TL; DR: Não sabemos ao certo, mas há indícios de que a flecha foi mágica.
No livro:
Ao longo dos trabalhos de Tolkien, fala-se muito de magia, mas na maioria dos casos, somos deixados a decidir por nós mesmos se a magia foi realmente usada em uma dada situação. Isso é verdade aqui.Os arqueiros de Dale dispararam inúmeras flechas contra o dragão, mas sem sucesso:
Roaring [Smaug] swept back over the town. A hail of dark arrows leaped up and snapped and rattled on his scales and jewels, and their shafts fell back kindled by his breath burning and hissing into the lake. No fireworks you ever imagined equalled the sights that night. At the twanging of the bows and the shrilling of the trumpets the dragon's wrath blazed to its height, till he was blind and mad with it. No one had dared to give battle to him for many an age; nor would they have dared now, if it had not been for the grim-voiced man (Bard was his name), who ran to and fro cheering on the archers and urging the Master to order them to fight to the last arrow...
Soon all the town would be deserted and burned down to the surface of the lake. That was the dragon's hope. They could all get into boats for all he cared. There he could have fine sport hunting them, or they could stop till they starved. Let them try to get to land and he would be ready. Soon he would set all the shoreland woods ablaze and wither every field and pasture. Just now he was enjoying the sport of town-baiting more than he had enjoyed anything for years. But there was still a company of archers that held their ground among the burning houses.
- The Hobbit, by J.R.R. Tolkien
O próprio Bardo - um grande homem descendente da nobreza - usa todas as suas flechas, exceto uma. Esta última flecha é especial, disse ter sido feita pelos anões sob a montanha onde Smaug agora vive. Um pássaro - a mesma espécie que ajudou o grupo de Bilbo a encontrar a porta para a montanha - chega a Bard e diz a ele para onde mirar sua última flecha:
Their captain was Bard, grim-voiced and grim-faced, whose friends had accused him of prophesying floods and poisoned fish, though they knew his worth and courage. He was a descendant in long line of Girion, Lord of Dale, whose wife and child had escaped down the Running River from the ruin long ago. Now he shot with a great yew bow, till all his arrows but one were spent. The flames were near him. His companions were leaving him. He bent his bow for the last time. Suddenly out of the dark something fluttered to his shoulder. He started-but it was only an old thrush. Unafraid it perched by his ear and it brought him news. Marvelling he found he could understand its tongue, for he was of the race of Dale.
"Wait! Wait!" it said to him. "The moon is rising. Look for the hollow of the left breast as he flies and turns above you!" And while Bard paused in wonder it told him of tidings up in the Mountain and of all that it had heard.
Then Bard drew his bow-string to his ear. The dragon was circling back, flying low, and as he came the moon rose above the eastern shore and silvered his great wings. "Arrow!" said the bowman. "Black arrow! I have saved you to the last. You have never failed me and always I have recovered you. I had you from my father and he from of old. If ever you came from the forges of the true king under the Mountain, go now and speed Well!"
The dragon swooped once more lower than ever, and as he turned and dived down his belly glittered white with sparkling fires of gems in the moon-but not in one place. The great bow twanged. The black arrow sped straight from the string, straight for the hollow by the left breast where the foreleg was flung wide. In it smote and vanished, barb, shaft and feather, so fierce was its flight. With a shriek that deafened men, felled trees and split stone, Smaug shot spouting into the air, turned over and crashed down from on high in ruin.
- ibid
Mais tarde, Rac, filho de Crac e senhor dos Corvos, voa até os anões para lhes dizer que Smaug estava morto; ele está familiarizado com o tordo que aconselhou Bard, o que sugere que o tordo era mais do que apenas um pássaro falante bem informado:
"I am Rac son of Carc. Carc is dead, but he was well known to you once. It is a hundred years and three and fifty since I came out of the egg, but I do not forget what my father told me. Now I am the chief of the great ravens of the Mountain. We are few, but we remember still the king that was of old. Most of my people are abroad, for there are great tidings in the South - some are tidings of joy to you, and some you will not think so good. "Behold! the birds are gathering back again to the Mountain and to Dale from South and East and West, for word has gone out that Smaug is dead!"
"Dead! Dead?" shouted the dwarves. "Dead! Then we have been in needless fear-and the treasure is ours!"
They all sprang up and began to caper about for joy.
"Yes, dead," said Rac. "The thrush, may his feathers never fall, saw him die, and we may trust his words. He saw him fall in battle with the men of Esgaroth the third night back from now at the rising of the moon."
- ibid