Lack of Ultrasonic Components in the Flight Noise of Owls

DURING the course of some observations on the ultrasonic components of the songs and call notes of a number of species of birds1, we noticed to our surprise the extent to which the wing beats of small birds produced ultrasonic ‘noises’. This raised interesting problems as to how far the silent flight of the smaller and medium-sized owls is also ‘silent’ above the normal upper limit of the human ear: for if the owls' wings are noisy at the higher frequencies, then most of the supposed advantage to owls of this adaptation would be lost, since it is likely that the vast majority of small mammals on which these birds prey possess good hearing in the ultrasonic range2.