The yellow-billed stork (Mycteria ibis), sometimes also called the wood stork or wood ibis, is a large African wading stork species in the family Ciconiidae. It is widespread in regions south of the Sahara and also occurs in Madagascar.
The yellow-billed stork (Mycteria ibis ), sometimes also called the wood stork or wood ibis, is a large African wading stork species in the family Ciconiidae. It is widespread in regions south of the Sahara and also occurs in Madagascar.
The yellow-billed stork has appeared on postage stamps in several African countries.
The yellow-billed stork is closely related to three other species in the genus Mycteria: the American woodstork (Mycteria americana), the milky stork (Mycteria cinerea) and the painted stork (Mycteria leucocephala). It is classified as belonging to one clade with these three other species because they all display remarkable homologies in behavior and morphology. In one analytical study of feeding and courtship behaviours of the wood-stork family, M.P. Kahl attributed the same general ethology to all members of the genus Mycteria, with relatively few species-specific variations. These four species are collectively referred to as the wood-storks, which should not be confused with one alternative common name (wood-stork) for the yellow-billed stork.
Before it was established that the yellow-billed stork was closely related to the American wood stork, the former was assigned to the genus Ibis, together with the milky stork and painted stork. However, the yellow-billed stork has actually long been recognised as a true stork and along with the other 3 related stork species, it should not strictly be called an ibis.
This medium-sized stork stands 90–105 cm (35–41 in) tall. Its body is white with a short black tail that is glossed green and purple when freshly moulted. The bill is deep yellow, slightly decurved at the end and with a rounder cross-section than in other stork species outside the Mycteria. Feathers extend onto the head and neck just behind the eyes, with the face and forehead being covered by deep red skin. Both sexes are similar in appearance, but the male is larger and has a slightly longer heavier bill. Males and females weigh approximately 2.3 kg (5.1 lb) and 1.9 kg (4.2 lb) respectively.
Colouration becomes more vivid during the breeding season. In the breeding season, the plumage is coloured pink on the upperwings and back; the ordinarily brown legs also turn bright pink; the bill becomes a deeper yellow and the face becomes a deeper red.
Juveniles are greyish-brown with a dull, partially bare orange face and a dull yellowish bill. The legs and feet are brown and feathers are blackish-brown all over. At fledging, salmon-pink colouration in the underwings begins to develop and after about one year, the plumage is greyish-white. Flight feathers on the tail and wing also become black. Later, the pink colouration typical of adult plumage appears.
These storks walk with a high-stepped stalking gait on the ground of shallow water. Their approximate walking rate has been recorded as 70 steps per minute. They fly with alternating flaps and glides, with the speed of their flaps averaging 177–205 beats per minute. They usually flap only for short journeys and often fly in a soaring and gliding motion over several kilometres for locomotion between breeding colonies or roosts and feeding sites. By soaring on thermals and gliding by turns, they can cover large distances without wasting much energy. On descending from high altitudes, this stork has been observed to dive deeply at high speeds and flip over and over from side to side, hence showing impressive aerobatics. It even appears to enjoy these aerial stunts.
This species is generally non-vocal, but will utter hissing falsetto screams during social displays in the breeding season. These storks also engage in bill clattering and an audible “woofing” wing beat at breeding colonies Nestlings make a loud continual monotonous braying call to beg parental adults for food.
The yellow-billed stork occurs primarily in Eastern Africa,but is widely distributed in areas extending from Senegal and Somalia down to South Africa and in some regions of western Madagascar. During one observation of a mixed species bird colony on the Tana River in Kenya, it was found to be the commonest species there, with 2000 individuals being counted at once.
The species does not generally migrate far, at least not outside its breeding range; but does usually make short migratory movements which are influenced by rainfall. It makes local movements in Kenya and has also been found to migrate from North to South Sudan with the rainy season. It may also migrate regularly to and from South Africa. However, little is actually known about this bird's general migratory movements. Due to apparent observed variation in migratory patterns throughout Africa, the yellow-billed stork has been termed a facultative nomad. It may migrate simply to avoid areas where water or rainfall conditions are unfavourable for feeding on prey. Some populations migrate considerable distances between feeding or breeding sites; usually by using thermals to soar and glide. Other local populations have been found to be sedentary and remain in their respective habitats all year round.
Preferred habitats include wetlands, shallow lakes and mudflats, usually 10–40 cm deep[4] but it usually avoids heavily forested regions in central Africa. It also avoids flooded regions and deep expansive bodies of water because feeding conditions there are unsuitable for their typical grope and stir feeding techniques.
This species breeds especially in Kenya and Tanzania. Although it is known to breed in Uganda, breeding sites have not been recorded there. It has been found to breed also in Malakol in Sudan and often inside walled cities in West Africa from Gambia down to northern Nigeria. Still other breeding sites include Zululand in South Africa and northern Botswana, but are rarer below northern Botswana and Zimbabwe where sites are well-watered. Although there is no direct evidence of current breeding in Madagascar, young birds unable to fly have been observed near Lake Kinkony during October.
This stork's diet mainly comprises small, freshwater fish of about 60-100mm length and maximally 150g, which they swallow whole. They also feed on crustaceans, worms, aquatic insects, frogs and occasionally small mammals and birds.
This species appears to rely principally on sense of touch to detect and capture prey, rather than by vision. They feed patiently by wading through the water with partially open bills, probing for prey at intervals. Contact of the bill with a prey item prompts a rapid snap-bill reflex, whereby the bird snaps shut its mandibles, raises its head and swallows the prey whole. The speed of this reflex in the closely related American woodstork (Mycteria americana) has been recorded as 25 milliseconds and although the corresponding reflex in the yellow-billed stork has not been quantitatively measured, the yellow-billed stork's feeding mechanism appears to be at least qualitatively identical to that of the American wood stork.
In addition to the snap-bill reflex, the yellow-billed stork also uses a systematic foot stirring technique to sound out evasive prey. It prods and churns up the bottom of the water as part of a “herding mechanism” to force prey out of the bottom vegetation and into the bird's bill. The bird does this several times with one foot before bringing it forwards and repeating with the other foot. Although they are normally active predators, they have also been observed to scavenge fish regurgitated by cormorants.
The yellow-billed stork has been observed to follow moving crocodiles or hippopotami through the water and feed behind them, appearing to take advantage of organisms churned up by their quarry. Feeding lasts for only a short time before the bird obtains its requirements and proceeds to rest again.
Parents feed their young by regurgitating fish onto the nest floor, whereupon it is picked up and consumed by the nestlings. The young eat voraciously and an individual nestling increases its body weight from 50 grams to 600 grams during the first ten days of its life. Hence, this species has earned the German colloquial common name “Nimmersatt”; meaning “never full”.
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