an old thrush. Unafraid, it perched by his ear and brought him news. Marveling he found that he could understand its tongue, for he was of the race of Dale.” The thrush tells him “The moon is rising. Look for the hollow of the left breast as he turns and flies above you.” Only then is Bard able to see the weak point and utilize it, bringing the dragon down once and for all as if falls out of the sky and crashes down on the houses below.
​​​​​​​The Town is of course ruined, and many lives are lost, but those who made it are able to regroup and rebuild a new abode thanks to the help of the old thrush, and the bargaining of Bilbo Baggins, who helps them to get the recompense they deserve from beneath the mountain once Thorin has fallen to the dragon-sickness.