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Owls are birds from the order Strigiformes, which includes about two hundred species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers adapted for silent flight. Exceptions include the diurnal northern hawk-owl and the gregarious burrowing owl.

Owls hunt mostly small mammals, insects, and other birds although a few species specialize in hunting fish. They are found in all regions of the Earth except Antarctica and some remote islands.
Anatomy

Burrowing owl (Athene cunicularia)

Captive short-eared owl chick at about 18 days old
Owls possess large forward-facing eyes and ear-holes; a hawk-like beak; a flat face; and usually a conspicuous circle of feathers, a facial disc, around each eye. The feathers making up this disc can be adjusted in order to sharply focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey have eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting. Although owls have binocular vision, their large eyes are fixed in their sockets—as are those of other birds—so they must turn their entire head to change views. As owls are farsighted, they are unable to see clearly anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes—like feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.

Owls can rotate their heads and necks as much as 270 degrees. Owls have fourteen neck vertebrae compared to seven in humans which makes their necks more flexible. They also have adaptations to their circulatory systems, permitting rotation without cutting off blood to the brain: the foramina in their vertebrae through which the vertebral arteries pass are about ten times the diameter of the artery, instead of about the same size as the artery as in humans; the vertebral arteries enter the cervical vertebrae higher than in other birds, giving the vessels some slack; and the carotid arteries unite in a very large anastomosis or junction, the largest of any bird's, preventing loss of blood supply from being cut off while they rotate their necks. Other anastomoses between the carotid and vertebral arteries support this effect.[1][2]

Sexual dimorphism
Sexual dimorphism is a physical difference between male and females of a species. Reverse sexual dimorphism is when females are larger than males and it has been observed across multiple owl species.[9] The degree of size dimorphism varies across multiple population and species, which is measured through various traits, such as wing span and body mass.[9] Overall, female owls tend to be slightly larger than males. The exact explanation for this development in owls is unknown. However, there are several theories explaining the development of sexual dimorphism in owls.

One theory suggests that selection has led males to be smaller because it allows them to be efficient foragers. The ability to obtain more food is advantageous during breeding season. In some species, female owls stay at their nest with their eggs while it is the responsibility of the male to bring back food to the nest.[10] However, if food is scarce, the male will first feed itself before feeding the female.[11] Small birds, which are agile, are an important source of food for owls. Male Burrowing owls have been observed to have longer wing chords than females, despite being smaller than females.[11] Furthermore, owls have been observed to be roughly the same size as their prey.[11] This has also been observed in other predatory birds.[10] This suggests that owls with smaller bodies and long wing chords have been selected for because of the increased agility and speed that allows them to catch their prey.

Another popular theory suggests that females have not been selected to be smaller like male owls because of their sexual roles. In many species, female owls may not leave the nest. Therefore, females may have a larger mass to allow them to go without starving for a longer period of time. For example, one hypothesized sexual role is larger females are more capable of dismembering prey and feeding it to their young, hence why female owls are larger than their male counterparts.[9]

A different theory suggests that the size difference between male and females is due to sexual selection: Since large females can choose their mate and may violently reject a male's sexual advances, smaller male owls that have the ability to escape unreceptive females are more likely to have been selected.[11]