The Old Man and Ross discuss events in Act II scene 4 that have been happening since King Duncan was killed. The Old Man says that he has not seen anything like recent events in his 70 years. For instance, the day is very dark as if it were night. Ross tells of an owl that ate a falcon. Also, the Old Man recounts how King Duncan's horses ate each other while in a frenzy. All of these events are symbolic representations of Duncan's murder. The owl, for instance, typically hunts mice, not falcons. The fact that an owl attacked a falcon is extraordinary because the falcon is a much more powerful bird. Symbolically, Macbeth (owl) acted in the same way by attacking a more powerful person (falcon).
Also, when the men discuss the weather, Ross says, "Is't night's predominance, or the day's shame, / That darkness does the face of earth entomb, / When living light should kiss it?" Ross is asking if the day is dark because the light is ashamed to show itself or if it is because darkness is too powerful that the light cannot show itself, referring back to Macbeth's evil and dark actions. The Old Man responds "'Tis unnatural, / Even like the deed that's done." The effect is to emphasize how Macbeth's and Lady Macbeth's actions have completely undone the natural order of nature.