Wood stork
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The wood stork (Mycteria americana) is a large wading bird in the family Ciconiidae (storks). Originally described in 1758 by Carl Linnaeus, this stork is native to the subtropics and tropics of the Americas where it persists in habitats with fluctuating water levels. It is the only stork species that breeds in North America. The head and neck are bare of feathers, and dark grey in colour.<ref>Template:Cite web</ref> The plumage is mostly white, with the exception of the tail and some of the wing feathers, which are black with a greenish-purplish sheen.
Globally, the wood stork is considered to be of least concern by the International Union for Conservation of Nature. In the United States, the wood stork was previously classified as Endangered due to loss of suitable feeding habitat in the Florida Everglades, its historical population stronghold in the country.<ref>Ogden J. C. & Patty B. W. (1981) "The recent status of the wood stork in Florida and Georgia". In: Odom RQ, Guthrie JW, editors. Proceedings of the nongame and endangered wildlife symposium (Technical Bulletin WL5). Atlanta, GA, USA: Georgia Department of Natural Resources Game and Fish Division. pp. 97–102.</ref> The species has been subsequently downlisted to Threatened after northward range expansion and increased population size.
Taxonomy and etymology
The wood stork was first formally given its binomial name Mycteria americana by Linnaeus in 1758.<ref name="Linn">Template:Cite book</ref> Linnaeus originally named two separate species, M. americana and Tantalus loculator, based on different and slightly erroneous accounts, in his book Systema Naturae.<ref>Template:Cite book</ref><ref>Template:Cite book</ref> It was later identified that these binomials referred to the same species, making M. americana and T. loculator synonymous.<ref name="Allen1908">Template:Cite journal</ref> M. americana takes priority as it occurs before T. loculator.
The accepted genus name Mycteria derives from the Greek μυκτήρ : myktēr, meaning snout or trunk, and the species name americana references the distribution of this stork.<ref name="Nellis2001" />
Likely because of its decurved bill, the wood stork has formerly been called the "wood ibis", although it is not an ibis.<ref name="Yow2012">Template:Cite book</ref> It also has been given the name of the "American wood stork", because it is found in the Americas.<ref name="hbw" /> Regional names include "flinthead", "stonehead", "ironhead", "gourdhead", and "preacher".<ref name="Nellis2001">Template:Cite book</ref>
The wood stork is classified within the tribe Mycteriini (which encompasses all species of genera Anastomus and Mycteria) based on morphology and behaviour.<ref>Template:Cite journal</ref>
Description
The adult wood stork is a large bird which stands Template:Convert tall with a wingspan of Template:Convert. The male typically weighs Template:Convert, with a mean weight of Template:Convert; the female weighs Template:Convert, with a mean weight of Template:Convert.<ref name="BrantJagoe2002">Template:Cite journal</ref><ref>CRC Handbook of Avian Body Masses, 2nd Edition by John B. Dunning Jr. (Editor). CRC Press (2008). Template:ISBN.</ref> Another estimate puts the mean weight at Template:Convert.<ref>Palmer, R. S. (1962). Handbook of North American birds, Volume 1, Loons through Flamingos. Yale Univ. Press, New Haven, CT.</ref> The head and neck of the adult are bare, and the scaly skin is a dark grey. The black downward-curved bill is long and very wide at the base. The plumage is mostly white, with the Template:Birdgloss, Template:Birdgloss, and tail being black and having a greenish and purplish iridescence. The legs and feet are dark, and the pink/beige-coloured toes are pink during the breeding season. The sexes are similar.<ref name="BNA">Template:Cite journal</ref>
Newly hatched chicks have a sparse coat of grey down (Template:Birdgloss) that is replaced by a dense, wooly, and white down (Template:Birdgloss) in about 10 days. Chicks grow fast, being about half the height of adults in three to four weeks. 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.<ref name="Kahl1962" />
Distribution and habitat
The current range of the wood stork includes the southeastern United States, Mexico, Central America, Cuba, and South America.<ref name=":1">Template:Cite journal</ref>
Within the United States, small breeding populations exist in Florida, Georgia, and the Carolinas.<ref name="ReutersJune2014" /> Post-breeding birds in the United States can be found as far west as Alabama and Mississippi.<ref>Template:Cite journal</ref>
In Mexico, non-breeding birds can be found along both the Pacific and Atlantic coasts, while breeding colonies are restricted to the Pacific coast.<ref name=":2">Template:Cite journal</ref> Most descriptions of wood stork breeding colonies in western Mexico are over 35 years old, but recent sources have confirmed active nesting colonies in the southwestern states of Oaxaca and Colima.<ref name=":2" />
Cuba contains the only two known breeding locations for wood storks in the Caribbean.<ref>Template:Cite journal</ref> Both colonies exist in important wetlands - Zapata Swamp and the Sabana-Camaguey Archipelago. Outside of Cuba, sightings of wood storks are rare in the Caribbean as the birds were extirpated from Hispaniola and are vagrants on other Caribbean islands.<ref>Template:Cite book</ref>
In South America, the wood stork is found south to northern Argentina. Most breeding colonies in Brazil are concentrated in the Pantanal wetland and the northern coastal region.<ref>Template:Cite journal</ref> Birds that breed in west-central Brazil often disperse to southern Brazil and northern Argentina after breeding.<ref>Template:Cite journal</ref>
The wood stork is able to adapt to a variety of tropical and subtropical wetland habitats<ref name="RodgersSchwikert1996" /> having fluctuating water levels, which initiate breeding.<ref name="Kahl1964">Template:Cite journal</ref> It nests in trees that are over water or surrounded by water. In freshwater habitats, it primarily nests in forests dominated by trees of the genus Taxodium (in the US), while in estuaries, it generally nests on trees in the mangrove forests.<ref name="RodgersSchwikert1996">Template:Cite journal</ref> To feed, the wood stork uses freshwater marshes in habitats with an abundance of Taxodium trees, while in areas with mangrove forests, it uses brackish water. Areas with more lakes attract feeding on lake, stream, and river edges.<ref name="Kahl1964" />
Behaviour and ecology
Breeding
A resident breeder in lowland wetlands, the wood stork builds large (Template:Convert) nests in trees.<ref name=EDW18 /> In freshwater habitats, it prefers to nest in trees that are larger in diameter.<ref name="RodgersSchwikert1996" /> It nests colonially, with up to 25 nests in one tree.<ref name=EDW18 /> The height of these nests is variable, with some nests located in shorter mangrove trees being at heights of about Template:Convert, compared to a height of about Template:Convert for taller mangrove trees.<ref name="Llanes-QuevedoRodríguez-Ochoa2015">Template:Cite journal</ref> For Taxodium trees, it generally nests near the top branches, frequently between Template:Convert above the ground.<ref name="Kahl1964" /> On the tree itself, forks of large limbs or places where multiple branches cross are usually chosen.<ref name="González1999">Template:Cite journal</ref>
The nest itself is built by the male<ref name="Clark1980" /> from sticks<ref name="Hauber2014" /> and green twigs<ref name="RodgersWenner1988" /> collected from the colony and the surrounding area.<ref name="Clark1980" /> The greenery usually starts to be added before the eggs are laid but after the main structure of twigs is completed. The frequency at which it is added decreases after the eggs hatch. This greenery functions to help insulate the nest.<ref name="RodgersWenner1988">Template:Cite journal</ref> When complete, the nest is about Template:Convert in diameter,<ref name="Clark1980" /> with a central green area having an average diameter of about Template:Convert. The thickness of the edge of the nest usually measures from Template:Convert.<ref name="González1999" />
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 usually waits for its mate to try to take the nest back over.<ref name="BryanCoulter1991">Template:Cite journal</ref>
Breeding is initiated by a drop in the water level combined with an increased density of fish (with the former likely triggering the latter). 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.<ref name="Kahl1964" /> This can occur anytime between November and August. After it starts, breeding takes about four months to complete.<ref name="hbw">Template:Cite journal</ref>
This bird lays one clutch of three to five cream coloured eggs that are about Template:Convert in size.<ref name="Hauber2014" /> These eggs are usually laid one to two days apart<ref name="Clark1980" /> and incubated for 27 to 32 days<ref name="Hauber2014">Template:Cite book</ref> by both sexes.<ref name="Clark1980" /> This incubation period starts when the first egg is laid.<ref name="Kahl1964" /> 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.<ref name="Clark1980">Template:Cite journal</ref> The eggs hatch in the order in which they were laid, with an interval of a few days between when each egg hatches.<ref name="Kahl1964" />
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The chicks hatch altricial, unable to move, and weigh an average of Template:Convert.<ref name="Kahl1962" /> They are brooded for the first week after hatching, and after that when it is raining and at night.<ref name="Clark1980" /> 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,<ref name="Kahl1962">Template:Cite journal</ref> and reach sexual maturity at four years of age, although they usually do not successfully fledge chicks until their fifth year of age.<ref name="hbw" />
The hatching success, the percentage of birds that had at least one egg that hatched in a year, of the wood stork is around 62%. 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.<ref name="RodgersSchwikert1997">Template:Cite journal</ref> Overall, about 31% of nests produce at least one fledged bird.<ref name="González1999" /> Raccoons<ref name="hbw" /> and caracaras, especially crested caracaras, 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.<ref name="González1999" />
Diet and feeding
During the dry season, the wood stork eats mostly fish, 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.<ref name="González1997">Template:Cite journal</ref> The wood stork eats larger fish more often than smaller fish, even in some cases where the latter is more abundant.<ref name="OgdenKushlan1976">Template:Cite journal</ref> It is estimated that an adult wood stork needs about Template:Convert per day to sustain itself. For a whole family, it is estimated that about Template:Convert are needed per breeding season.<ref name="Kahl1964" />
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The wood stork usually forages in flocks when not breeding, and alone and in small groups when it is breeding. In the dry season, the stork generally forages by slowly walking forward with its bill submerged in water while groping for food. During the wet season, this method is used about 40% of the time to catch food. During this period, foot stirring, where the stork walks very slowly with its bill in the water while pumping its foot up and down before every step, is used about 35% of the time. Both these hunting methods are non-visual.<ref name="González1997" />
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 averages about Template:Convert in depth, while during the wet season, the water usually is about Template:Convert deep. 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 Template:Convert above the surface on average.<ref name="González1997" /> This bird can travel over Template:Convert to reach foraging sites, lending it access to a wide variety of habitats.<ref name="Kushlan1986">Template:Cite journal</ref>
Both parents feed the chicks by regurgitating food onto the nest floor. The chicks are mainly fed fish that are between Template:Convert in length, with the length of the fish typically 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 Template:Convert before it fledges.<ref name="Kahl1962" />
Predators and parasites
Raccoons are predators of wood stork chicks,<ref name="CoulterBryan1995">Template:Cite journal</ref> especially during dry periods where the water beneath nesting trees dries up.<ref name="hbw" /> Where it occurs, the crested caracara is a significant predator of eggs. Other caracaras, and hawks and vultures, also prey on both eggs and chicks.<ref name="González1999" />
In the United States, Haemoproteus crumenium, a blood protozoan, can be found in subadult and adult wood storks.<ref name="ZabranskyWebb2008">Template:Cite journal</ref> Other species of Haemoproteus also infect wood storks in Costa Rica,<ref name="ValkiūnasIezhova2004">Template:Cite journal</ref> in addition to Syncuaria mycteriae, a nematode found in the gizzard of the wood stork.<ref name="ZhangBrooks2003">Template:Cite journal</ref>
In Florida, wood storks may be eaten by some growth stage of invasive snakes such as Burmese pythons, reticulated pythons, Central African rock pythons, Southern African rock pythons, boa constrictors, yellow anacondas, Bolivian anacondas, dark-spotted anacondas, and green anacondas.<ref>Template:Cite book</ref>
Flight
When flying, this bird utilizes two different techniques. When it is not sufficiently warm and clear, such as in the late afternoon or 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 gains an altitude of at least Template:Convert through continuously flapping its wings. It can then glide for distances ranging from Template:Convert. It does not have to flap its wings during this time because the warm thermals are strong enough to support its weight.<ref name="Kahl1964" /> Because of the energy that is conserved by soaring, this stork usually uses this method to fly to more distant areas.<ref name="BryanCoulter1987" /> It flies with its neck outstretched and its legs and feet trailing behind it.<ref name="BNA" />
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Flapping -
Gliding
When flying to foraging areas, the wood stork averages a speed of about Template:Convert. In flapping flight it does Template:Convert, and about Template:Convert by gliding.<ref name="BryanCoulter1987">Template:Cite journal</ref>
Excretion and thermoregulation
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During the breeding season, the wood stork commonly defecates over the edge of its nest, while the chicks usually defecate inside.<ref name="RodgersWenner1988" /> The method of defecation of the adult differs based on temperature. Normally, it excretes by leaning forward and slightly raising its tail, with the waste either going straight down or slightly backward. When it is hot, though, the adult takes a different position, quickly moving its tail downwards and forwards while twisting its body around to aim at a leg that is bent backward (this is called urohidrosis). Which leg is aimed at is alternated. The excrement aimed at the legs is fluid and watery. It generally hits the legs around the middle of the unfeathered tibia, and runs down the leg as it is being directed by the scales. This results in evaporation, making this a method of thermoregulation. The temperature at which this starts is slightly above the threshold for panting, the latter of which takes place at temperatures of about Template:Convert and above, compared to the normal body temperature of about Template:Convert.<ref name="Kahl1963">Template:Cite journal</ref> In hot weather, breeding adults will also shade their chicks with their wings.<ref name="RodgersWenner1988" />
Evolution
Fossil record
The species most likely evolved in tropical regions and 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, which would be distinguished from the wood stork on the basis of size and on the basis of M. wetmorei's less curved mandible. This was probably a sister species; both occurred sympatrically on Cuba at the end of the Pleistocene.<ref name="SuárezOlson2003">Template:Cite journal</ref>
Phylogenetics
Template:CladogramWithin storks (Ciconiidae), the genus Mycteria, which includes the wood stork and three other extant species, is the sister group to the clade containing the genera Ephippiorhynchus, Jabiru, and Ciconia.<ref name=":0" /> Based on molecular and cytogenetic data, the wood stork is the basal group, or the earliest diverging group, within the Myceria genus.<ref name=":0">Template:Cite journal</ref><ref name="SheldonSlikas1997">Template:Cite journal</ref>
Genome
The wood stork has 72 chromosomes (2n = 72; 35 pairs of autosomes and one pair of sex chromosomes) as determined by karyotype analysis.<ref name=":0" /><ref>Template:Cite journal</ref> Like all birds, the wood stork has a ZW sex-determination system in which males have a pair of similar chromosomes, ZZ, and females have a pair of dissimilar chromosomes, ZW. Thus, females are the heterogametic sex while males are the homogametic sex. A highly contiguous genome assembly of the wood stork has been produced with 31 autosomal pairs and both sex chromosomes identified.<ref name=":1" /> Four chromosomal pairs were not identified; this is attributed to the small size of these particular chromosomes. Avian genomes tend to have approximately 10–12 pairs of large chromosomes and many smaller microchromosomes, gene-dense chromosomes with little repetitive genetic content.<ref>Template:Cite journal</ref>
Conservation status
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Globally, the wood stork is considered least concern by the International Union for Conservation of Nature due to its large range.<ref name="iucn status 12 November 2021" /> In the United States, this bird is considered to be threatened. This is a recovery from its former status as endangered, which it held from 1984 to 2014 because of a decline in its population<ref name=ReutersJune2014 /> caused by habitat loss and drought.<ref name="npr">Template:Cite AV media</ref> 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.<ref name=AmorimPiacentini2006 /> 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.<ref name=Bencke2007 />
Threats
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 ecotourism; disturbance by tourists can have an effect on nesting success, with a study finding that nests that had boats passing by them within about Template:Convert had an average of 0.1 chicks fledging, compared to the normal rate for that area of about 0.9 chicks fledging per nest.<ref name="BoutonFrederick2005">Template:Cite journal</ref> Pedestrians watching from a distance of at least Template:Convert did not significantly affect nesting success.
Habitat alteration has been implicated as the main threat to wood stork populations in the United States. In the Everglades, levee and drainage systems have caused the timing of water fluctuations to change, thus shifting the timing of nesting and consequently a decrease in population.<ref name="Kushlan1986" /> Template:Clear
References
External links
- Template:EBirdSpecies
- Wood stork – Mycteria americana – USGS Patuxent Bird Identification InfoCenter
- Florida Bird Sounds at Florida Museum of Natural History
- Template:VIREO
- Template:NeotropicalBirds
Template:Storks Template:Taxonbar Template:Authority control