The wood stork (Mycteria americana) is a large American wading bird in the stork family Ciconiidae. It was formerly called the "wood ibis", though it is not an ibis. It is found in subtropical and tropical habitats in the Americas, including the Caribbean. In South America, it is resident, but in North America, it disperses to as far as South America. It was originally described by Carl Linnaeus in 1758. It likely evolved in tropical regions. Of the extant members of its genus, the wood stork is basal, being the first off-shoot from the genus' common ancestor. The head and neck are not feathered and dark grey in colour. All of the plumage is white, with the exception of the primaries, secondaries, and tail, which are black with a greenish-purplish sheen. The bill is black. The juveniles differ from the adult, with the former having a feathered head and a yellow bill. The sexes are similar.
The habitat of the wood stork can vary, but it must be tropical and with fluctuating water levels. It prefers those surrounded by water or over water. The one metre (3.3 ft) nest is found in trees, especially mangroves and trees of the genus Taxodium. Wood storks nest colonially. The nest itself is made from sticks and greenery. During the breeding season, which is initiated when the water levels drop and can occur anytime between November and August, a single clutch of three to five cream coloured eggs is laid. These are incubated for around 30 days, and hatch altricial. The chicks are brooded for the first week after hatching, and are always in the presence of one of the parent wood storks until three weeks. They fledge 60 to 65 days after hatching. Only about 31% of nesting pairs fledge a chick in any given year. Chicks are fed fish. The diet of the adult changes throughout the year. During the dry season, fish and insects are eaten, whereas fish, crabs, insects, and frogs are the predominate foods during the wet season. Because it forages by touch, it needs shallow water to effectively catch food. This is also the reason why the wood stork breeds when water levels start to fall.
Globally, the wood stork is considered to be least concern by the International Union for Conservation of Nature (IUCN). This is due to its large range. In the United States it is considered to be threatened. Predators of the wood stork include raccoons, which predate chicks, crested caracaras, which prey on eggs, and other birds of prey, which feed on both eggs and chicks. Hunting and egg-collecting by humans has been implicated as a factor in the decline of South American wood storks. Humans also cause nest failures through bio-tourism, although observation through binoculars about 75 metres (246 ft) away does not have a large effect on nesting success. In both North and South America, habitat alteration has caused the wood stork to decline, with levee and drainage systems in the Everglades causing a shift in the timing of breeding and thus a decrease in breeding success.
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Taxonomy and etymology
The wood stork was first described by Linnaeus in 1758, who gave it the current binomial Mycteria americana. Linnaeus based this description on an account of the jabiru-guacu. He also described Tantalus loculator after M. americana, the former having basis in a description of the wood pelican. But, it was discovered that these binomials actually referred to the same species. Thus, M. americana, being found before the description of T. loculator, took precedence and was thus established as the name of the wood stork.
This species seems to have evolved in tropical regions; its North American presence probably postdates the last ice age. A fossil fragment from the Touro Passo Formation found at Arroio Touro Passo (Rio Grande do Sul, Brazil) might be of the living species; it is at most from the Late Pleistocene age, a few 10,000s of years ago. North American fossils from that time are of an extinct larger relative, M. wetmorei. This was probably a sister species; both occurred sympatrically on Cuba at the end of the Pleistocene. Of the extant members of the genus Mycteria, this bird is basal. The clade that it is basal to is the clade that has the yellow-billed stork being basal to the milky stork and the painted stork.
Likely because of its decurved bill, the wood stork has formerly been called the "wood ibis", although it is not an ibis. It also has been given the name of the "American wood stork", because it is found in America. Various regional names that it has been given include "flinthead", "stonehead", "ironhead", "gourdhead", and "preacher".
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Description
The adult is a large bird which stands 83 to 115 cm (33-45 in) tall and spans 140 to 180 cm (55-71 in) across the wings. Males typically weigh 2.5 to 3.3 kg (5.5-7.3 lb), with a mean weight of 2.7 kg (6.0 lb); females weigh 2.0 to 2.8 kg (4.4-6.2 lb), with a mean weight of 2.42 kg (5.3 lb). Another mean estimated weight for the species is 2.64 kg (5.8 lb). The head and neck do not have any feathers, and are scaly and coloured a dark grey. The downward-curved bill is long and very wide at the base. It is black in colour. The legs and feet are dark, and the toes are pink during the breeding season, being flesh-coloured otherwise. The plumage is mostly white, with the primaries, secondaries, and tail being black and having a greenish and purplish iridescence. The sexes are similar.
When the chicks hatch, they have a sparse coat of grey down, protoptiles, which is replaced by a dense, wooly, and white down, mesoptiles, in about 10 days. Chicks grow fast, being about half of the height of the adult by three to four weeks after hatching. By the sixth and seventh weeks, the plumage on the head and neck turns smokey grey. When fledged, they resemble the adult, differing only in that they have a feathered head and a yellow bill.
Distribution and habitat
This is a subtropical and tropical species which breeds in much of South America, Central America and the Caribbean. The wood stork is the only stork that presently breeds in North America. In the United States there is a small breeding population in Florida, Georgia, and South Carolina, along with a recently discovered rookery in southeastern North Carolina. It is resident in South America, but in some places in North America, it disperses, sometimes as far as South America.
This stork is suggested to be able to adapt to a variety of habitats that are in the tropics and have fluctuating water levels. The main habitats are those where it nests over water and those where it is surrounded by water. In freshwater habitats, it generally prefers forests that are dominated by trees of the genus Taxodium, while in estuaries, it prefers mangrove forests.
Behaviour
Breeding
A resident breeder in lowland wetlands with trees, the wood stork builds a large stick nest in a forest tree. In freshwater habitats, it prefers to nest in trees that are larger in diameter. It nests colonially, with up to 25 nests in one tree. The height of these nests is variable, with some nests located in shorter mangrove trees being at heights of about 2.5 metres (8.2 ft), compared to a height of about 6.5 metres (21 ft) for taller mangrove trees. For Taxodium trees, it generally nests near the top branches, frequently between 18 and 24 metres (59 and 79 ft) above the ground. On the tree itself, forks of large limbs or places where multiple branches cross are usually chosen.
The nest itself is built by the male from sticks and greenery collected from the colony and the surrounding area. The greenery usually starts to be added before the eggs are laid but after the main structure of twigs is completed. It is continued to be added during incubation, with the addition of greenery decreasing after the eggs are hatched. The greenery functions to insulate the nest. When complete, the nest is about one metre (3.3 ft) in diameter. The central area where greenery is, on average, about 28 centimetres (11 in) in diameter, and the thickness of the edge usually measures from 12 to 20 centimetres (4.7 to 7.9 in).
Breeding is initiated by a drop in the water level combined with an increased density of fish. This is because a decrease in the water level and an increased density of fish allows for an adequate amount of food for the nestlings. This can occur anywhere between November and August. After it starts, breeding takes about four months to complete.
This bird lays one clutch of three to five cream coloured eggs that are about 68 by 46 millimetres (2.7 by 1.8 in) in size. These eggs are usually laid one to two days apart and incubated for 27 to 32 days, starting with when the first egg is laid, by both sexes. During the first week of incubation, the parents do not go far from the colony, with the exception of the short trips to forage, drink, and collect nesting material carried out by the non-incubating bird. After the first week, the non-incubating bird spends less time in the colony, although the eggs are never left unattended. After a few hours of incubation, this bird sometimes takes a break to stretch, preen itself, rearrange nest material, or turn the eggs. The eggs hatch in the order in which they were laid, with an interval of a few days between when each egg hatches.
The chicks hatch altricial, unable to move, and weigh an average of 62 grams (2.2 oz). They are brooded for the first week after hatching, and after that when it is raining and at night. The chicks are not left alone until at least three weeks of age, with one parent foraging while the other guards the nest and chicks. When the chicks are at least three weeks old, they are large enough to stay and protect the nest. This coincides with the chicks getting more aggressive when presented with foreign objects or organisms. They fledge 60 to 65 days after hatching, and reach sexual maturity four years of age, although they usually do not successfully fledge chicks until their fifth year of age.
The hatching success of the wood stork is around 62%, meaning that about 62% of birds that laid at least one egg had one or more eggs eventually hatch. This can vary widely, though, with colonies ranging from about 26% to 89% hatching success. The period when chicks are most vulnerable to death is from hatching to when they are two weeks old. Overall, about 31% of nests produce at least one fledged bird. Crested caracaras and raccoons are prominent predators of eggs and chicks. Other causes of nesting failure is the falling of nests, thus breaking the eggs inside. This can be caused by many events, the most prominent being poor nest construction and fights between adults.
Feeding
During the dry season, the wood stork eats mostly fish and is supplemented by insects. During the wet season, on the other hand, fish make up about half the diet, crabs make up about 30%, and insects and frogs make up the rest. The wood stork eats larger fish more often than smaller fish, even in some cases where the latter is more abundant. It is estimated that an adult wood stork needs about 520 grams (1.15 lb) per day to sustain itself. For a whole family, it is estimated that about 200 kilograms (440 lb) is needed per breeding season.
The wood stork usually forages in flocks when not breeding, and alone and in small groups when it is breeding. In the dry season, this bird generally forages by slowly walking forward with its bill submerged in the water while groping for food. During the wet season, this method is used about 40% of the time to catch food. During this time, foot stirring, where the stork walks very slowly with the bill in water while pumping its foot up and down before every step, it used about 35% of the time. Both of these methods are non-visual.
Because of its non-visual foraging methods, the wood stork requires shallow water and a high density of fish to forage successfully. The water that it forages in during the dry season average about 17 centimetres (6.7 in) in depth, while during the wet season, the water usually is about 10 centimetres (3.9 in). In the dry season, this stork prefers to forage in waters with no emergent vegetation, whereas in the wet season, it prefers areas with vegetation emerging between 10 and 20 centimetres (3.9 and 7.9 in) above the surface on average. This bird can travel over 80 kilometres (50 mi) to reach foraging sites, lending it access to a wide variety of habitats.
Both sexes feed the chicks by regurgitating meals onto the nest floor. The chicks are mainly fed fish that are between 2 and 25 centimetres (0.79 and 9.84 in) in length, with the length of the fish usually increasing as the chicks get older. The amount of food that the chicks get changes over time, with more being fed daily from hatching to about 22 days, when food intake levels off. This continues until about 45 days, when food consumption starts to decrease. Overall, a chick eats about 16.5 kilograms (36 lb) before it fledges.
Aggressiveness
Wood storks without a nest occasionally try to take over others' nests. Such nest take-overs are performed by more than one bird. The young and eggs are thrown out of the nest within about 15 minutes. If only one stork is attending the nest when it is forced out, then it will usually wait for its mate to try to take the nest back over. Chicks do not display aggression until about three weeks after they hatch.
Flight
When flying, this bird utilizes two different techniques. When it is not sufficiently warm and clear, such as in the late afternoon and on cloudy days, this stork alternates between flapping its wings and gliding for short periods of time. When it is warm and clear, this bird glides after it reaches an altitude of at least 610 metres (2,000 ft) through continuously flapping its wings. It can glide in this manner for distances ranging from 16 to 24 kilometres (9.9 to 14.9 mi). It does not have to flap its wings during this time because the warm, rising air currents are strong enough to support its weight. Because of the energy that is conserved by gliding, this stork usually uses this method to fly to more distant areas. It flies with its neck outstretched and its legs and feet trailing behind it.
When flying to foraging areas, the wood stork has an average speed of about 24.5 kilometres per hour (15.2 mph). When it flies by flapping its wings, it goes about 34.5 kilometres per hour (21.4 mph), whereas when it glides, it flies at speeds of about 20 kilometres per hour (12 mph).
Predators and parasites
Raccoons are predators of wood stork chicks, especially during dry periods where the water beneath nesting trees dries up. Where it occurs, the crested caracara is a significant predator of eggs. Other caracaras and hawks and vultures predate both eggs and chicks.
In the United States, Haemoproteus crumenium, a blood protozoan, can be found in subadult and adult wood storks. Other species of Haemoproteus also infect wood storks in Costa Rica, in addition to Syncuaria mycteriae, a nematode found in the gizzard of the wood stork.
Status
Globally, the wood stork is considered least concern by the IUCN due to its large range. In the United States, this bird is considered to be threatened, a recovery from its endangered status, which it held from 1984 to 2014 because of a decline in its population. Similarly, in the state of Santa Catarina, Brazil, its decline seems to have been reversed: after an absence between the late 1960s and the mid-1990s, the species is now again regularly encountered there, in particular in the Tubarão River region. It is likely that the Paraná River region's wetlands served as a stronghold of the species, from where it is now re-colonizing some of its former haunts.
Threats
Disturbance by tourists can have a large effect on nesting success, with a study with nests that boats passed by within about 20 metres (66 ft) of fledging an average of .1 chicks, compared to the normal rate for that area of about .9 chicks fledged per nest. Pedestrians watching at a distance of at least 75 metres (246 ft) did not significantly affect nesting success. In the Everglades, levee and drainage systems have caused the timing of water fluctuations to change, thus causing a shift in the timing of nesting and consequently a decrease in population. In South America, habitat alteration is the primary threat, in addition to hunting and egg collecting.
Source of the article : Wikipedia
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