Why can owls fly almost silently?

Three features together. The leading edge of the flight feathers carries a stiff comb of fine projections that breaks the incoming air into small streams instead of letting it roll up into one noisy vortex. The trailing edge is soft and frayed, so the flows rejoin gently rather than snapping together. And the whole surface of the wing is covered in a velvety down that absorbs the remaining high-frequency sound.

The reason is not really stealth against the prey's ears, or not only. An owl hunts by listening, and the loudest thing in its world is itself; silencing the wings is largely about being able to hear a mouse under snow while flying. Barn owls can locate prey in complete darkness by sound alone, using ears set at different heights on the skull so a sound arrives at each at a slightly different time and angle.

The trick has been borrowed. Serrated trailing edges modelled on owl feathers now appear on wind turbine blades and have been trialled on aircraft, and the noise reduction is real. It costs something, though: the same soft feathering makes owl plumage poorly waterproofed, which is why an owl caught in heavy rain is in genuine trouble.