A few years back I heard a lecture from a prominent British Aerodynamics/Acoustics expert named Geoffrey Lilley, who had done a lot of studies on owls and their lack of noise. It was fascinating.
Owls have very fluffy downy feathers under their wings that help absorb noise, and their feathers are setup to make only very low frequency noise, lower than their prey (and humans) can even hear. And the owls own ears block out their wing noise too. Their hearing range is the same as their prey. So when they swoop in on prey, it's a silent operation -- the owl can hear the prey but the prey can't hear anything and the owl doesn't even hear it's own noise.
The other interesting thing he said is that owls are capable of very steep flight paths, like as steep as 45 degrees. They can fly down towards prey at angles no fixed wing airplane could come close to doing, which is part of their amazing capability.
The end result is that owls hit their targets very fast and silent, as if out of nowhere. It's pretty amazing.
Lilley was an interesting guy too, who lived quite a life:
Obituaries: Professor Geoffrey Lilley, OBE | The Scotsman
