Ardeidae

(vertebrate zoology) The herons, a family of wading birds in the order Ciconiiformes.

(Ardeidae)

Class: Aves

Order: Ciconiiformes

Suborder: Ardeae

Family: Ardeidae

Thumbnail description
Medium to very large wading birds, typically with long legs and toes, long bills, and long necks, which are folded over the back when flying

Size
9.7–58.5 in (25–150 cm); 0.16–9.9 lb (73 g–4.5 kg)

Number of genera, species
16 genera; 62 species

Habitat
Inland and coastal wetlands, lakes and streams, grasslands, wet forests, coasts and estuaries, islands and agricultural areas such as rice fields and aquaculture ponds

Conservation status
Endangered: 3 species; Vulnerable: 5 species; Near Threatened: 1 species

Distribution
Worldwide in tropical and temperate zones

Evolution and systematics

Herons, as far as is known, originated in the Eocene era about 60–38 million years ago. Specimens attributed to herons occur infrequently in later deposits. Herons are not well represented in the fossil record prior to the Pleistocene, probably owing to the slight structure of their bones. Missing from the record are birds clearly ancestral to what are now considered the more basal lineages of the heron family tree, the boat-billed heron (Cochlearius cochlearius), agami heron (Agamia agami), and the tiger herons. Therefore, the details of the evolutionary history of herons must be inferred primarily from modern species.

Present-day herons are a rather homogenous group of birds that have been lumped together taxonomically since Linnaeus's day. As part of the order Ciconiiformes, they are related most closely to storks, ibises, and spoonbills. They are related distantly to waterfowl, shorebirds, and pelicans, and even more distantly to such birds as loons, petrels, and albatrosses. The details of higher-level systematics among these birds may be amenable to modern molecular research techniques.

Over the decades, there has been much discussion about placement of a few relatively odd species, the shoebill (Balaeniceps rex), the hammerhead (Scopus umbretta), and the boat-billed heron. The first is most closely allied to storks, the second to pelicans, but the last is definitely a heron, but of a different sort. Together with the tiger herons, the boat-billed heron and agami heron appear to represent remnants of basal limbs of the heron evolutionary tree. In a recently proposed taxonomic scheme, these species are considered to be representatives of distinct subfamilies—Cochleariinae, Agamiinae, and Tigrisomatinae.

The remaining herons encompass the typical herons and the bitterns. The bitterns, subfamily Botaurinae, appear to represent the herons most divergent from the primitive heron stock, a complete turnaround of systematic thinking of a few decades ago. Herons of the subfamily Ardeinae include the herons (tribe Ardeini), the egrets (tribe Egrettini) and the night herons (tribe Nycticoracini).

Within the five subfamilies and three tribes, about 62 currently recognized species are partitioned among 16 genera. Over the past several decades, anatomical and molecular studies have led to reassignment of species among infrafamilial categories, particularly among genera. Progress in heron systematics can be measured by noting that only a few decades ago, the large egrets were placed in the monotypic genus Casmerodius or in the genus Egretta and the terrestrial cattle egret was placed in a monotypic genus Bubulcus and later considered an Ardeola or an Egretta. It has recently been proposed that the yellow-crowned night heron (Nyctanassa violacea) is also more closely related to the egrets than to other birds called night herons, but additional study is called for.

Physical characteristics

Typical herons are relatively tall and thin, with relatively long necks and legs, large sharply pointed bills, large moveable eyes, and broad wings. Their plumage is generally complex, featuring black, white, grays, or browns, and they have distinctive plumes. These fundamental heron features are primarily adaptations for wading in water in order feed on fish and other aquatic prey and for communicating with other birds.

The long neck has 20–21 cervical vertebrae, the fifth through seventh having the articulation that gives the neck its characteristic kink. The neck is long enough to be folded over the back in an "S" shape when the bird in prolonged flight, making a flying heron easily recognizable.

The long legs have feathers on the thighs but are otherwise featherless. The toes are long (including the back toe, which is on level with the rest) and slightly webbed. The claw of the middle toe has a serrated edge, which facilitates care of the plumage.

The heron bill is one of its defining characteristics. Most are elongated to effect the capture of quickly moving prey in a tweezer-like fashion following a rapid strike. Thin rapier-like bills are adaptations for fish eating, piercing through the water to capture fleeing prey. Bill color depends on species, and can vary with age and season. In some species bill color brightens and becomes more colorful during courtship.

Herons have well-developed eyes with substantial capacity for movement. Typical herons have a tall but narrow field of binocular vision that is aimed forward and includes the zone under the bill down to the feet, to aid in sighting prey. The color of the iris of some species changes seasonally, or when agitated, and so is used for social signaling. The head is fully feathered and often distinctively marked by species, except for the area between the bill and eye, which often is featherless. These bare loral patches are colored characteristically among the species, and their colors typically change during courtship and other interpersonal encounters.

Some herons are entirely white, gray, or black, while others have exceptionally complicated plumage. Plumage pattern is generally correlated with lifestyle. White-bodied herons are often highly social, feeding in flocks and nesting close together in colonies. Dark-plumaged herons tend to be more solitary, or have the capacity to be social or not as the situation requires. In other species the plumage is predominantly cryptic, featuring brown, white or buff stripes, speckles, and spots. This color scheme predominates among species that hide in reeds and bushes.

Heron plumage changes with age. The first downy plumage, which is generally gray or light brown, is immediately replaced by the first juvenal plumage. Generally an adult plumage is gained by the time of first breeding. Pond herons change plumage seasonally.

As a general rule, the sexes are not distinguishable by the coloration of their plumage. During the breeding season, however, herons use feathers on the nape, back, crest, and crown as display plumes. These are particularly functional in aggressive encounters. Plumes also occur on the front of the neck, upper neck, and back. The major types of heron plumes include filoplumes, which are elongated and hairy appearing; aigrettes, which have long shanks and few barbs so that they appear frayed; and lanceolate plumes, which are more like typical long body feathers but with frayed edges.

Herons also have patches of friable downy feathers that provide a powder used for grooming. Most herons have three pairs of down patches. The powder produced apparently keeps the plumage water-repellent and probably cleans it as well.

The wings are broad, with nine to 11 primaries. The tail is short with 12 tail feathers in most species. Large herons appear to take flight with some difficulty, holding their head out with legs dangling until they gain altitude. Smaller herons can take flight more rapidly. Once in flight, herons fly well and with endurance using slow, quiet wing beats and can travel long distances both on migration and to and from feeding grounds.

Distribution

Herons occur around the world, on all continents but Antarctica, and also on islands in all oceans. Herons occur in the greatest numbers and diversity in tropical zones. Many species range into the temperate zones, but their limits depend on the species-specific ability to nest in progressively shortened summer periods.

The herons (Ardeini) are found around the world. Ardea appears to be primarily an Old World genus, with 12 species there and only four in the New World, two of which are shared. The three great herons, including the goliath heron, occur in Africa and Asia. Butorides herons also are widely distributed continentally, and also on many islands. Pond herons (Ardeola) also are Old World species, mostly Asian with two representatives in Europe and/or Africa.

The egrets (Egrettini) are worldwide, with no particular concentration area. Several Egretta are New World species, probably originating in South America and subsequently invading North America. The Chinese egret is the only Asian species. The night herons (Nycticoracini) likely are an Old World group. The bitterns (Botaurinae) also appear to be an Old World group, with only four of 11 species occurring in the New World. The tiger herons, agami heron, and boat-billed heron (Tigrisomatinae, Agamiinae, Cochleariinae) are tropical species, four from tropical Americas and one each from Africa and New Guinea.

Temperate-zone herons are generally migratory. Some species, such as the large herons, remain rather far north such as in Canada and Great Britain, although periodic severe winters can cause substantial mortality in these populations. Many species that breed in the tropics also migrate regularly according to the wet and dry seasons. Many species of herons tend to wander after nesting leading to a post breeding dispersal away from their nesting areas. Due to both post breeding dispersal and overshoots on return migrations, herons often wander far from their normal range. As a result, herons stray to high latitudes, deserts, and mountains, as well as to far off islands and ships at sea.

The ranges of some species are changing. Some in the Northern Hemisphere are expanding their ranges northward. In contrast, the large bitterns are experiencing range contraction. Some species are changing their ranges transcontinentally.

Habitat

Herons are generally aquatic birds, and the typical heron is seen feeding by standing or walking in the shallow water of a marsh or pool. However, herons use a wide array of wet and dry habitats. They may be habitat specialists or habitat generalists.

Inland wetlands are typical habitats for herons. Tree swamps are particularly favored because they provide not only foraging habitat, but also trees and bushes for roosting and for nesting. Herbaceous marshes also are used by herons worldwide. Some species, such as bitterns and the purple heron, are specialists in living among dense emergent vegetation.

Herons also feed in more open areas, such as the shallow edges of lakes, ponds, pools, and lagoons, where they tend to feed along the edges in shallow water. In these situations it is not unusual to see species arranged out from shore according to leg length, with taller birds foraging in deeper water and shorter birds at shallower sites.

Running water is exploited by species that feed along the banks, either by perching on overhanging trees or by feeding from the bank itself. Herons also perch on rocks, and the larger herons can withstand current sufficiently to wade into running water, although within limits. Placid streams and ditches are more commonly used by many species.

Tidal environments are of critical importance for many species. Tidal creeks, tidal mudflats and bars, salt marshes, mangrove swamps, coastal lagoons, and beaches are all used by herons. The tidal cycle determines the daily schedule of species feeding in tidal environments. They feed when conditions are appropriate, usually around the outgoing tide, and then move to nontidal habitats to continue feeding or to roost during the high tide periods. Species depending on tidal flux, especially the night herons and large herons, may also feed at night.

Artificial environments have become essential habitats for many populations. Reservoirs, farm ponds, and ditches provide aquatic or semi-aquatic habitat. Lands designated for agriculture and aquaculture are of even more importance. Rice fields have become critical habitat for herons around the world. Aquaculture provides concentrated prey of the sort that herons customarily eat. Young birds particularly may be attracted by these sources.

Despite the aquatic origins of the group, herons also use terrestrial habitats either occasionally or predominantly. Some species, such the cattle egret and black-headed heron (Ardea melanocephala), are predominantly terrestrial and inhabit natural grasslands and pasturelands. Many other species of herons and egrets feed on dry land at least occasionally.

Herons nest in many sorts of habitats that afford proximity to feeding areas and protection from predators. Colonial herons use islands off shore and in lakes or rivers. Herons nest on islands of vegetation composed of trees or bushes surrounded by marsh or swamp. They also nest in tall trees, either within expansive forests or in coppices. On safely isolated islands, herons may nest on the ground, on rocks, on in cave entrances. Many species of herons nest in reed beds.

Behavior

For most herons standing is a principal feeding behavior and they spend much of their day or night resting or roosting. Many herons are sit-and-wait predators that lie in wait for prey to make itself apparent. Waiting herons can keep quite still for many minutes. Depending on species and circumstance, they stand crouched, upright, or fully erect. Crouched postures lower visibility to prey and allow the strike to occur closer to the water. In the upright posture, the body and neck are angled above the water allowing more scanning for prey than the crouched posture.

Herons may walk in the water, on the ground, over grass, or in bushes and trees. Walking herons generally move about in search of prey or stalking individual prey. Walking may be very quick or so slow as to be nearly imperceptible. Some herons run and hop from place to place in search of better feeding opportunities.

In feeding, the heron makes maximal use of its head and neck. Fish and other prey are caught after a quick movement of the head and neck. The usual method is a bill stab in which the heron issues a downward or lateral strike with the head and neck. Shorter-necked herons capture prey by bill, thrusting forward the bill, head, neck, and body, in a sort of a headfirst leap. Herons also feed by more subtle methods, such as probing into mud or vegetation, pecking the ground, or gleaning insects off plants.

In most species, herons need adequate vision to see their prey before stabbing it. Other than the challenge of identifying something in the water as edible, the heron's most substantive problem is refraction of light in the water, and they compensate for the fact that their underwater prey is not actually located where it appears to be. Herons also move their heads around to compensate for reflection off the water's surface. They move their heads side to side to better locate prey binocularly and also sway both the head and neck side to side or backward and forward, a behavior also used among land-dwellers.

Herons attract or startle prey in several ways. Several medium-sized egrets have yellow feet contrasting with darker legs. They stir, rake, and wiggle their feet to attract prey or stir it into movement. Butorides herons attract prey by placing food items (such as dog food or corn) or imitation food (such as sticks or feathers) in the water and catching prey that the bait attracts. Some egrets also attract fish by putting their bill in the water and opening and closing it quickly, creating ripples to attract fish.

Many herons are highly social. They tend to gather in feeding sites where prey is particularly available, forming multi-species aggregations that can number in the hundreds or even thousands of birds. Most herons, the bitterns being the primary exception, nest in colonies, often of mixed species. Other species are solitary or occur in more well-dispersed pairs. In a few instances family groups may occupy a single area. Many species choose between being solitary and aggregative depending on food availability.

All herons defend themselves and their immediate surroundings with ritualistic bill lunges called forward displays. Herons at a nest site will shake twigs vigorously. A bird may also attack another by running or flying at it to supplant it from its site. Herons fight with their bills, wings, and feet. When hundreds of herons of several species gather at feeding sites, they jockey for position, open their sharp beaks, spread their wings, and rush at their enemy. Individual fights can last half an hour.

Another behavioral interaction among herons is prey theft. Large herons steal prey from smaller ones, and other birds steal prey from herons. A heron with a large or uncooperative fish must subdue it by biting, bashing, or stabbing. This delays swallowing and allows other birds to steal the fish. The attacking heron runs or flies at a victim in its attempts to steal the prey. The threat of theft may even influence the choice of prey, and a heron may sometimes pass up a larger prey item in favor of a smaller one.

Feeding ecology and diet

Herons are primarily fish-eating birds. Most species wade about looking down into the water and capture fish they see by a rapid thrust of their long sharp bill. Given their feeding method, herons can catch fish at or near the water's surface. Fish must be visible and also shallow enough that they cannot swim away before the heron can get its beak around them. Large herons can capture large fish. Lungfish are an important prey for the largest herons. The mosquitofish (Gambusia affinis), introduced around the world, is taken by small and medium-sized herons in great numbers wherever it occurs. Overall, many fish species can be captured by herons.

The second most common type of prey for herons is crustaceans. Crabs occur primarily in marine and estuarine environments, although some occur well inland in the large rivers and on land. In freshwater, burrowing crayfish and small shrimp are important prey. Crayfish, shrimp, and prawns are farmed in many parts of the world; herons are attracted to these sites to partake of the easy feast.

Amphibians are another important prey for herons. Frogs and toads are frequently caught, as are their tadpoles. Salamanders are less common. Insects, especially aquatic insects, are an important food source for herons. Both adults and larval forms are picked up from the water or submerged plants and rocks. Terrestrial herons primarily eat insects. Flies, dragonflies, and similar insects are often taken. Other food may include snails, bivalves—both freshwater and marine—small mammals, birds, and reptiles. Reptiles are most often an important part of heron diet on islands.

Black-crowned night herons also feed on nesting gulls, terns, or other herons in colonies. Other herons are reported to take birds as they become available. As far as is known, herons are entirely carnivorous. However, there have been reports of herons purposefully eating fruits, and vegetation may be taken along with fish.

Many species of herons aggregate to forage for several reasons. Herons are adept at finding and then exploiting ephemeral patches of highly concentrated food. Therefore, aggregations develop at places where food supply is high. This sharing of food-finding information is called local enhancement. Feeding locations for social herons change hourly, daily, and seasonally.

In some cases of aggregate foraging, the participating herons gain an advantage in that the mass of birds stir up the prey, which makes the animals more vulnerable to capture. Herons also achieve commensal benefits from following the paths of other birds or mammals in search of prey those animals may have disturbed.

In contrast, some herons are always solitary or occur in pairs. The great herons and large herons (Ardea) tend to feed alone.

However, when the occasion presents itself, they will join mixed species aggregations. Birds that feed alone have freedom from the disturbance of their potential prey by other animals. These birds also defend their feeding areas from invasion.

Reproductive biology

Most species of herons are serially monogamous. Although some birds return to the same site to nest year after year, many birds tend to move nests or even change nesting areas from one season to the next. As a result, pair bonds tend to be formed anew each season. For social nesters, despite monogamous pairing, promiscuous mating behavior can be common. Extra-pair mating usually occurs among individuals nesting near each other.

Bitterns and tiger herons are generally solitary nesters. More species of herons are colonial, nesting in single-species or mixed-species aggregations that can number from a few birds to hundreds or even thousands of birds. Along with other herons, colonies may include pelicans, cormorants, ibises, spoonbills, storks, and also crows and raptors. Herons tend to partition the nesting habitat, often with the larger birds on the top of trees or bushes and the smaller species underneath. Nest defense by parents is essential in a colony. Some species, such as cattle egrets, are particularly aggressive and may take over nests of other species.

Males tend to arrive at colony sites first at the future colony and claim display sites, which they defend against other birds. They give spontaneous displays and calls that attract potential mates and defend the sites. During the advertising period, a stretch display is the most universal among typical herons and egrets. The display consists of lifting the head to vertical, perhaps calling, and then bringing it down again, perhaps with a snap of the bill. Later in the nesting season, this display is also used between the mating pair. With the snap display, the bird erects head and neck feathers and extends its neck with a snap of its bill. Herons may combine snaps and stretches, bow, shake twigs, fly about the colony in circles, flip their tails, shake their feathers loosely, preen, and mock preen. After pairing, the birds give landing calls and a greeting ceremony that permits the returning mate access to the nest site.

Following courtship, the pair builds the nest. Nests are made of sticks or reeds, depending on species and nesting site. The eggs are typically blue, but may be white, greenish, or olive-brown; a few species have spotted eggs. Clutch size can range up to 10, but for most species is three to five. Incubation lasts from two to four weeks depending on the species and size of the bird. Larger birds' eggs incubate longer. Except for large bitterns, both parents incubate, taking turns.

Newly hatched young are covered with sparse down, have closed eyes, and are unable to walk, but the birds grow quickly. Both parents tend the young. They bring food in their stomachs or throats to the nest and then regurgitate it first into the beaks of the young, and later onto the edge of the nest.

Because incubation begins before the clutch is completed, the young hatch at different times. The oldest get a head start and dominate the youngest siblings in competition for food, which the parents provide preferentially to the most persistent chick. Usually the younger chicks die, either by starvation or through harassment by older chicks. How many chicks die depends on the ability of adults to supply food and the health of older chicks.

One parent broods the young for one to two weeks and continues to guard them for a bit longer. While one parent is guarding, the other forages. Chicks learn to fly gradually within the colony. Colonial species do not feed young after they fledge.

Conservation status

Many heron species remain abundant, and some are expanding their ranges and populations. Large herons often occur in rural and even urban areas. Many species use rice paddies, farm ponds, and aquaculture facilities; in fact, some populations have become dependent on them. Herons nest in artificial lakes, urban parks, and zoos, and feed along roadside ditches. Reservoir construction has increased available habitat, especially in otherwise arid areas that are inhospitable to herons.

Some species or populations in certain areas, however, teeter on the verge of extinction. The 2000 IUCN Red List ofThreatened Species lists three species as Endangered (Ardea insignis, Gorsachius goisagi, and G. magnificus), five species as Vulnerable (Ardea humbloti, A. idea, Egretta eulophotes, E. vinaceigula, and Botaurus poiciloptilus), and one species as Near Threatened (Zonerodius heliosylus). The primary threat for all species is habitat destruction and alteration, in some cases exacerbated by hunting. The most important habitat alteration involves the widespread destruction of wetlands, lowland forests, and coastal swamps and lagoons. Hunters take eggs, chicks, or adults.

Protecting and managing habitat is by far the most critical issue in heron conservation. Colonial species require nesting substrate, usually bushes and trees. Because the nesting activity can stress the plants through breakage, defoliation, and excess nutrient deposition, colony sites degrade over time, requiring either active management or the provision of alternative sites. Most solitary nesting herons need relatively large patches of marsh or forest in which to nest, although some tiger herons nest in a single isolated tree. Herons need feeding habitat throughout the year. For migrating species, this means during nesting, migration, and wintering periods. Fortunately, the habitat needs of herons coincide with those of waterfowl and other aquatic birds, making it possible for heron habitat protection to be part of larger wetland and forest conservation strategies.

Hunting is an important issue in worldwide heron conservation. Hunting for food and for body parts continues in China and Madagascar. Herons are killed at fish farms and other aquacultural facilities. Herons are killed by accidents, sport shooting, and cases of acute chemical contamination.

Significance to humans

Humans have certainly always been aware of heronries, and were probably one of the few ground predators that could access them for easy food. Many colonies occurred in places that were relatively inaccessible to early humans. Official protection afforded herons in the recent years was probably the exception rather than the rule worldwide. Herons were seen along watercourses and other places where men fished, and in these circumstances humans fostered an attachment to the birds. This relationship probably has peaked with the people of Manchar, Pakistan, who for a thousand years or more have kept herons as honored pets.

Herons have not figured in folklore to the extent that storks or some other birds have. Mention of herons nonetheless goes back to the Old Testament, ancient Egypt, and Hindu culture. The booming of the large bitterns has long been held as a bad omen in several cultures. But the graceful heron is often used as a symbol of natural beauty and grace in contemporary societies, and so the bird is a subject of poems and books.

Humans have long appreciated and used heron feathers. The decorative plumes of the large white and little egrets served as expensive head decorations for the Hungarian nobility and the Turks in the Middle Ages. By the late 1800s and early 1900s, birds and their feathers were quite valuable and widely used in Europe and elsewhere for ornament, particularly on hats. London became the center of the European plume market, and colonies of birds on the Danube and Theiss Rivers were among those devastated by plume-collecting expeditions. Colonies were hunted in both southern North America and through tropical America. All birds with useful plumes were killed and their feathers plucked. Non-target birds nesting in the colonies were disrupted and orphaned young died. In 1902, 3,012 lb (1,366 kg) of egret plumes were sold in London. This meant that 192,960 egrets were killed to supply the demand.

Never in their history have herons been so dependent on another species as they are on humans today. Worldwide, colony sites are protected or threatened by people. Feeding sites are part of parks, refuges, and other protected environments. Elsewhere, human destruction of mature wet forests, wetlands, and coastal environments affect populations profoundly. It is likely that herons are more and more coming to depend on their relationships with humans for their continued survival.

Species accounts

Resources

Brown, Leslie. The Birds of Africa. Vol. 1. San Diego: Academic Press, 1983. del Hoyo, J., A. Elliot, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 1, Ostrich to Ducks. Barcelona: Lynx Edicions, 2001.

Hancock, J.A. Birds of the Wetlands. London: Academic Press, 1999.

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

Kushlan, J.A., and H. Hafner. Heron Conservation. London: Academic Press, 2001.

Kushlan, J.A., and J.A. Hancock. The Herons. Oxford: Oxford University Press, 2002.

Draulans, D., and J. van Vessem. "Some Aspects of Population Dynamics and Habitat Choice of Gray Herons (Ardea cinerea) in Fish-pond Areas." Gerfault 77 (1987): 43–61.

Kushlan, J.A. "Feeding Behavior of North American Herons." Auk 93 (1976): 86–94.

Kushlan, J.A. " Feeding Ecology of Wading Birds." Wading Birds, National Audubon Society Research Report 7 (1978): 249–297.

Maddock, M., and G.S. Baxter. "Breeding Success of Egrets Related to Rainfall, A Six Year Australian Study." Colonial Waterbirds 14 (1991): 133–139.

Marion, L. "Territorial Feeding and Colonial Breeding are Not Mutually Exclusive: The Case of the Gray Heron (Ardea cinerea)." Journal of Animal Ecology 58 (1989): 693–710.

Herons Specialist Group. Station Biologique de la Tour du Valat, Le Sambuc, Arles 13200 France. Phone: +33-4-90-97-20-13. Fax: 33-4-90-97-29-19. E-mail: hafnerh@aol.com Web site:

Waterbird Society. National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 USA. Web site:

Kushlan, J.A., and L. Garrett. A Bibliography of Herons.

[Article by: James A. Kushlan, PhD]