**1. Introduction**

Owls are one of the most distinctive-looking birds in the world. With their upright stance, large head with forward-facing round eyes, flat facial disc and soft fluffy plumage, they cannot be mistaken for anything else. This distinctive outward appearance is the result of many unique evolutionary adaptations, which have enabled the owl to become a highly efficient crepuscular and nocturnal predator. While they share the night skies with insectivorous Caprimulgiformes, such as Nightjars, Frogmouths, Potoos and Oilbirds, and with Owlet-nightjars of Aegotheliformes, the true nocturnal owls are unlike their diurnal raptor counterparts. Whereas the diurnal raptors, consisting of Eagles, Falcons, Hawks, and Buzzards, have separately evolved in response to a wide range of prey species and habitats, the owl is singularly the only true nocturnal raptor. Species of owls are found on every continent and nearly in every country of the world, except Antarctica and some small isolated islands, and can thrive in habitats as diverse as frozen tundra, equatorial rainforests, temperate northern forests, and even open grasslands and deserts.

#### **Figure 1.**

*Tytonidae—Strigidae skull comparison. L—Barn owl (Tyto alba). R—Little owl (*Athene noctua*). Photo: Alan Sieradzki.*

Owls belong to the taxonomic order of Strigiformes, which is divided into the two families of Strigidae (Typical Owls) and the much smaller family of Tytonidae (Barn, Bay and Grass Owls). While there are some distinct anatomical differences between the two families, most notably in the structure of the skull (**Figure 1**), both families share the same adaptations that make them owls. There are approximately 250 known species of owl in the world, ranging in size from the diminutive Elf Owl (*Micrathene whitneyi*) to the enormous Eurasian Eagle Owl (*Bubo bubo*). While there are minor variations in the individual physiology between certain species due to the type of prey they take and the habitat they operate in, they are all subject to the same unique adaptations in eyes, hearing, head rotation, feathers structure, digestive system, and hind limb musculature.

Recent research has used a genome-wide scan to uncover the genetic and selective mechanisms that are the basis of the owl's unique sensory adaptations. As predicted, a primary finding of the study was that genes involved in sensory perception showed a genome-wide signal of positive selection. This category included genes involved in acoustic and light perception, photosensitivity, phototransduction, dim-light vision, and the development of the retina and inner ear. Genes involved in circadian rhythms, which regulate the body's internal clock, also showed evidence of accelerated evolution, as did some genes related to feather production [1].
