Like many other birds, owls achieve flight by flapping their wings. Owls, however, have a special adaptation that allows them to fly nearly silently and take prey by surprise on dark nights.
Flapping flight in birds, including owls, involves a downstroke and an upstroke. The downstroke is a downward and forward motion that propels the bird forward. The ends of the primary feathers separate during the upstroke, and the wing folds inward towards the body to reduce drag.
In most birds, air passes over the wing surface, creating turbulence and a rushing sound. However, the leading edge of an owl primary is comb like, while the opposite edge has soft fringes. This fringing breaks up the turbulence and reduces sound, making owl flight near silent.
One benefit of near-silent flight is the ability to surprise prey in a dark environment. The absence of sound also means less interference with the owl's own sense of hearing, giving the bird better auditory acuity.
Not all owls are nocturnal. Some species native to far northern regions, like the snowy owl and great grey owl, are at least partly diurnal. This is partly the result of fewer hours of darkness during the arctic summer. In these species, the comb-like feather edges are absent.