Animal Diversity Web

Although their internal taxonomy is currently debated, two owl families are commonly recognized, the barn-owls (Tytonidae) and the typical-owls (Strigidae). Over 200 species are distributed among approximately 27 genera. Owls are found world-wide, covering nearly all types of terrestrial habitats.

A diverse order, owls range in size from sparrow- to eagle-sized. They are known for many characteristics, including their well-developed talons, soft plumage, and notoriously silent flight. Their legs are strong, and feathered in many species; they have zygodactyl raptorial feet. All species have a characteristic facial disk, which is circular among strigid owls and heart-shaped in the tytonid owls. Their holorhinal nostrils are found at the base of their short, downward-pointing bills, and their palates are desmognathous or schizognathous. They have unusually fleshy tongues, and lack crops, but have large caeca with club-shaped ends. Their large wings allow them to fly slowly, and their plumage is often cryptic, and many species have different color phases. Their feathers have either absent or rudimentary aftershafts. Most species are active hunters at night, and as such they have several important adaptations for this lifestyle. Their forward-facing eyes are large, elongated, and have slightly thickened corneas. The number of light-sensitive elements (rods) in their retinas is high, especially compared to other birds. This allows them a heightened sense of vision in dim light, but contrary to popular belief, they cannot see in total darkness. At least some species have been shown to have color vision, it is not known whether the more nocturnal species are able to see color. Their eyes are more or less fixed in their orbits, supported by a bony sclerotic ring. Therefore they have well-developed binocular vision, but they need to turn their heads to see on either side of them. Their hunting ability is also greatly augmented by their excellent sense of hearing, and some species use their auditory capabilities to hunt prey that they cannot see (such as rodents running under the snow in winter). The facial disk and facial ruff are both important in their sense of hearing. The facial ruff is composed of paired layers of densely packed feathers that have large rachides and reduced vanes; these are inserted behind the ear openings in a special flap of skin. The facial disk, which lies over the ruff, is composed of feathers with open vanes. Together these form parabolic troughs leading to the ears that can increase sound pressure tenfold. The facial ruff and disk are larger in highly nocturnal species, as well as those that hunt prey travelling under the snow. Their hearing is more sensitive to low-frequency sounds than most birds, and is sensitive across a wide range of frequencies. They have a wide outer ear tube and a large inner ear, and the auditory region of the brain has more nerve cells than that of other species of comparable size, which allows them to detect sounds more effectively.

Owls hunt a wide variety of prey, ranging from small mammals to birds, lizards, and insects; there are even fish-eating owls in Africa and Asia. Prey is often swallowed whole, and the fur, feathers, and bones are later regurgitated in pellets. Many owl species, especially those that hunt during night hours, are able to fly nearly silently. Several special feather adaptations make this possible. The leading edges of their outer primaries have stiff, comb-like fringes that reduce noise, the trailing edge of their primaries and secondaries have soft fringes that reduced turbulence behind the wings, and their primaries, secondaries, and wing coverts are covered in downy feathers that also reduce noise. Silent flight may be less important in those species that hunt over water or during the day, and many of these species do not have these adaptations.

Owls do not build nests, instead they use nests of other species or utilize tree cavities. Some nest on the ground and one species nests in underground burrows dug by mammals. In all species the female does the majority, if not all, of the incubation; the male hunts prey and feeds both the female and the young. In some species the male will present food to the female in a courtship display. Females are generally larger than males. Clutch size may vary with prey abundance; some species may not breed at all in times of food scarcity. Most owls are sedentary, some may occupy territories year-round, and pairs may remain together outside of the breeding season. Very few species are truly migratory, though some species may have migratory populations; other species may shift habitats seasonally, still others are nomadic and are continually moving.

Owl taxonomy has been particularly confusing. Because of convergence with many of the features found in species of the order Falconiformes, owls and falcons at one time were considered superfamilies within the same order. Linnaeus placed them within the same group, and this classification persisted for 130 years. They have alternatively been grouped together with the nightjars. The classification we use recognizes three distinct orders for owls, falcons, and nightjars. After much debate, owls are considered most closely related to the Caprimulgiformes, the nightjars, though the evidence is still somewhat unclear.

Owls are well represented in the fossil record, with several families dating to the Paleocene era, approximately 58 MYA. There was a major radiation of owls in the Eocene, producing four or more families, including Tytonidae. It appears that there was a great diversity of the tytonid owls in Europe during the Paleogene. The appearance of the family Strigidae is uncertain, though can currently be traced back to the lower Miocene (22-24 MYA) in French and North American deposits. Therefore, the tytonid owls appear to have evolved first, in the Paleogene, with the strigid owls originating in the Neogene. We have learned from the fossil record that in some cases different lineages of owls evolved "gigantism"; for example there were three species of giant barn owls. The largest owl known to have existed comes from the Pleistocene in Cuba; Ornimegalonyx oteroi was larger than a meter tall with the most powerful claws of all owls. In addition, during the Holocene 4 species of owls in the genus Gallistrix evolved on the Hawaiian islands; all had long legs and short wings and are thought to have hunted birds, convergent with the accipter hawks; all are extinct now.

Owls have been part of human folklore for thousands of years, across many cultures. The first known cave paintings of owls were produced 15,000-20,000 years ago in France. Wall paintings and mummified owl remains of have been discovered in ancient Egyptian tombs, and the owl motif was used in Egyptian as well as Mayan hieroglyphics. In some cultures, owls are associated with wisdom and courage while in others they represent trickery and evil. Australian aboriginals believed in an owl that was the son of a half-human spirit. In some parts of Asia the owl was thought to ward off evil spirits. Negative associations with owls are probably in part due to their nocturnal habits; a variety of myths are based on the ideas that owls bring bad luck, death, or take away souls. African lore considers owls to be possessed by demons. Many of these superstitions persist to this day.

Many tropical owl populations are at risk from habitat destruction such as clearcutting and forest fragmentation. Pesticides have also had deleterious effects on owl populations, particularly on local scales when dangerous chemicals are used to control rodents. In many cases, we simply do not know enough about the species to know what impact we are having on their populations. Owl conservation, however, is becoming more popular, and various conservation groups have started installing nest boxes to facilitate breeding. For those species that will use man-made structures, programs to install nest boxes or platforms can have positive effects on population sizes. For the many species that depend on tree cavities or other natural nest sites, only efforts to protect their habitat will ultimately help the owls.

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del Hoyo, J., Elliott, A., & J. Sargatal. 1992. Handbook of the birds of the world, volume 5. Lynx Edicions, Barcelona.

Feduccia, A. 1999. The origin and evolution of birds. Yale University Press, New Haven.

Johnsgard, P.A. 1998. North American owls: biology and natural history. Smithsonian Inst. Press, Washington.

Sibley & Ahlquist. 1990. Phylogeny & classification of birds: a study in molecular evolution. Yale Univ. Press, New Haven.

Thomson, A. 1964. A new dictionary of birds. British Ornithologists' Union. McGraw-Hill Book Co., NY.

Contributors

Danielle Cholewiak (author).