This song is about someone who has finally realised that they are inconsequential to the world they live in, and I have inferred this from the line "we are ghosts amongst these hills". The hills mentioned in the same line are a metaphor for the troubles of the singer. I also believe that the line "a bird without it's wings is a low and a tragic thing" is linked to my earlier thoughts as they cannot effect their situation, or escape it, much like a grounded bird. "The mountain song" is the sounds of the world around them, and the animalistsic feelings that the singer seems to be feeling. So, to summarise everything above (James) feels much like a caged animal, who has no power over it's fate, and cannot escape it's destiny, or the bars that it is held behind.