roller

[roh-ler]

roller, common name for brightly colored Old World birds noted for performing somersaults in flight. They include the rollers proper (subfamily Coraciinae) and ground rollers (subfamily Brachypteraciinae) of the family Coraciidae, as well as the monotypic cuckoo roller (Leptostomus discolor) of another family, Leptostomatidae. The rollers comprise approximately a dozen species of solitary, jaylike birds, widespread throughout the tropical and temperate areas of the Old World. They are stout-bodied and large-headed birds, ranging from 91/2 to 13 in. (24-33 cm) long, with long, straight beaks that end in hooked tips. Their colors run to greens, blues, and reddish or yellowish browns, with little distinction between sexes. Rollers are strong flyers and feed while on the wing, usually on insects and small birds but occasionally on fruit. They lay their three to six white eggs in tree or rock holes, to which they add bits of grass, straw, or feathers. The slightly smaller tropical broad-billed rollers (genus Eurystomas) do not actually tumble or roll in flight. The five species of ground rollers are confined to the island of Madagascar. They differ from the true rollers in being ground feeders and thus show the expected adaptations of this way of life: longer and stouter legs; shorter, more rounded wings; and less bright but more cryptic coloration. Four species inhabit the forest floor, and one, the 18-in.-long (46-cm) Uratelornis chimaera, dwells in arid scrub. Ground rollers feed on insects and small animals and build their hole nests in the ground. The cuckoo roller is also found on Madagascar, as well as on the nearby islands of Comoros and Mayotte. It is about 17 in. (43 cm) in length and somewhat resembles the cuckoo in its coloration and its crested head. It differs from all other rollers in the possession of an outer toe capable of being turned backwards and a bill overhung with large tufts of feathers. A creature of forest and brushland, it feeds on large insects and lizards and lays its eggs in a tree-hole nest. Rollers are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Coraciiformes, families Coraciidae and Leptostomatidae.

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roller-skating

Recreation and sport in which the participants use roller skates (shoes with sets of wheels attached) to move about on special rinks or paved surfaces. The invention of roller skates is traditionally credited to the Belgian Joseph Merlin in the 1760s, but the first practical four-wheel skate was designed in 1863 by James Plimpton of Medford, Mass. Roller-skating speed events became popular in the early 20th century. Later, team competitions in “roller derbies” on banked tracks became a spectator sport. Other roller-skate contests, such as acrobatics and hockey, followed. In the late 20th century, roller skates gave way to in-line (Rollerblade) skates, in which a single row of wheels is used in place of the standard rectangular configuration.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

roller bearing

One of the two types of rolling, or antifriction, bearings, the other being the ball bearing. Like a ball bearing, a roller bearing has two grooved tracks, but the balls are replaced by rollers. The rollers may be cylinders or shortened cones. If the rollers are cylindrical, only radial loads (perpendicular to the axis of rotation) can be carried, but with conical rollers both radial and thrust, or axial, loads (parallel to the axis of rotation) can be carried. In a given space, a roller bearing can carry a greater radial load than a ball bearing can.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Roller

The rollers are an Old World family of near passerine birds, related to the kingfishers and bee-eaters. The group gets its name from the aerial acrobatics some of these birds perform during courtship or territorial flights.

Description

Rollers resemble crows in size and build, ranging from 25 to 27 centimetres in length. They share the colourful appearance of those groups, blues and browns predominating. The two inner front toes are connected, but not the outer one.

They are insect eaters, with Eurystomus species taking their prey on the wing, and those of the genus Coracias catching it on the ground. They often perch prominently whilst hunting, like giant shrikes.

These are birds of warm climates. They nest in an unlined tree-hole, and lay 2-4 eggs. The eggs hatch after 17-20 days, and the young remain in the nest for approximately another 30 days.