How do owls defend themselves?

Robins have a couple of warning calls that they make when they spot a predator, which helps other robins to notice the danger. If a hawk flies over, robins sometimes freeze in position, and can hold still for a long time if necessary. One summer I spent a lot of time watching a pair of Merlins (bird-hunting falcons) who were nesting in an area adjoining a robin nest territory. Whenever the male robin sang, he sang at half volume, so he sounded very far away when he was right there!