Thick-knees

(Burhinidae)

Class Aves

Order Charadriiformes

Suborder Charadrii

Family Burhinidae

Thumbnail description
Medium-sized, long-legged, terrestrial birds with highly developed vocalizations, striking patterns on the spread wing, and stout, pointed bills for catching, manipulating, and breaking up tough prey

Size
12.5–23 in (32–59 cm); 0.65–2.4 lb (0.293–1.13 kg)

Number of genera, species
2 genera; 9 species

Habitat
Semi-arid to arid areas with low or sparse vegetation, watersides including rivers and sea coasts

Conservation status
No species endangered or vulnerable; Near Threatened: 3 species

Distribution
South, southeast, and southwest Asia, southern and western Europe, Australia, Africa, the Middle East, and central and northern South America

Evolution and systematics

Taxonomically, thick-knees sit uneasily between the stilts (Recurvirostridae) and the coursers and pratincoles (Glareolidae) in most systematic lists; they also have some characteristics akin to the bustards (Otididae). The bustard-like appearance, which is reflected in old, local names for some species and in past classification in the Gruiformes, is likely to be a result of convergent evolution. In their ground-breaking checklist, Burt Monroe and Charles Sibley listed thick-knees after the Chionidae (sheathbills) and before the Charadriidae.

Two species in the genus Esacus are generically separable from the seven species in the genus Burhinus. The two Esacus species are larger than the others, and both have particularly large, heavy bills and similar diets and habitats. In the past they have themselves been separated into two genera, based largely on bill structure and plumage pattern. In the Peters 1934–1986 taxonomy, the two species are broken into two genera: Esacus and Orthoramphus, each with one species.

Variation within species is limited, although 19 subspecies are recognized. Division into subspecies is based on small differences in size and color, especially in the widespread species such as the stone-curlew (B. oedicnemus) of Europe, North Africa, and Asia.

Physical characteristics

All thick-knees are long-legged and have three short, thick toes and a pointed bill. The two Esacus species have a powerful bill with a markedly upswept lower mandible. All thick-knees have a round head on a slim, waisted neck, a broad, bulky body, and long tails that are narrow and tapered when closed and held pointing slightly downwards. Strikingly large, round eyes, a pale bill base, and various combinations of stripes above, through, and below each eye create bold plumage patterns. The head stripes tend to be more eyecatching at long range, but the piercing eye is most arresting up close. A wide iris contracts greatly in poor light to open a very large pupil but makes a vivid yellow or amber disk in bright sun.

Plumages are pale and sandy brown with white undersides; most thick-knees have a dark-edged, pale panel across the folded wing. Wing patterns are streaked, spotted, or plain, but in flight all reveal black-and-white patterns above and below the wings and tail. Sexes are almost alike and juvenal plumages are similar to those of adults. Downy chicks' complex patterns mimic stony, sandy ground.

Distribution

Most Burhinus species are Old World birds; two are Neotropical. The double-striped thick-knee (B. bistriatus) is found in central and northern South America from Mexico to Brazil, whereas the Peruvian thick-knee (B. superciliaris) occupies a Pacific coastal strip from southern Ecuador through Peru. In Europe, Africa, and Asia, distribution of thick-knees is much wider, ranging from as far north as

Britain in the case of the stone-curlew (B. oedicnemus) to the southern tip of South Africa (the spotted dikkop [B. capensis]), and from the Canary Islands off northwest Africa east to Southeast Asia and Australia, where the bush thick-knee (B. grallarius) is common. The great thick-knee (E. recurvirostris) ranges from Iran east to China, and the beach thick-knee (E. magnirostris) is found more southerly, from Malaysia and the Philippines to coastal Australia.

Habitat

Several species have adapted to agricultural development in open, flat landscapes, but they often lose eggs and chicks to agricultural operations and to trampling by livestock, and feeding opportunities are reduced as extensive agriculture is intensified. Original habitats of double-striped and Peruvian thick-knees, the stone-curlew, and the bush thick-knee are bushy to open grassy landscapes, bordering on semi-arid in hotter areas, with exposed sandy soil. Stone-curlews in Britain once occupied coastal shingle ridges but disappeared from this habitat as human activity increased. The African spotted dikkop inhabits quite arid places away from water, as well as open savanna woodland. The water dikkop (B. vermiculatus) of southern Africa and the Senegal thick-knee (B. senegalensis) from farther north in Africa are typically waterside birds, although they usually live close to rivers that run through semi-arid or even desert countryside. Groups of Senegal thick-knees are often seen on sand-banks beside the Nile against a backdrop of the golden cliffs of the Egyptian desert. Senegal thick-knees nest on flat roofs in big cities such as Cairo, water dikkops may be found on village playing fields, and double-striped thick-knees gather close to ranches and are frequent victims of road traffic at night.

Behavior

The Burhinus thick-knees become active at dusk, having stayed quiet and immobile by day. They call loudly as night approaches, with far-carrying, strident, or fluty calls. As pairs fly to feeding places their bold wing patterns show well in fading light. Stone-curlews breed in isolation or in loose groups where limited habitat concentrates a few pairs. They are mostly solitary except when gathering to molt just before autumn migration. Senegal thick-knees and water dikkops, however, are found in small, close groups. In midday heat, they frequently find deep shade beneath bushes and are difficult to find. Senegal thick-knees make the air ring with their eerie calls around city roofscapes after dark. Bush thick-knees melt into the background with their cryptic coloration on open ground with scattered dead branches and fallen leaves.

Great thick-knees tend to avoid open sand and resort to rocks, stony banks, and muddy places along rivers or around large lakes; they are mostly active by night. Beach thick-knees are mostly seen by day, but it is not clear when they are most active.

All thick-knees are terrestrial birds and perch no higher than on a fallen log or rock (except for the rooftop-perching Senegal thick-knee). They fly low but strongly when moving between nesting, roosting, and feeding places or if disturbed; otherwise, they are ground-dwelling birds. Courtship and territorial aggression are ground activities with loud calls, presumably because their nocturnal nature precludes extensive display flights. If undisturbed, a thick-knee may rest on its tarsi, or stand with its body markedly sloping, tail down, head withdrawn into the shoulders, but a long-striding, feeding bird has a special elegance, if a somewhat furtive character.

Feeding ecology and diet

Thick-knees walk slowly, looking for prey on the ground. Large beetles and birds' eggs are slow-moving or immobile prey that are easily approached. When a thick-knee spots food, it tilts forward to pick up the item in its bill. Thick-knees use their sharp bill tip to hammer large prey and to break it up if necessary before it is swallowed.

The beach thick-knee eats crabs and uses its massive, powerful bill to break them into manageable pieces. Otherwise, preferred foods are similar across species: the Senegal thick-knee eats beetles, crickets, grasshoppers, crustaceans, mollusks, worms, frogs, and a few small rodents, while the stone-curlew also eats earwigs, snails, slugs, lizards, frogs, and even shoots of low-growing plants. Much food is associated with animal dung, and stone-curlews fly up to two miles at dusk to suitable pastures where sheep graze or pigs forage. Grass becomes too long and dense for stone-curlews if it is not grazed, and some nature reserves specially managed for stone-curlews are fenced to concentrate and protect dense populations of rabbits that keep vegetation very short.

Reproductive biology

Northern species nest in late spring whereas tropical species breed whenever the opportunity arises, which is often related more to river and lake levels than to time of year. Stone-curlews are monogamous and pair for life, and other species may do the same. Stone-curlews that winter in southern Europe or Africa arrive in spring in the United Kingdom already paired, reinforcing strong pair bonds by ritual displays. The black-and-white patterns of wings and tail probably form an important part of nocturnal or crepuscular displays, but there appears to be little in the way of posturing or aggression. While feeding, pairs keep in close contact by regular calling.

Nests are mere scrapes in soft earth, selected by the pair as they bow together towards the preferred spot. The male finally pinpoints the site and the female shuffles onto it to scrape earth away with her feet. Small twigs, pebbles, and bits of detritus are scattered around the scrape, but several others may be created in this way before a final choice is made.

The beach thick-knee lays one egg; all other species lay two, occasionally three, in a clutch. Eggs are pale with brownish spots and streaks in a camouflage pattern. Senegal thick-knees and spotted dikkops surround the eggs with twigs, stones, and animal droppings. Incubation, by both parents,

lasts from 24 to 27 days; empty eggshells are carried away by parent birds to prevent predators from locating the nest.

Newly-hatched chicks dry and leave the nest before they are a day old. They are fed by their parents for a short time but quickly learn to find food themselves, although they are still protected by their parents and are often brooded beneath a wing. If a predator threatens the young, adult thick-knees will display with fanned wings and tail, but they rarely feign injury to draw the intruder away. Sheep, which may trample eggs or chicks, are driven off by thick-knees with fanned wings and tail. The chicks make their first flight at around 42–50 days old. They do not normally breed until they are two or three years old.

Conservation status

Stone-curlew populations have declined in Europe and populations in the United Kingdom have recovered only with intensive efforts involving cooperation between conservationists and farmers. Farming operations are apt to destroy nests and intensification of farms, especially a reduction in the variety of crops and increased height and density of plants in spring, removes breeding opportunities. Security of the United Kingdom population may require annual efforts to protect individual pairs and nests, and the species would decline without such special consideration. Whether the United Kingdom populations are sustainable remains to be seen.

South American species have declined with increased disturbance and development, and the Peruvian thick-knee may now be in a precarious state. Recent assessments of its status are somewhat optimistic according to Ridgely and Greenfield. In Australia, bush thick-knees have suffered long-term declines in numbers and range because of habitat loss. Claims that fox predation, poisoned baits, shooting, and egg collecting have been the cause of the decline confuse the issue. The beach thick-knee is thinly scattered and its beach habitat is especially vulnerable to development and disturbance. The open ground occupied by most species is always likely to be subject to human disturbance and development and many habitats are considered "waste" by most people.

Significance to humans

Birds that are scarcely seen by day but make loud noises at night give rise to stories in simple rural communities. However, as people have become more dissociated from the land and its wildlife, awareness of the bird or even its voice has declined. Old names for stone-curlews, such as goggle-eyed plover, thick-kneed bustard, and Norfolk plover, suggest some familiarity with the species, but few people living in the twenty-first century would recognize such names. Bush thick-knees appear in folklore in Australia, with onomatopoeic names such as Weeloo and Willaroo, and they gave rise to suspicion and unease among early white settlers. The double-striped thick-knee, however, has at times been kept as a semi-domesticated bird to reduce insect infestations. It was welcome around farms and settlements.

Species accounts

List of Species

Spotted dikkop

Burhinus capensis

taxonomy

Oedicnemus capensis Lichtenstein, 1823, Cape of Good Hope. Four subspecies.

other common names

English: Spotted thick-knee, Cape dikkop, Cape thick-knee; French: Oedicnème bistrié; German: Dominikanertriel; Spanish: Alcaraván Venezolano.

physical characteristics

16–17.5 in (37–44 cm) Most obviously spotted thick-knee, rich rusty-brown with bold black arrow-head spots, whiter below with fine streaks and white on the face, behind the eye, and on the throat. The bill base, eyes, and legs are rich yellow. This is an elongated species, long-winged and long-tailed.

distribution

Found in a narrow sub-Saharan band from Senegal on the west coast to the Horn of Africa in the east, with a small, isolated population in Arabia; extends south, more widely and commonly, through East Africa to Zambia and in a broad zone across the south of the continent. The distribution reflects unsuitable areas of the Sahara in the north and the Congo basin forests in West Africa.

habitat

Semi-arid areas away from marshes and rivers, but sometimes more or less wooded in dry savannas or around clumps of trees and bushes near open grasslands; such cover is important in providing daytime shade and shelter. Sometimes found on playing fields, parkland, and near coastal beaches.

behavior

Seeks shade and cover by day; pairs or small groups remain quiet and still until dusk except on dull, cloudy days when they may feed.

feeding ecology and diet

Typical plover-like feeding action involves a steady forward walk and a swiveling, tilting motion to pick up food from the ground in the bill. Large insects are favored, but a number of crustaceans and frogs are eaten, as well as a limited amount of vegetable matter, mainly seeds. It feeds mostly at night and entirely on the ground.

reproductive biology

Pairs are monogamous, probably established for several seasons, breeding in the dry season or at the beginning of seasonal

rains. The nest is a simple hollow made on bare ground with the feet, sparsely lined with small pebbles, grass, leaves, and dried animal droppings. Lays two eggs (less often one or three) and incubation lasts 24 days; chicks fly at eight weeks.

conservation status

Its ability to cope with human activity and limited alteration of open habitats allows the spotted dikkop to thrive in quite well-populated areas, so long as disturbance and persecution remain slight and housing or industrial development is absent. Generally quite numerous in suitable areas, one of the more common and most secure thick-knees.

significance to humans

Most African people take little or no specific notice of the spotted dikkop in areas where nocturnal noise from an assortment of birds and mammals is common.

Beach thick-knee

Esacus magnirostris

taxonomy

Edicnemus magnirostris Vieillot, 1818, Depuch Island, Western Australia. Monotypic.

other common names

English: Beach stone curlew, beach curlew, Australian stoneplover; French: Oedicnème des récifs; German: Rifftriel; Spanish: Alcaraván Picogruesco Australiano.

physical characteristics

21–22.5 in (53–57 cm) The largest thick-knee, massively built and thick-legged, with a long, strongly-upcurved bill. Plain gray-brown above, marked with a blackish shoulder bordered below by a thin white line, above pale gray wing coverts; rusty patch under tail. Dark crown and black cheek stripe separated by wide white stripe over eye; white lower cheek and throat patch. Bill largely black with small yellow base; eyes and legs yellow.

distribution

Scattered through the Andaman Islands to the Philippines, Indonesia, New Guinea, and many islands of the Southwest Pacific, and south to the north and northeast coasts of Australia.

habitat

Found on island shores and mainland beaches, whether of sand, shingle, rocks, or mud, in wide open spaces or restricted to narrow beaches fringed by mangroves or rocks. Often feeds in intertidal areas and equally at home on windswept estuaries, sheltered river mouths, and exposed rocks.

behavior

Usually found along the beach close to the water's edge, the beach thick-knee tends to fly off over the sea if disturbed before sweeping back to the shore. It may resort to undisturbed dunes and sand flats a little way inland, or even to the shores of shallow, coastal lagoons. It will rest on offshore reefs, and even enters quite tall and moderately dense mangroves.

feeding ecology and diet

The main food is crabs, where they are common, but other crustaceans may also be taken when crabs are scarce. Larger crabs are broken up before being swallowed. Typical feeding technique is a slow, quiet stalk, followed by a lunge or sudden fast run to snatch up prey, but the beach thick-knee will also probe mud and sand. It does not wade in water.

reproductive biology

Monogamous. Breeds at low density in isolated pairs, frequenting regular territories for many seasons. The nest, on a sand-bank or spit, is a simple depression, occasionally ringed (but not lined) with bits of vegetation. Lays just one egg, which is incubated for 30 days; young develop slowly and fly after 12 weeks, but they remain with adults for up to a year.

conservation status

Locally secure but in general faces increasing threats from disturbance as beaches are subject to human pressures, including tourist and hotel development, off-road vehicles on beaches, and other disruptive activities. Faces potentially large, locally catastrophic, but uncertain threats from rising sea levels with global climate change.

significance to humans

None known.

Resources

Books

Blakers, M., S.J.J.F. Davies, and P.N. Reilly. The Atlas of Australian Birds. Melbourne: Melbourne University Press, 1984.

Cramp, S., and K.E.L. Simmons, eds. Birds of the Western Palearctic. Vol. 3, Waders to Gulls. Oxford: Oxford University Press, 1983.

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 3, Hoatzin to Auks. Barcelona: Lynx Edicions, 1996.

Hayman, P., J. Marchant, and T. Prater. Shorebirds. London: Croom Helm, 1986.

Marchant, S., and P.J. Higgins, eds. Handbook of Australian, New Zealand and Antarctic Birds. Vol. 2, Raptors to Lapwings. Melbourne: Oxford University Press, 1993.

Monroe, Burt L. Jr., and Charles G. Sibley. A World Checklist of Birds. New Haven, CT: Yale University Press, 1993.

Peters, James Lee. Check-list of Birds of the World. Cambridge: Harvard University Press, 1934.

Ridgely, Robert S., and Paul J. Greenfield. The Birds of Ecuador. London: Christopher Helm, 2001.

Sibley, Charles G., and Burt L. Monroe Jr. Distribution and Taxonomy of Birds of the World. New Haven, CT: Yale University Press, 1990.

Urban, Emil K., C. Hilary Fry, and Stuart Keith, eds. The Birds of Africa. Vol 2. London: Academic Press, 1986.

White, C.M.N., and Murray D. Bruce. The Birds of Wallacea. London: The British Ornithologists' Union, 1986.

Rob Hume