Below is the link to the illustration. It is one of about 3 or 4 defining poster images of the Death of Smaug.
Rivertown is burning, Bard is running around, organizing a resistance, and trying to save lives. He gets a clear view of the dragon, and he pulls his bow, readies his shot, and lets loose the arrow, heeding the hint from his feathered friend, and takes down the dragon.
How could have P. Jackson screw this up? Even if you take away the Thrush, the point is is that using the arrow blessed by the dwarven king, he takes a one in a million shot, and hits the weakness in the armor.
So, I am guessing having a Human upstart Legolas was just too much for the producer? I mean, we added Legolas into the film, and we've already established he's a supernatural archer to begin with... so having Bard be the same is too much, right?
So, suddenly, our Yew Bow (An English Longbow, for pete sake!) is replaced with a Ballista, and we set up a chase in the streets to get the Bolt and to go back to the tower... ...
And omg, this is dumb. This is not what Bard is. He is not a man sitting in a tower hiding and not being a leader. This is horrible. This is the plot point that upsets me. It diturbed me at the theater, and it bothers me even more now.
Don't get me wrong. The film is fine, overall. It's a light romp, Smaug is gorgeous and talks just enough to make you feel you paid your money's worth, and there are niggles about the rest of the film (I will not embellish how I felt the Necromancer scene was ill-played, also...), and I feel it is worthy of your time watching, but I also feel that the Producer is pulling favorites, and messing up things just for the sake of doing it different.
The downgrading of Bard from leader of the people, to smuggler, is a bit of a downgrade.