Ciconiiformes

(Herons, storks, spoonbills, ibis, and New World vultures)

Class Aves

Order Ciconiiformes

Number of families 6

Number of genera, species 43 genera; 120 species

Evolution and systematics

Among the six families that make up the order Ciconiiformes, the New World vultures (Cathartidae) occupy the most incongruous of positions. Historically, they were classified as belonging to the order Falconiiformes. Physical and behavioral similarities to carrion-feeding, hook-beaked, bare-headed Old World vultures made their inclusion in this order seem obvious. But similarities between New and Old World vultures are actually a case of convergent evolution. DNA hybridization studies show that the New World vultures' closest relatives are storks and thus they are generally accepted by taxonomists as Ciconiiformes, placed in the suborder Cathartae.

Yet ornithologists re-classify this family in its new home with great reluctance. Even in Raptors of the World, published in 2001, Ferguson-Lees and Christie admit that "however they are treated taxonomically, they also continue to be thought of as raptors" and unashamedly include the New World vultures in this authoritative species guide.

Their closest relatives, the true storks, share their place in the suborder Ciconiae with the ibis and spoonbill family. Fossil remains of storks are extensive, with the first identifiable stork dating to the Upper Eocene.

The shoebill (Balaeniceps rex) is, as of 2001, generally placed in the same suborder as storks, but like the hammerhead (Scopus umbretta), which sits on its own in the suborder Scopi, it defies convenient classification. The shoebill has physical characteristics that could place it among storks, herons, flamingos, or plovers.

The heron family, in the suborder Ardeae, is a very old group whose origins are in the Lower Eocene, 55 million years ago. It consists of four subfamilies, broadly defined as day herons, night herons, tiger herons, and bitterns.

Physical characteristics

All Ciconiiformes share the basic characteristics: a long bill and neck, a bulky body with a short tail, large, broad wings, and long legs and toes. Members of the family Ardeidae share a pectinate, or comblike, middle claw. Most herons, egrets, and bitterns have dagger-shaped bills. Those of storks are thicker and generally considerably longer. In Threskiornithidae, the bill is the defining morphological characteristic: the ibis bill is decurved, while the spoonbill does indeed have a flattened, spoon-shaped bill. New World vultures have developed meat-eating beaks, with hooked tips and sharp edges.

Without exception, Ciconiiformes are medium to very large birds. Herons show the widest variation among genera, ranging from the Ardea day herons, with the largest, the goliath heron (Ardea goliath), reaching 55 in (140 cm), to the Ixobrychus bitterns. But even the smallest of these, the dwarf bittern (Ixobrychus sturmii), is a respectable 11 in (28 cm) in length. The closely related stork and New World vulture families include some of the largest birds in the world. The male marabou stork (Leptoptilos crumeniferus) stands 4.9 ft (1.5 m) high; Leptoptilos storks and cathartids have very large wings adapted for soaring flight; the Andean condor (Vultur gryphus) has the largest wingspan at 10.5 ft (3.2 m).

This order tends to lack brightly colored plumage, with most species possessing a combination from gray, brown, black, or white. Most show no sexual dimorphism other than size differences, with males up to 10% larger. Those species which feed diurnally and gregariously, including many egrets, storks, and ibises, tend to be light in color. This may be so that they are less visible to prey looking up from water into the light, or for thermoregulation, since white plumage does not absorb heat as quickly. Also, waterbirds with striking light upper parts, easily seen at great distances, attract other gregarious

feeders to exploit feeding opportunities. Conversely, nocturnal, and shade-feeding species, such as bitterns and night herons, have dark underparts or cryptic plumage, and blend with murky surroundings. These solitary feeders have no wish to attract attention from other Ciconiiformes.

Coloration of bare parts is often of great importance in the Ciconiiformes. In many, color changes in the legs, bill, and lore (small area between the eye and bill) take place immediately before courtship, with these bare parts becoming brighter or even—most frequently in the egrets—changing altogether, generally from yellow to red or black. The colors fade or alter again after eggs are laid, suggesting that these changes are an important part of breeding display.

The effects of color changes are most pronounced in those families where bare parts can extend to include the face, throat, neck and even the breast. Some storks, most ibises and all vultures have extensive featherless areas that usually intensify in color during courtship. A few storks and vultures can exaggerate the display by inflating large air pouches in the neck.

A feature of many heron species before courtship is the development in both males and females of ornamental plumes. Night herons and some day herons such as the great blue (Ardea herodias) and capped heron (Pilherodius pileatus), gain backward-facing head plumes which are rounded at the base and tapered at the ends (lanceolate). Some grow filoplumes, feathers resembling fine, thin hair, down the neck and breast. Most egrets have long, delicate aigrettes, ornamental plumes which trail loosely from their backs. Herons, bitterns, and egrets have powder downs, a type of feather that is not shed but becomes powder, conditioning and protecting the other feathers.

Distribution

While Ciconiiformes are found in all but the northern and southernmost parts of the earth, most species are found in tropical or sub-tropical regions. A number of those in northern temperate zones are partial or true migrants.

Habitat

With the possible exception of cathartid vultures, this order predominantly, but not exclusively, favors wetlands (103 out of 113 species), from tidal creeks, rivers, and forest streams to swamps, marshes, paddy fields, and damp meadows. Some species also forage on land-neighboring waterbodies. A few are adapted to feeding wholly in dry habitats such as savannah, light woodland, or moorland.

The range of New World vultures is governed by their ability to soar and find carrion, rather than by temperature, so they can occupy cold, mountainous areas as well as tropical forests. Scavenging abilities have enabled some species to spread into areas of dense human habitation.

Behavior

Most Ciconiiformes exhibit gregarious behavior, although the extent varies considerably among species and in function. Roosting is one of the most sociable activities within this order. Conspecifics may roost in the same colony as other Ciconiiformes and Pelicaniiformes. Roosts of gregarious herons, ibises, and storks are usually in trees, occupied year after year, and may number hundreds or even thousands of birds. Most New World vultures roost communally; American black vultures (Coragyps atratus) roost together in large numbers and reports from the 1940s of 20 or more California condors (Gymnogyps californianus) together suggest that this species' roosts were also sizeable in previous centuries. Even the otherwise solitary hammerheads roost colonially. Studies on American black vultures suggest that colonial roosting has at least one important function; inexperienced or less successful birds leave the roost to follow good hunters to sources of food. For most species, it may also be an antipredator measure.

Potential threats from predators may influence the choice of many species to nest colonially too. A total of 61% of Ciconiiformes are known to nest colonially. Various other hypotheses have been put forward; for example, social nesting may be a way of ensuring that birds have a good choice of mates for breeding.

Ciconiiformes have a limited vocal repertoire. At one extreme, cathartid vultures have no syrinx, the "voice box" that enables most birds to call and sing. The soft wheezes and whistles given at the approach of an intruder are probably only air passing through their mouth and nasal passages. Likewise, storks, shoebills, ibises, and spoonbills have little to say. Storks are silent for most of the year. Only during the breeding season do they emit a range of whistles, croaks, squeals, and grunts—usually when one birds greets another at the nest. Storks and shoebills indulge in noisy bill snapping and clattering as part of their courtship display. Ibises and spoonbills engage in bill clattering too, generally during confrontations.

Herons are the most vocal of Ciconiiformes, with most producing a range of grunts, honks, and croaks. These are emitted in greeting ceremonies during courtship, when disturbed, during antagonistic encounters, or in flight. The bitterns are exceptional in having a modified esophagus, which enables them to produce a booming call to attract a mate and proclaim a territory.

Since many Ciconiiformes must cope with extremes of temperature and humidity, they have evolved a number of thermoregulatory patterns of behavior. The extensive bare parts in many species have a large number of blood vessels close to the skin, enabling birds to radiate heat from the body. Conversely, these species are at risk from heat loss in colder conditions. Some storks of the genus Ciconia spend part of the year in temperate areas. In cold conditions, they stand on one leg, or tuck the bill beneath body plumage in an effort to reduce heat loss. Storks and vultures engage in sunning to warm up, standing with the wings outstretched. Among the various hypotheses for wing-stretching—including nest-shading, exposure of plumage parasites to ultraviolet light, social displays, skin conditioning during molting, maintenance of balance, and straightening of feathers after long periods of soaring—the two foremost are wing-drying and thermoregulation.

Storks, shoebills and vultures share the unusual characteristic of cooling down by urohydrosis—excreting urine on the legs to increase evaporation. Most herons, ibises, and spoonbills use the more common method of heat reduction in tropical areas, by gaping or fluttering their gular pouches.

Few Ciconiiformes are truly sedentary. A number, which spend part of the year in north temperate areas are migratory. Most species show some degree of dispersive behavior or irregular movements, usually to exploit food sources.

Most migratory species follow a north-south post-breeding route from temperate latitudes to tropical or subtropical areas, with cold weather the factor that drives migration. Some, such as American wood storks (Mycteria americana), move from southern breeding areas to more northern areas, to feed following breeding. Populations in warmer southern parts of the breeding range of species such as great blue herons (Ardea herodias), turkey vultures (Cathartes aura) and white-faced ibises (Plegadis chihi) do not migrate.

Ciconiiformes' large, broad wings are adapted for soaring on thermals and this is the most common method involved in distance travel over land among vultures, storks, ibises, and spoonbills: some of the world's biggest concentrations of these migrants are recorded passing over narrow strips of land between continents, such as Panama, or short sea crossings, including the Straits of Gibraltar and the Bosporus. Herons, however, have a slow, strong, flapping flight, enabling them to fly large distances over oceans. Purple herons (Ardea purpurea), for example, migrate over the widest parts of the Mediterranean Sea between Africa and Europe.

Feeding ecology and diet

The Ciconiiformes are mostly carnivorous. Those feeding in aquatic habitats eat predominantly fish, amphibians, crustaceans, insects, and mollusks. Other terrestrial and more catholic feeders take small mammals and birds, reptiles and, in a very few species, fruit and berries. The cathartid vultures,

marabou stork (Leptoptilos crumeniferus) and greater adjutant (Leptoptilos dubius) are unique among Ciconiiformes in obtaining most of their food by scavenging.

All species forage for food by sight or by touch. Vultures also use their strong sense of smell. Most herons and storks stand still or wade slowly through shallow water to stalk prey and rely on striking quickly. Ibises, spoonbills, and storks of the genus Mycteria hunt by touch, either probing in water with slightly open bills, or moving them from side to side.

Solitary feeders defend feeding territories, which can be large in bigger species. Goliath heron (Ardea goliath) territories in South Africa average one bird per 3.7 mi² (6 km²). Gregarious feeders, often in mixed groups together with other Ciconiiformes and Pelicaniiformes, may exploit a temporary glut of food; the presence of a large number of birds attracts more to share the bounty. Birds on the edges of a feeding group spend longer looking out for predators. Flocks move seasonally to take advantage of optimal conditions—as one feeding area dries out or floods, they move to find more suitable habitat.

Reproductive biology

The pattern of nesting behavior is similar for most colonial and solitary nesting species. Most are monogamous. Nesting is timed to coincide with the period of peak prey availability. In temperate areas this is spring and summer. Subtropical species tend to nest during the dry season to avoid the threat of flooding. Tropical species usually nest in the wet season, when food is more plentiful.

The nest is nearly always a rough stick construction made by the female using material brought by the male. In most species, it is built in a tree, bush, low vegetation, or on the ground. The male arrives at the nest site first and defends his territory, often with stretching displays or wing flapping. The arrival of the female prompts greeting ceremonies that can be complex and may include bill snapping and tapping, mutual preening, and presenting of nest material.

In colonial-nesting species, egg-laying is almost simultaneous. Incubation is usually carried out by both species. Young are hatched blind and almost naked. Some ibis and spoonbill chicks hatch with a fine down. Often, both adults brood the young continuously for the first few weeks. The parent bird returning to the nest with food either regurgitates it onto the nest floor, or waits for the young to reach into its mouth and fish out a regurgitated meal.

After fledging, young of some species begin feeding with their parents, but return to the nest to rest. When the young finally abandon the nest, they often disperse over significant distances. Young black-crowned night herons (Nycticorax nycticorax) travel distances of up to 621 mi (1,000 km) from the colony where they were hatched. This reduces risk of overcrowding and enables species to exploit new habitats.

Vultures are exceptional, since they build no nest. The egg or eggs are laid on the ground in a cave, under a bush, in a large tree hole, or in an abandoned building. Hatchlings are covered with white down (or buff in black vultures). Both parents feed the young regurgitated food. Vulture young remain dependent on their parents for long periods—in the case of condors, it is six months before they even learn to fly.

Conservation status

Just over a fifth of Ciconiiformes (21 species) are under a serious level of threat, according to the IUCN, ranging in increasing degree from Endangered and Vulnerable to Critically Endangered. All but three of these species show downward population trends. An additional seven species are classified as Near Threatened.

The vulnerability of Ciconiiformes is due to a number of factors. Their large size, relatively sedentary habits, and slow flight make them easy targets for human persecution. Noisy, colonial nesting in regularly used sites by a number of species, and regular gathering in high concentrations at feeding sites increases their vulnerability, as chicks and eggs are taken and adults snared or shot by humans for food. In Asia, the white-shouldered (Pseudibis davisoni) and giant (Pseudibis gigantea) ibis, greater adjutant and milky stork (Mycteria cinerea) are among those species severely threatened by hunting. As human populations rise, pressures from hunting and disturbance increase. So too, do development threats. In the late 1990s, Japanese night herons (Gorsachius goisagi) lost habitat in Japan to golf courses, housing, and factories.

Waterbird species are particularly vulnerable to pollution. Throughout the world, there is evidence of its impact on Ciconiiformes. One of the main causes of the decline of the Oriental white stork (Ciconia boyciana) on the upper Amur in Russia has been pollution from industrial waste, oil leakages, pesticides, and fertilizers. The black-faced spoonbill (Platalea minor) is threatened in China, where an estimated 63.1% of the rivers in the seven main river systems are polluted, according to Shen Maocheng in 2000.

The trend towards agricultural exploitation of ciconiiform habitats, particularly in Asia, has accelerated. Most of the Mekong floodplain in Laos, once a stronghold of the white-shouldered ibis, has been converted to rice paddy. Forest clearance and conversion of wetlands to agriculture in densely populated China, has fragmented the habitats of species such as the white-eared night heron (Gorsachius magnificus). In Indonesia and Malaysia, coastal wetlands, such as mangrove swamps, are increasingly logged and cleared to make way for fish farms and tidal rice cultivation.

Conservation efforts are addressing this generally bleak picture. For some species, the immediate aim is to carry out surveys to locate and quantify remaining populations and conduct research to understand breeding requirements, demography, and seasonal movements. Conservation groups and governments are working to establish protected areas, encompassing large tracts of habitat found to support populations of endangered species. The Japanese ibis (Nipponia nippon) is one such species, now severely restricted in range, but thanks to protection, one of only three Critically Endangered species showing an upward population trend.

In Laos and Cambodia, stricter enforcement of regulations and public campaigns to reduce the hunting of large water-birds are underway. At Prek Toal reserve, in Cambodia, egg and chick theft of greater adjutant storks fell by 80% in 1997. Attempts at restoration of habitats include a replanting scheme in Assam to replace felled nesting trees of lesser adjutant storks (Leptoptilos javanicus).

Captive-breeding may offer hope for species whose declines are not habitat-related. The California condor, driven to extinction in the wild, largely by hunting pressures and poisoning by lead shot, will benefit from a successful reintroduction program in North America if such human pressures can be controlled. But this option is not open to the increasing number of species whose habitat is being obliterated.

Significance to humans

Myths and superstitions have historically safeguarded the populations of many Ciconiiformes. Native peoples of the Americas venerated condors and vultures as gods. Even today the Andean condor appears on the coats of arms of Bolivia, Ecuador, Chile, and Colombia. Positive associations also benefited the European white stork (Ciconia ciconia), Abdim's stork (Ciconia abdimii), and the sacred ibis (Threskiornis aethiopicus). White storks were considered lucky in Europe. To have one nesting on one's roof would increase fertility and wealth. Abdim's storks arrived on their breeding grounds in central Africa at the same time as life-giving rains. The "rain-bringer" was given the run of every village. Similarly, the sacred ibis returned annually to the Nile when the river flooded. The ancient Egyptians therefore worshipped it as a god. Fear motivated the protection of bitterns and hammerheads. The strange booming call of the Australasian bittern (Botaurus poiciloptilus) marked it as a bird of ill omen among Australian aboriginals. African villagers considered it unlucky to harm the crepuscular hammerhead or its inexplicably huge nest.

Ciconiiformes have suffered because of their perceived economic threat or value to humans. Heavily commercialized hunting of egrets, ibises, and spoonbills for their feathers during the nineteenth and early twentieth century saw millions of birds slaughtered annually. In North America, vultures were killed because they were seen, unjustifiably, as a threat to cattle farming. Herons were shot, principally in North America and Europe, since they were regarded as a threat to fishing interests.

Today, there is generally a greater acceptance of Ciconiiformes and a tolerance of those birds that live near humans. Night herons can be found in the center of built-up Hong Kong.

Resources

Books

Collar, N. J., et al. Threatened Birds of Asia: BirdLife International Red Data Book. Cambridge: BirdLife International, 2001.

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 1, Ostrich to Ducks. Barcelona: Lynx Editions, 1992.

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 2, New World Vultures to Guinea Fowl. Barcelona: Lynx Editions, 1994.

Hancock, J. A., H. Elliott, and R. T. Peterson. The Heron's Handbook. New York: Harper Trade, 1984.

Hancock, J. A., J. A. Kushlan, and M. P. Kahl. Storks, Ibises and Spoonbills of the World. San Diego: Academic Press, 1992.

Sibley, C. G., and J. E. Ahlquist. Phylogeny and Classification of Birds: A Study in Molecular Evolution. New Haven and London: Yale University Press, 1990.

Other

BirdLife International Saving Species. Birdlife International. 1 July 2001 (1 Feb. 2002). <http://www.birdlife.org.uk/species/index.cfm>.

Georgia Wildlife Web. The Georgia Museum of Natural History. 2 May 2000 (30 Jan 2002). <http://museum.hhm.uga.edu/gawildlife/birds/ciconiiformes/ciconiiformes.html>.

The Peregrine Fund. 23 Jan. 2002 (1 Feb. 2002). <http://www.peregrinefund.org/conserv.html>

Derek William Niemann, BA