In this scene, the Old Man tells Ross of the unnatural events that have been occurring lately.  He tells of a falcon being killed by an owl.  A falcon is a bird that an owl would not usually be able to kill.  A falcon is a bird associated with royalty who participated in the sport of falconry down through the ages.  King Duncan is the figurative falcon killed by the figurative owl, Macbeth.  Macbeth is, like the owl who hunts at night, one who prefers the dark in which to do his business. Macbeth asked the stars to hide their fires before he killed Duncan and it was a starless night when he committed the deed.

In my previous post, I failed to include a link to the etext of the tragedy.  You can access the text to review the dialogue at http://www.enotes.com/macbeth.

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.