In the Old Man's Dialogue in Act II, scene iv in Macbeth, what symbolizes King Duncan?

The old man states, "On Tuesday last/A falcon, tow'ring in
her pride of place,/Was by a mousing owl hawked at and killed."  The falcon is King
Duncan, and the owl is Macbeth.  Within this dialogue, references are also made to
disturbances in Nature, which represent the unnatural nature of the
murder.