Species of bird
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The maguari stork (Ciconia maguari) is a large species of stork that inhabits seasonal wetlands over much of South America, and is very similar in appearance to the white stork; albeit slightly larger. It is the only species of its genus to occur in the New World and is one of the only three New World stork species, together with the wood stork and the jabiru.
Taxonomy.
The maguari stork was formally described in 1789 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's "Systema Naturae". He placed it with the herons, cranes and storks in the genus "Ardea" and coined the binomial name "Ardea maguari". Gmelin's description was ultimately based on the "Maguari Brasiliensibus" that had been described and illustrated in 1648 by the German naturalist Georg Marcgrave in his book "Historia Naturalis Brasiliae". The maguari stork is now one of seven storks placed in the genus "Ciconia" that was introduced in 1760 by the French zoologist Mathurin Jacques Brisson. The genus name is the Latin word for "stork". The specific epithet "maguari" and the common name are from the Tupi language, "Maguarí" or "Baguarí" means "heavy bill". The species is monotypic: no subspecies are recognised.
This stork was formerly placed in its own genus "Euxenura", but was later reclassified as belonging to "Ciconia" because of its strong morphological and ethological similarity to other storks of this genus. However, the maguari stork most closely resembles both the white stork and the Oriental stork in morphology and behaviour; and strongly resembles the white stork in its manner of performing the up-down greeting display. Plumage pattern and soft part colourations are also very similar between these three stork species. Further, phylogenetic analyses based on a portion of the cytochrome b oxidase gene have suggested that the maguari stork is evolutionarily paired with the white stork-Oriental stork sister group; although the morphological similarity between the maguari stork and the Oriental stork has been considered to be greater than between the white stork and the Oriental stork. Incidentally, the maguari stork also shares the prominent forkedness of its tail with the woolly-necked stork ("Ciconia episcopus").
Fossils of the extinct Pleistocene stork "Ciconia maltha" discovered in North America appear to be morphologically intermediate between the maguari stork and white stork and may therefore represent a common ancestral link between these two species that inhabit different continents. "Ciconia maltha" was probably highly dispersive and could have extended its range to Venezuela from North America during the Pliocene, part of the range of the extant maguari stork.
Description.
Adult appearance.
The bird stands at tall and has a wingspan of around , making it similar in size to the congeneric white stork. Nine male maguari storks weighed an average of while five females weighed an average of . It is intermediate in size between the smaller wood stork and the larger jabiru, the two other stork species with which the maguari stork exists in sympatry in some parts of its range. Much of the adult plumage is white, with black flight feathers and a forked blacked tail which is shorter than the stiff white under-tail coverts, so that these protrude from underneath the tail and may function aerodynamically in flight. The forked tail clearly distinguishes the maguari stork from the white stork, and is easily observed in flight from the ground thanks to the white under-tail coverts. During flight, this stork offers an impressive sight. It soars at least a hundred metres above the ground with an outstretched neck and extended legs, intermittently beating its broad wings to gain momentum for long glides. It flaps its wings at a rate of 181 beats per minute and the wingspan measures 150–180 cm. This stork needs to make three long jumps before it can take off from the ground.
Individuals undergo a fresh moult in the breeding season to produce brighter plumage in preparation for courtship. Head and neck feathers consist of long semi-plumes measuring up to 18 cm long that become erectile and are important in courtships and aggressive displays. Barbs on the neck feathers also have no barbules, thus making the neck appear like a translucent net.
The bill is straight, bluish grey, and lined with red. The final third portion along the bill length is dark maroon. The iris is lemon-yellow or cream white and the legs are purplish red. The skin of the throat and the pebbly-textured lores are orangey red, becoming deeper red during courtship. The sexes are largely indistinguishable externally, except that the male is slightly larger with a slightly upward curved bill. Individuals usually weigh 3800-4200 g, with males being heavier.
Juvenile appearance.
A distinct characteristic unique to this stork species is intermediately dark plumage in the young, which persists for most of the nestling period. In the chicks’ first few days after hatching, their sparse down is snowy white. The chicks subsequently undergo two basic moults before entering adulthood. The first moult usually begins after four days, in which black semi-plumes on the head and neck begin to emerge; followed shortly by the emergence of greyish-black down feathers over the body after one week of age. Some white down feathers initially remain attached to give a temporary mottled black and white appearance before finally receding to leave a darkish grey downy plumage. During development of the dark down, the bill, legs and feet are shiny black. A pale yellow stripe extends up the venter, the gular pouch is bright orange, and the iris is dark brown. At hatching, nestlings weigh 76-90g.
After the chicks pass their third week of age, the black down develops olive streaks and the only parts of the body that do not appear dark are the bright orange gular skin and a small patch of white down above and below the tail. Just over 10 days later, black flight feathers develop, shortly followed by black contour feathers. At this point, the chicks are fledged and have their first full juvenile plumage after about a month of hatching. Hence, juveniles remain black at fledging, by which time the orange of the gular pouch has also turned pale scarlet.
In the second moult, white down begins to return when the chicks are seven weeks old, followed by white semi-plumes then white contour feathers. The first white basic plumage is complete after three months and resembles that of adults. By this time, the legs and feet are now an incipient pink and the bill has become bicoloured (pale blue and maroon). Although the young now resemble adults, the skin surrounding the eye remains black for about a year before turning red, and the dark brown iris does not begin to yellow until after two years old. These are two potentially useful features that identify juvenile maguari storks; but in the field, the dark iris of the juveniles is the most reliable feature that distinguishes them from adults.
Two hypotheses have been proposed to explain the dark colouration in the juveniles. First, the blackish feathers may serve as camouflage against both ground and aerial predators. Alternatively, the black plumage may serve a thermoregulatory function by enhancing its absorption of the sun's radiation. This could arguably be advantageous because this stork breeds during the winter in the part of its range in the southern hemisphere, so that the chicks can be exposed to low environmental temperatures. However, Thomas regards the anti-predator explanation for the dark plumage to be more plausible.
Vocalisations.
Adults utter wheezing, hissing, disyllabic whistles during up-down greeting displays at the nests about every 1–1.5 seconds. These whistles are slower and lower pitched than the corresponding vocalisations of other Ciconia, but sound similar to those of the Abdim's stork and the woolly-necked stork. The quality of these vocalisations is probably linked with the maguari stork's ground nesting habits and may be an adaptation to minimise attraction of predators’ attention. Young make begging calls that have been described as "Ehehe-ehehe".
Distribution and habitat.
The maguari stork has a relatively wide distribution over much of South America, and occurs primarily to the east of the Andes. It lives in the Llanos of Venezuela and eastern Colombia; Guyana; eastern Bolivia; Paraguay; Brazil, but rarely in the Amazon and the north-east,); Uruguay and Argentina. The most southerly part of the range lies in the Chubut province. It more rarely occurs to the west of the Andes (e.g. in Chile) and probably does not breed there. It is a rare visitor to the Suriname coast from March until May and also been reported as a vagrant on Trinidad and Tobago.
It is especially common and widespread in the Chaco of Argentina, which appears to be a popular destination for flocks of about 30-40 migrant individuals from the southern hemisphere part of the range that come from the south in winter to seek warmer temperatures. The stork is also common in Brazil, especially in the state Rio Grande do Sul, Paraguay and the Pampas of Argentina. It occurs seasonally and is common in the Pantanal of Argentina. Large numbers migrate to the Pantanal in the wet season, probably from the Parana Basin and Rio Grand do Sul. However, overall migration patterns for this species across its range have thus far not been determined exactly.
Its habitat largely comprises open lowland and shallow-water wetland such as tropical wet savannah grasslands, marshes, mudflats, and flooded fields. It more occasionally frequents dry fields, but invariably avoids forested regions. Numerous maguari stork assemblages have been observed in their habitat during the dry season, where they forage in low-level bodies of water where prey is concentrated.
The maguari stork lives in sympatry with jabiru and wood stork where the ranges of these three species overlap, especially in the Venezuelan llanos. Of all American stork species, the maguari stork has the smallest geographical range.
Behavior and ecology.
Feeding.
This stork has a markedly broad diet and is considered to be a generalist compared to the sympatric wood stork and jabiru. It feeds on fish, frogs, eels, earthworms, invertebrates, insect larvae, snakes, freshwater crabs, small mammals such as rats, and bird eggs. More rarely, it may take smaller birds; as one case has shown where a large, intact rail was discovered in the gullet of an individual from Patagonia. Despite the maguari stork's apparently generalist diet, one study from Brazil has suggested that this stork may actively target worm lizards ("Amphisbaena") as prey items. This may be because of the elongated body shape of such taxa that occupies a relatively small volume in the bird's stomach and hence can more compactly fit inside the stomach to optimise the bird's energy intake.
The maguari stork preferentially forages in shallow waters about 12 cm deep, and more rarely at water depths of up to 30 cm. This may be because shallow waters harbour higher numbers of prey taxa, or are high in dissolved carbon and nutrients. This species is primarily a visual forager. Its usual manner of hunting consists in walking slowly through wetlands with its bill close to the surface of the water, ready to seize any prey it comes into contact with. The bird breeds early during the seasonal rains while the water in the wetland habitat is still undisturbed by fresh rainwater, so that prey items are more visible through the water and success of prey capture, especially as food for nestlings, is higher. However, this stork has also been observed to grope with its bill in the water, although this may be more common toward the end of the breeding season when water bodies begin to dry up and become turbid.
Especially during the breeding season, the maguari stork forages solitarily or in pairs. However, it may also feed in larger aggregations outside the breeding season, often in association with other wading bird species. Because prey are concentrated in the shallowness of these pools, tactile foraging would be highly efficient in this situation.
Although the maguari stork largely depends on shallow freshwaters as a source of prey, it has also been observed to forage solitarily on dry plains outside the breeding season where mice and toads (candidate prey) sometimes occur in large numbers. It also forages on dry cultivated fields where invertebrates may have been disturbed. During the evenings in the dry season from December until April, individuals form large assemblages around low-lying water features where prey density is high but prey abundance is not. Notably