In Act II, scene iii of Macbeth, Macduff and Lennox have come to wake King Duncan, just outside his room. Macduff goes inside the chamber, leaving Lennox outside to make small talk with Macbeth.
Lennox's report of the conditions outside Dunsinane reveals unnatural and horrifying imagery: "lamentings" and "strange screams of death." It sounds like hell on earth. His reference to the "obscure bird" refers to an owl, for it flies and hunts at night without being seen. It is a reference to an earlier mention by Lady Macbeth in Act II, scene ii:
It was the owl that shriek'd, the fatal bellman, / Which gives the stern'st good night
The screech of the owl coincides with the death of Duncan just as the sound of the bellman would sound a prisoner's hanging in a prison. The use of supernatural animal imagery is a Gothic convention to signal a disruption in the natural order.
Later, in Act II, scene iv the Old Man will tell Ross that a mousing owl killed a falcon, the third mention of owl in this act. This too is a disruption in nature and goes against the food chain: a falcon should kill an owl. So, a thane (Macbeth) who kills a king (Duncan) is like the inferior predator (owl) killing the superior one (falcon). He does so only under cover of night (or using blackness, evil, supernatural, the occult).