Owls number about 200 species belonging to the families Tytonidae (barn owls) and Strigidae (the other owls). The former are distinguished by their heart-shaped facial disk completely encircling face; their bill comparatively long and slender; their legs rather long; their middle claw with comb and the absence of ear tuft. They are placed as sister families in the phylogenomic study of Hackett et al (fig 2).
Owls have evolved remarkable specializations to optimize the capture of preys at night. Their head is broad to accommodate the exceptionally large eyes. Virtually immobile, the eye is rigidly encased. Wide flexibility of the neck compensates for the fixed position of the eyes; an owl can turn its head more than 180° in either direction and can thus look directly backward. The vision is binocular, and depth perception is often enhanced by moving the head away from the central plane. Various owls have only rods in the retina, resulting in an absence of colour vision but a great increase in visual acuity and light sensitivity in the dark.
The upper surfaces of the flight feathers of most species are provided with a nap that makes the flight perfectly noiseless, allowing the owl to hear prey without interference caused by the sound of flight. The ears are large and surrounded by a ruff of papery feathers that serves to concentrate the sound. The feathers covering the ear opening are lacy and permeable to sound. A movable flap (operculum) on the front margin of the opening may function as a baffle to focus sounds. Some owls, including barn owls, can locate and capture prey in total darkness, relying on their ability to localize the rustle of a mouse in leaves and to fly to that spot.
Marshall J.T. and Gill F.: Owl. (2011). Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica.
http://en.wikipedia.org/wiki/Owl An interesting review which nevertheless overlooks recent phylogenomic studies on owl systematics. See the following reference by Hackett et al.
Austin Jr, O.L, (1961), Birds of the World, Golden Press, New York, p 154