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In The Hobbit Bard kills Smaug the dragon. In the book he is informed by a thrush of the dragon's one unprotected spot. But who sends the thrush to help Bard?

The thrushes were allies of the people of Dale and the dwarves of the mountain since olden times. The thrush overhears what Bilbo has to say about his trip in to meet Smaug, including the location of the dragon's weak point. Having heard this, the bird decides (apparently based on a sarcastic suggestion made by Bilbo) to fly down to Esgaroth and to alert Bard to the location of the wyrm's weak spot.

From "Inside Information":

But the hobbit was worried and uncomfortable, and they had difficulty in getting anything out of
him. On thinking things over he was now regretting some of the things he had said to the dragon, and
was not eager to repeat them. The old thrush was sitting on a rock near by with his head cocked on one
side, listening to all that was said. It shows what an ill temper Bilbo was in: he picked up a stone and
threw it at the thrush, which merely fluttered aside and came back.

“Drat the bird!” said Bilbo crossly. “I believe he is listening, and I don’t like the look of 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 of 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.”

“Well, he’ll have news to take to Lake-town all right, if that is what he is after,” said Bilbo; “though I don’t suppose there are any people left there that trouble with thrush-language.”

“Why what has happened?” cried the dwarves. “Do get on with your tale!”

So Bilbo told them all he could remember, and he confessed that he had a nasty feeling that the
dragon guessed too much from his riddles added to the camps and the ponies. “I am sure he knows we
came from Lake-town and had help from there; and I have a horrible feeling that his next move may
be in that direction. I wish to goodness I had never said that about Barrel-rider; it would make even a
blind rabbit in these parts think of the Lake-men.”

“Well, well! It cannot be helped, and it is difficult not to slip in talking to a dragon, or so I have
always heard,” said Balin anxious to comfort him. “I think you did very well, if you ask me — you
found out one very useful thing at any rate, and got home alive, and that is more than most can say
who have had words with the likes of Smaug. It may be a mercy and a blessing yet to know of the bare
patch in the old Worm’s diamond waistcoat.”

That turned the conversation, and they all began discussing dragon-slayings historical, dubious,
and mythical, and the various sorts of stabs and jabs and undercuts, and the different arts devices and
stratagems by which they had been accomplished. The general opinion was that catching a dragon
napping was not as easy as it sounded, and the attempt to stick one or prod one asleep was more likely
to end in disaster than a bold frontal attack. All the while they talked the thrush listened, till at last
when the stars began to peep forth, it silently spread its wings and flew away. And all the while they
talked and the shadows lengthened Bilbo became more and more unhappy and his foreboding grew.