The circles of feathers around an owl's eyes do not, as you might expect, help it to see; they help it to hear. Known as facial discs, the circles have tightly packed feathers around their rims. The feathers channel high-frequency sounds, such as the squeak of a mouse, into the owl's ears, which lie behind the discs. They perform essentially the same function as they fleshy external ears of mammals such as rabbits and cats.
With their enormous eyes, owls are particularly well-equipped for hunting in dim light. But their remarkable vision cannot help them in the complete darkness they encounter under trees at night. They then have to rely on their extremely acute hearing.
Owls locate sounds much as we do; a sound from the right reaches the right ear fractionally before it reaches the left, and their brains analyse this tiny difference to give the direction of the sound. In the same way, some species of owl can sense the exact height a sound is coming from, as one of their ears is higher than the other. Owls turn their heads to balance a sound in both ears - this gives them the precise direction of a sound, and allows them to pinpoint their prey with complete accuracy in total darkness.
As if all this was not enough, owls have extremely soft plumage, allowing them to make their final gliding assault in almost complete silence. A mouse scuttling through the undergrowth has very little chance of escape.