What adaptation helps owls fly silently?

The story behind the answer

Owls fly silently thanks to specialized soft-edged feathers: the leading edges of their primary feathers have comb-like serrations, and the upper surfaces are velvety. That stealth lets them approach prey undetected in the dark.

The other traits help owls in different ways: hollow bones reduce weight in most birds, broad rounded wings enable slow, maneuverable flight, and facial discs focus sound to the ears for pinpoint hearing.