I found quite interesting the usage of the word "Reel" in this verse. A "reel"is a reference to the gyre mentioned in the first verse of the poem. A reel is a a cylinder, circular object used in an artistic format; threads, wires, and other flexible materials are wound tightly around it. I believe that Yeats is suggesting, through both the gyre and reel, the evolution of art through history, but also the culmination and or end of history. When something is wound up too tight, it can break. Further, a thread is fragile; the thread signifies time, the universe, and perhaps even a life line. I think it is also important to note that he uses "reel" in order to describe shadows that are "circular" in shape; this is peculiar because a bird should not have this particular shape. Further, the birds are the only creatures left besides the sphinx.. this is symbolic because birds suggest hope, yet they are described as indignant. There is no hope left.