In scene 4 of the second act, Banquo is talking with an old man about how nature/the has been working in strange ways. They mention these acts to show how everything is going against its own nature; the day is dark regardless of the time, an owl killed a falcon (an owl which usually goes for smaller animals, like mice), and the horses ate each other. These sinister occurrences happened to convey how unnatural was the death of the King.
