It means: "There is no trace of birds in the sky", which means that the environment is quiet at this time, and birds are dynamic things. They will not stop flying because they all have their own directions and know where they should fly.
"But I have flown", a "Dan", shows that the "bird" in the previous sentence is metaphorical, and the author compares himself to a bird in the sky, which also has a profound and broad ideal and direction; Flying over describes the author's struggle for his goal. From this sentence, we can realize the author's happiness in fighting for his ideal and his pride in making unremitting efforts for something.