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. As of June 26, 2014 it is classified as a threatened species in the United States by the U.S. Fish & Wildlife Service.
Taxonomy and etymology
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.
Description
The adult is a large bird which stands 83 to 115 cm (33â€"45 in) tall and spans 140 to 180 cm (55â€"71 in) across the wings. Males typically weigh 2.5 to 3.3 kg (5.5â€"7.3 lb), with a mean weight of 2.7 kg (6.0 lb); females weigh 2.0 to 2.8 kg (4.4â€"6.2 lb), with a mean weight of 2.42 kg (5.3 lb). Another mean estimated weight for the species was 2.64 kg (5.8 lb). However, exceptionally large males are sometimes found and these can weigh up to 4.5 kg (9.9 lb). It appears all white on the ground, with blackish-gray legs and pink feet. In flight, the trailing edge of the wings is black. The head is dark brown with a bald, black face, and the thick down-curved bill is dusky yellow. The bare head and the long bill, which can measure up to 25.5 cm (10.0 in) in length, render the wood stork distinctive from other large waders in its range. The standard scientific measurements of the wood stork are as follows: the wing is 42â€"49 cm (17â€"19 in), the culmen is 19â€"25 cm (7.5â€"9.8 in) and the tarsus is 17.5â€"21.5 cm (6.9â€"8.5 in).
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. Juvenile birds are a duller version of the adult, generally browner on the neck, and with a paler 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.
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.
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 (3.3Â ft) in diameter.
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. 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 (2.2Â 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.
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. It is estimated that an adult wood stork needs about 520 grams (1.15Â lb) per day to sustain itself. For a whole family, it is estimated that about 200 kilograms (440Â 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 (6.7Â in) in depth, while during the wet season, the water usually is about 10 centimetres (3.9Â 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 7.9Â in) above the surface on average.
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 9.84Â 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 (36Â lb) before it fledges.
Aggressiveness
Wood storks occasionally without a nest occasionally try and 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 and 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 (2,000Â ft) through continuously flapping its wings. It can glide in this manner for distances ranging from 16 to 24 kilometres (9.9 to 14.9Â 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.
Predators and parasites
Each adult will defend its nest against various predators. Corvids, vultures, grackles and striped skunks will attempt to pick off eggs. Raccoons are the leading predator of nests, and can cause almost complete colony nesting failure when water dries under nests in drought years since they can easily access the nest using dry ground under the tree.
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 nests that boats have passed within about 20 metres (66Â ft) of fledging an average of .1 chicks, compared to the normal rate of about .9 chicks fledged per nest. Pedestrians watching at a distance of at least 75 metres (246Â ft) does not significantly affect nesting success.
References
Further reading
- Alsop, Fred J. III (2002). Birds of Texas. New York City: DK Publishing, Inc. ISBNÂ 0-7894-8388-2.Â
External links
- Wood Stork Species Account â€" Cornell Lab of Ornithology
- Wood stork - Mycteria americana - USGS Patuxent Bird Identification InfoCenter
- Florida Bird Sounds at Florida Museum of Natural History
- "Wood stork media". Internet Bird Collection.Â
- Wood stork photo gallery at VIREO (Drexel University)
- Wood stork species account at NeotropicalBirds (Cornell University)
- Interactive range map of Mycteria americana at IUCN Red List maps