Answer: Owls

Though they look silent as they glide through the air high above our heads, most birds are anything but. At close range, bird wings generate a substantial amount of noise as the wind cuts over their feathers; startle a group of pigeons in the park, for example, and you’ll hear the sound of feathers cutting through the air very clearly.

You won’t hear the same thing from an owl, however, as owls are the veritable stealth bombers of the avian world. Owls have adapted over time to have feathers shaped in such a fashion (the edges of their front flight feathers are serrated) that the passage of air currents over them is almost completely soundless. This adaptation helps the owl glide down behind prey with very sensitive hearing, like field mice, and snatch them up without so much as a whisper giving them away.

The adaptation is only present among owls whose primary diet is mammalian. Species of owls that feed primarily off fish (of which the stealth granted by adapted feathers would be of no advantage) have no adaptation and their wing beats sound just like those of other birds.