Rollers

(Coraciidae)

Class: Aves

Order: Coraciiformes

Suborder: Coracii

Family: Coraciidae

Thumbnail description
Medium-sized, brightly colored birds with stout, hook-tipped bills and small, strong syndactylous (toes merged with no intermediate web) feet

Size
9–20 in (22–50 cm); 0.16–0.5 lb (80–250 g)

Number of genera, species
5 genera; 17 species

Habitat
Forests, woodlands, savanna, urban

Conservation status
Vulnerable: 4 species

Distribution
Africa, southern Europe, and southern Asia to northeastern and southeastern Asia, and Australasia, east to the Solomon Islands

Evolution and systematics

Although common throughout much of their range, rollers are poorly known. This scarcity of information is reflected in a sketchy understanding of their evolutionary history. The present predominantly Afrotropical distribution suggests that they originated in ancient Africa, but in The Origin and Evolution of Birds (1999), Alan Feduccia points out that the few fossils identified confidently as belonging to the family come from Europe. Dating from the late Eocene and Oligocene (35 million years ago), the fossil Geranopterus alatus was unearthed from deposits in France, and is similar to modern Coracias species. Three unnamed species date from the middle Eocene (45 million years ago), and were taken from deposits in Germany. Older roller-like birds, which may be nearer to ground rollers, are known from deposits in Britain and the United States (Wyoming).

Three groups of birds known collectively as rollers traditionally are placed in the suborder Coracii, and share a series of skull characteristics that tend to separate them from other groups in the diverse Coraciiformes. Although some anatomical features denote affinities with kingfishers (Alcedinidae), motmots (Momotidae), and bee-eaters (Meropidae), a lack of many derived muscle characteristics found in other Coraciiformes and a primitive condition of the middle-ear bone suggest that rollers form a relatively primitive group. It is speculated, though apparently with little supporting evidence, that they represent generalized, primitive perching birds, and from ancient roller-like birds the now-prevalent passerines or perching birds may have evolved.

Ornithological debate has focused on relationships among "true" rollers, ground-rollers, and the strikingly aberrant courol or cuckoo rollers (Leptosomus discolor), with three separate families—Coraciidae, Brachyteraciidae, and Leptosomatidae— as the commonly adopted arrangement. Morphological evidence argues that true rollers and ground rollers had a more recent common ancestor than either had with the courol, so a more relevant arrangement could retain Leptosomatidae, but include within Coraciidae two separate subfamilies— Brachypteraciinae for the five species of ground-rollers and Coraciinae for the 12 species of true rollers.

Physical characteristics

True rollers constitute a fairly homogeneous assemblage of medium-sized, heavy-bodied birds characterized by a proportionately large head, short neck, and stout robust bill. Syndactylous feet are proportionately small but strong. Broad, long wings reflect strong powers of flight. The rather broad tail may be squarish, slightly rounded, or somewhat forked, sometimes with markedly elongated outermost feathers. Variation in size is not great, and ranges from 10 in (26 cm) and 3 oz (90 g) for the blue-throated roller (Eurystomus gularis), to 14.5 in (37 cm) and 5.3 oz (150 g) for the blue-bellied roller (Coracias cyanogaster). All species are handsomely plumaged in tones of blue or lilac, with olive, chestnut, or pink markings; young birds resemble adults. Modifications in bill structure that evolved in response to different foraging techniques are the key distinguishing feature separating the two genera. In Coracias, the shrike-like bill is strong, arched, and hook-tipped, and is suited to grasping prey captured on the ground. Eurystomus species catch flying insects, and the short, wide bill is well adapted to aerial feeding.

Ground-rollers are medium-sized terrestrial birds resembling Coracias rollers, but readily distinguished by long legs, distinctive plumage patterns, and short, rounded wings. All are stout-bodied, with a proportionately large head and a strong, robust bill. The large eyes possibly enable these birds to forage effectively in heavily shaded undergrowth, at dusk, or even during the night. Total length for four species with broad, rounded tails is 8.7–13 in (22–33 cm); a very long, graduated tail gives the long-tailed ground roller (Uratelornis chimaera) a disproportionate total length of 18 in (46 cm). Bold markings are a feature of the rich plumage coloration, with rufous or dark-green upperparts contrasting with finely patterned underparts and striking facial patterns. Sexes are alike, and young birds resemble adults.

Distribution

In 1971, when commenting on restriction of the courol to Madagascar and the nearby Comoro Islands, Joel Cracraft postulated that the first invasion of Madagascar was by primitive rollers that probably were terrestrial and arboreal. These gave rise to a lineage that became predominantly arboreal, eventually resulting in the evolution of Leptosomus. A second invasion by a more coraciid-like stock gave rise to ground-rollers, which now survive only on Madagascar. The only true roller to reach Madagascar is a recent colonizer from Africa.

Africa is the home of true rollers; eight of the 12 species occur on the continent as breeding residents. Outside Africa, they are dispersed widely through much of the Old World, from Ireland and Britain in the west to the Solomon Islands and eastern Australia in the east, and from about 55° N in the western Palaearctic to approximately 38° S in southeastern Australia. Nowhere do more than two species occur together. Western and eastern sectors of the world range each are occupied by a widespread species from different genera: the European roller (Cocacias garrulus) in the west, and the dollarbird (Eurystomus orientalis) in the east. Restricted insular ranges in the Indonesian archipelago are occupied by Temminck's roller (Coracias temminckii) and the azure roller (Eurystomus azureus).

Habitat

A strong preference for evergreen forest is shown by four species of ground-rollers, which keep to the heavily shaded undergrowth or forest floor. Occurring only in a narrow coastal strip of arid southwestern Madagascar, the long-tailed ground-roller prefers low, fairly dense deciduous woodland with a sparse groundcover on sandy soil.

Although occurring at high altitudes when crossing major mountain ranges during migration, true rollers are lowland birds, frequenting habitats that vary from tropical or subtropical woodland to most types of open country, including grassy hillsides with scattered trees, plains, scrublands, cultivated fields, and urban parks or gardens. In West Africa, the blue-bellied roller is very much an inhabitant of Isoberlinia woodlands, and the racket-tailed roller (Coracias spatulata) from southern Africa shows a preference for woodlands in which Brachystegia, Colophospermum, or Baikaea trees dominate. Other species appear to exhibit no marked habitat preferences.

Behavior

Little is known of the life and habits of shy, secretive ground-rollers, but available information indicates they are essentially terrestrial, especially when feeding, and usually encountered singly or in pairs. Instead of flying, they prefer to escape danger by running or standing motionless in a well-concealed position. The brief, guttural call notes are heard mainly during breeding season.

While on migration, some true rollers gather into loose flocks that may come together in large aggregations. However, for most of the year they are strongly territorial, normally encountered in pairs or post-breeding family parties. In the breeding territories, they are noisy and conspicuous, with loud cackling call notes that constantly accompany spectacular aerobatics. This contrasts with quiet, often lethargic behavior in wintering areas. Eurystomus species advertise territorial occupation by flying high above treetops and calling loudly, but the spectacular rolling flights undertaken by Coracias species give the birds their collective name. The flights feature prominently in territorial and courtship displays. They are made with powerful wingbeats as the bird flies up at a steep angle, then suddenly tips forward and plunges down with rapid wingbeats while rolling the body from side to side before leveling out and moving away to repeat the sequence. In fine warm weather, true rollers are active throughout the day, spending much time sitting on vantage perches awaiting the appearance of prey. Activity levels decline, often significantly, during rain. Movements undertaken by true rollers vary from regular, seasonal migration, sometimes over vast distances, to local, irregular wanderings. Toward the tropics, seasonal migration tends to be replaced by irregular local movements or post-breeding dispersals.

Feeding ecology and diet

Courols are arboreal foragers, searching amid tree or shrub branches for large insects and small reptiles, especially chameleons. Captured prey is struck repeatedly against a stout limb before being swallowed. Ground-rollers are almost exclusively ground-foraging insectivores, capturing prey by searching amid leaf litter or probing with the bill into soft soil. They take small vertebrates such as frogs or lizards. The more arboreal short-legged ground-roller (Brachypteracias leptosomus) prefers to forage from perches in low-to-mid levels of the forest.

Vantage perches are of the utmost importance to true rollers, for it is from these that birds pounce to capture prey on the ground or sally forth to catch flying insects. Coracias species take mainly large arthropods, almost always from the ground; Eurystomus rollers are almost exclusively aerial feeders, taking flying insects after sallying forth from a high, exposed perch. The latter method is considered to be the more advanced technique. Small, soft-bodied prey are swallowed whole; larger prey are brought back and struck repeatedly against the perch before being swallowed.

Reproductive biology

A summer breeding season has been recorded for ground-rollers, with egg-laying occurring mostly in December. Pairs defend nesting territories, and courtship feeding of females by males has been observed. Little is known of nesting behavior, and the nest of the short-legged ground-roller—the only species known to nest in tree hollows—was not discovered until 1996. Other species nest in a chamber at the end of a burrow excavated by the birds in the ground, and usually two, but up to four, white eggs are laid. Incubation seems to be undertaken only by the female, and nestlings are fed by both parents.

Loud vocalization during flight features prominently in courtship displays of true rollers, but spectacular aerobatics are performed only by Coracias species. Bowing displays are performed by paired birds while perched facing each other. Copulation may occur after display flights, or more frequently after the bowing display. For long-distance migrants, courtship begins during spring migration, and nesting gets underway soon after pairs arrive in breeding territories. Nests mostly are in holes in trees, but sometimes in crevices in cliff faces or building walls. Clutches are 3–5 white eggs, and for 18–20 days incubation is undertaken by both sexes, though mostly by the female. Newly hatched chicks are naked, with pin feathers first appearing at about seven days and full feathering being acquired between 17 and 22 days. Both parents feed nestlings for some 30 days and for up to 20 days after fledging.

Conservation status

Although generally common and apparently able to survive in relatively small, fragmented forest patches, the courol is adversely affected by widespread land clearing. The subspecies gracilis is restricted to mountain slopes on Grand Comoro Island, and is estimated to number about 100 pairs. Deforestation poses a more serious threat to ground-rollers, with four species listed as Vulnerable. The long-tailed ground-roller has one of the most restricted ranges of all Madagascar birds. As a group, true rollers have fared well, and most remain common throughout all or part of their ranges. Only the azure roller (Eurystomus azureus) is considered Vulnerable because of loss of forest habitat within its restricted insular range. Since the 1960s, significant local declines of European rollers have occurred in central and eastern Europe, but the species remains numerous elsewhere, with counts made in the 1970s of an estimated two to three million overwintering birds in eastern Kenya.

Significance to humans

There is little evidence of the courol being subjected to human interference, though in some districts body parts are used in love potions. Traditionally, it is seen as a bird of good omen or as representing the bond between loving persons. Only hunters show an interest in ground-rollers, which are considered easy targets. All species are killed for food. Hunting for sport, food, or taxidermy affects migrating European rollers passing through the Mediterranean region, especially on Cyprus and Malta where significant numbers are taken each year. The species is widely admired because of its colorful plumage; in agricultural districts it is considered beneficial as a destroyer of insect pests. Elsewhere, there appears to be little interest in rollers among local people.

Species accounts

Resources

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 6, Mousebirds to Hornbills. Barcelona: Lynx Edicions, 2001.

Feduccia, A. The Origin and Evolution of Birds. 2nd ed. New Haven: Yale University Press. 1999.

Forshaw J. M., and W. T. Cooper. Kingfishers and Related Birds. Vol. 3, Leptosomatidae, Coraciidae, Upupidae, Phoeniculidae. Sydney: Lansdowne Editions, 1993.

Fry, C.H., K. Fry, and A. Harris. Kingfishers, Bee-eaters and Rollers. London: Christopher-Helm, 1992.

Balanca, G., and M. N. de Visscher. "Observations sur la Reproduction et les Déplacements du Rollier d'Abyssinie Coracias abyssinica du Rollier Varié C. naevia et du Rollier Africaine Eurystomus glaucurus au Nord du Burkina Faso." Malimbus 18 (1996): 44–57.

Cassola, F., and S. Lovari. "Food Habits of Rollers During the Nesting Season." Bollettino di Zoologia 46 (1979): 87–90.

Forbes-Watson, A. D. "Observations at a Nest of the Cuckoo-roller Leptosomus discolor." Ibis 109 (1967): 425–430.

Garbutt, N. "Madagascar's Ground Rollers. Jewels among the Shadows." Africa Birds and Birding 5 (2000): 52–57.

Robel, D., and S. Robel. "Zum Verhalten der Blauracke (Coracias garrulus) Gegenüber Anderen Vogelarten im Brutgebiet." Beitr. Vogelkd. 30 (1984): 361–382.

Sosnowski, J., and S. Chmielewski. "Breeding Biology of the Roller Coracias garrulus in Puszeza Pilicka Forest (Central Poland)." Acta Ornithologica 31 (1996): 119–131.

Thorstrom, R., and J. Lind. "First Nest Description, Breeding, Ranging and Foraging Behavior of the Short-legged Ground-Roller Brachypteracias leptosomus in Madagascar." Ibis 141 (1999): 569–576.

Coraciiformes Taxon Advisory Group. Web site:

[Article by: Joseph M. Forshaw]

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