Many nocturnal owls in flight make little noise. The article points out that this is due to the types of feathers on the wings of these owls. These feathers combine to reduce the amount of noise by breaking up the air flowing over the wings and absorbing the some of the remaining sound. The reduced noise makes it easier for the owl to detect prey and to avoid being detected by potential meals.

The author of the article makes the inevitable comparison to a stealth bomber so I'll oblige with a set of pics.