Strigidaetypical owls

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The family Strigidae is the larger of the two families of owls, with close to 190 species distributed among 25 genera. The internal taxonomy is very complicated, owing in part to the similarities between species, and in part to the fact that many have been little studied. They are found worldwide, on every continent of the world except Antarctica, but 80% of the strigids are found in the tropics. Strigid owls cover nearly all terrestrial habitats, though 95% are forest-dwelling species. Most of the typical owls are non-migratory. Less than 10% of species have migratory populations within some part of their range, although some species do have seasonal habitat shifts.

The strigid owls are a diverse family, ranging in size from 40 g to 4 kg. They differ from their tytonid relatives in a number of minor osteological features. The characteristic facial disk in strigid owls is circular, rather than heart-shaped, and their eyes are relatively larger. Many species have thickly feathered legs; their well-developed talons have a smooth edge on the claw of the third toe (in contrast to the pectinate claw of the tytonids), which is longer than the second toe. These owls have large heads, large, slightly elongated eyes, a short, hooked bill that points downwards. Their sternum has four notches, and is not fused with the furcula. Nonetheless, they do share many features with their tytonid relatives. They have large wings and strong legs. Their talons are sharp and hooked and their feet are zygodactyl with a reversible fourth toe. Plumage is soft and dense; feathers lack an aftershaft and have a downy base. Strigids tend to be cryptically colored and some species have dark nape patches that resemble false eyes. Many species have ear tufts that have been suggested to have behavioral functions; they do not function in sound acquisition.

Strigid owls feed on a variety of prey items, with small mammals forming a large part of the diet of some species. Eagle owls, the most powerful of strigid owls, can even handle larger mammalian prey such as foxes, young roe deer, and monkeys. However, not all species focus on mammals. Many are insect specialists, some hunt birds or bats, and both the fish-owls and the fishing-owls prey mainly on freshwater fish. Although several of the strigids hunt during daylight, all species are nocturnal to some degree. Like the tytonid owls, they are well-adapted to hunting under low light conditions. Their eyes have many more rods than cones, which is unusual compared to most birds,and makes them more sensitive to light. The eyes are long and are more or less fixed in place, supported by a bony sclerotic ring (see Strigiformes). Their hearing is extremely sensitive, owing in part to the structure of their facial ruff and disk, and many species have feather adaptations that allow them to fly nearly silently (see Strigiformes).

Courtship involves aerial displays in some species; in many others it may only involve ritualized feeding in which the male brings food to the female. The owls are extremely adverse to building nests; they will utilize nests of other species or tree cavities; some species nest on the ground. Some smaller species are very particular in the their nesting choices, nesting mainly in cavities excavated by woodpeckers. The burrowing owl is unique in that they nest in underground burrows dug by mammals. Females generally lay a clutch of 4-7 eggs, though some smaller species have smaller clutches, and in times of abundant food, some species may lay larger clutches. In the northern saw-whet owl the female may reach 150% of her non-breeding body mass while egg-laying! Females usually begin incubating with the first egg laid, resulting in a large size skew among young once all are hatched. Once incubating, females will generally leave the nest only once or twice a night to defecate; the male will provide food to the female and to newly hatched young. Females will not resume hunting until later in the brood-raising period. Because they are the only incubators, females are usually the primary nest-defenders in the face of an enemy. Some species are extremely aggressive towards humans, especially during this crucial period. When threatening an intruder, the owl will crouch down, lower its head, droop its wings, and ruffle its feathers. It will usually vocalize or bill-clap as well, in an effort to scare away the unwanted visitor or predator. Most of the strigid owls spend their time alone or in mated pairs all year. Only four species are considered 'social' when breeding, though none are truly colonial. Several species roost in colonies during the winter.

The strigid owls have been divided into 2 subfamilies; Buboninae (with ~21 genera) and Striginae (with 6 genera). A current classification, based on skull morphology, divides them into 3 subfamilies; Striginae (13 genera), Surniinae (8 genera), and Asioninae (2 genera).

The fossil record for owls dates back to the Paleocene, with a major radiation of families and species in the Eocene (~50 MYA). The appearance of the family Strigidae is somewhat uncertain; several fossils that were originally thought to belong to this family were actually tytonid owls. The first real evidence for the strigid owls appears in the lower Miocene (22-24 MYA) in both North America and Europe, after which these owls probably superceded the tytonid owls.

Owls have been part of human folklore for thousands of years. From beneficial protectors to deliverers of evil, owls have been important in human superstitions across cultures throughout the world. The first known cave paintings of owls were produced 15,000-20,000 years ago in France. The variety of myths surrounding owls range from the idea that they bring bad luck, announce death, or take away souls, to the belief that they provide cures for ailments, ward off evil spirits, and guide the dead on their journeys. Many superstitions persist to this day.

In rural communities throughout North America and Europe, however, farmers are realizing the benefits of having nesting owls nearby, and conservation groups in various places have started installing nest boxes to facilitate breeding. In general, we have little information on many strigid populations, but it is likely that many species are at risk. Because the majority of strigid owls are found in the tropics, in rainforests or on islands, they are extremely vulnerable to habitat destruction. By one estimate, 21 species are in danger of extinction, and 14 are near-threatened. The spotted owl ,*Strix occidentalis*, threatened by logging in the Pacific Northwest, is only one of many species that are affected by forest fragmentation. Some species are able to coexist with humans, especially those that live in more open areas; for many others it will take a concerted effort to preserve their habitat to protect the species.

Campbell, B. & E. Lack, eds. 1985. A dictionary of birds. Buteo Books, Vermillion, South Dakota.

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).

Glossary

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate