Owls are fierce predators, but even they have to sleep from time to time. When they do, they are exposed to bigger predators as well as 'mobbing' by smaller prey animals. Mobbing means smaller animals band together to fight off the owl's feared attackes.
Certain types of birds, when identifying an owl on their turf, might band together to harass and fight the owl and chase it off. For hunting, owls rely on stealth and surprise, and so a good camouflage can be the difference between dinner and going to sleep on an empty stomach.