0045 043204434

Yellow-billed Stork

Mycteria ibis


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.

It is a medium-sized stork standing 90–105 cm (35–41 in) tall. The body is white with a short black tail that is glossed green and purple when freshly moulted.[4] The bill is deep yellow, slightly decurved at the end and has 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 all over the body are blackish-brown. 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 begins to appear.

These storks walk with a high-stepped stalking gait on the ground of shallow water and 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 utters 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.

Fun Facts

Behaviour And Ecology
Food And Feeding

Their diet comprises mainly small, freshwater fish of about 60-100mm length[3] 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 mainly on sense of touch to detect and capture prey, rather than by vision.They feed patiently by walking through the water with partially open bills and probe the water for prey. Contact of the bill with a prey item is followed by 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 woodstork.

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,[8] they have also been observed to scavenge fish regurgitated by cormorants.[8]

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”.

Breeding Behaviour

Breeding is seasonal and appears to be stimulated by the peak of long heavy rainfalln and resultant flooding of shallow marshes, usually near Lake Victoria.This flooding is linked to an increase in prey fish availability;and reproduction is therefore synchronised with this peak in food availability. In such observations near Kisumu, M.P. Kahl’s explanation for this trend was that in the dry season, most prey fish are forced to leave the dried-up, deoxygenated marshes that cannot support them and retreat to the deep waters of Lake Victoria where the storks cannot reach them. However, fish move back up the streams on the onset of rain and spread out over the marshes to breed, where they become accessible to the storks. By nesting at this time and providing that the rains do not end pre-maturely, the storks are guaranteed a plentiful food supply for their young.

The yellow-billed stork may also begin nesting and breeding at the end of long rains. This occurs especially on flat extensive marshlands as water levels gradually decrease and concentrate fish sufficiently for the storks to feed on.However, unseasonal rainfall has also been reported to induce off-season breeding in northern Botswana[19] and western and eastern Kenya. Rainfall may cause local flooding and hence ideal feeding conditions.This stork appears to breed simply when rainfall and local flooding are optimal and hence seems to be flexible in its temporal breeding pattern, which varies with rainfall pattern throughout the African continent.

As with all stork species, male yellow-billed storks select and occupy potential nest sites in trees, whereupon females attempt to approach the males. The yellow-billed stork has an extensive repertoire of courtship behaviours near and at the nest that may lead to pair formation and copulation. Generally, these courtship behaviours are also assumed to be common to all Mycteria species and show remarkable homology within the genus Mycteria. After the male has initially established at the nesting-site and the female begins to approach, he displays behaviours that advertise himself to her. One of these is the Display Preening, whereby the male pretends to strip down each of his extended wings with the bill several times each side and the bill does not effectively close around the feathers. Another observed display among males is the Swaying-Twig Grasping. Here, the male stands on the potential nesting-site and bends over to gently grasp and release underlying twigs at regular intervals.This is sometimes accompanied by side-to-side oscillations of the neck and head and he continues to pick at twigs in between such movements.

Reciprocally, approaching females display their own distinct behaviours. One such behaviour is the Balancing Posture, whereby she walks with a horizontal body axis and extended wings toward the male occupying the nesting-site. Later, when the female continues to approach or already stands near an established male, she may also engage in Gaping. Here, the bill is gaped open slightly with the neck inclined upward at about 45o and often occurs in conjunction with the Balancing-Posture. This behaviour ordinarily continues if the male accepts the female and has allowed her to enter the nest, but the female usually closes her wings by this time.[3] The male may also continue his Display-Preening when standing next to the female in the nest.

During copulation, the male steps onto the female’s back from the side, hooks his feet over her shoulders, holds out his wings for balance and finally bends his legs to lower himself for cloacal contact, as happens in most birds. In turn, the female holds out her wings almost horizontally. The process is accompanied by bill clattering from the male as he regularly opens and closes his mandibles and vigorously shakes his head to beat his bill against the female’s.[3] In turn, the female keeps her bill horizontal with the male’s or inclined downward at approximately 45 degrees.[3] Average copulation time in this species has been calculated as 15.7 seconds.[3]

The male and female build the nest together either in high trees on dry land away from predators, or in small trees over water.[4] Nest building takes up to 10 days.[4] The nest may be 80–100 cm in diameter and 20–30 cm thick.[4] The female typically lays 2-4 eggs (usually 3)[4] on alternate days[20] and average clutch size has been recorded as 2.5.[21] The male and female share duties to incubate the eggs, which takes up to 30 days.[4] As in many other stork species,[22][23] hatching is asynchronous (usually at 1- to 2-day intervals), so that the young in the brood differ considerably in body size at any one time.[3] During food shortage, the smaller young are at risk of being outcompeted for food by their larger nest-mates.

Both parents share duties of guarding and feeding the young until the latter are about 21 days old.Thereafter, both parents forage to attend to the young’s intense food demands.Alongside parental feeding by regurgitation of fish, parents have also been observed to regurgitate water into the open bills of their nestlings,[24] especially on hot days. This may aid the typical thermoregulatory strategy of the young (common to all stork species) to excrete dilute urine down their legs in response to hot weather. Water regurgitated over the young serves as a water supplement in addition to fluid in their food, so that they have sufficient water to continue urinating down their legs to avoid hyperventilation. Additionally, parents sometimes help keep the young cool by shading them with their open wings.

The nestlings usually fledge after 50–55 days of hatching[ and fly away from the nest. However, after leaving the nest for the first time, the offspring often return there to be fed by their parents and roost with them for another 1–3 weeks It is also thought that individuals are not fully adult until 3 years old and despite lack of data, new adults are thought to not breed until much later than this.

Fledglings have also been observed to not differ considerably in their foraging and feeding strategies from adults. In one investigation, four adult, hand-reared yellow-billed storks kept in captivity showed typical grope-feeding and foot stirring shortly after they were introduced to bodies of water.[3] Hence, this suggests that such feeding techniques in this species are innate.

These birds breed colonially, often alongside other species;] but the yellow-billed stork is sometimes the only occupant species of a nesting site. A subset of up to 20 individuals may nest close together in any one part of a colony;with several males occupying potential nest sites all in the same place. If many of these males do not acquire mates, the whole group moves on with the unpaired females to another tree. These “bachelor parties” are a noticeable feature of colonies of this species and usually consist of 12 or more males and at least as many females.As many as 50 nests have been counted all at once in a single breeding area.

Other Behaviours

Despite their gregariousness during breeding, most individuals generally ignore each other outside nesting-sites;although some hostile encounters may occur. Some of these encounters involve one individual showing an unambiguous attack or escape response if there is a large difference in social status between the two individuals.  However, if two individuals are equally matched, they slowly approach each other and show a ritualised display called the Forward Threat. Here, one individual holds its body forward horizontally and retracts the neck so that it touches the crown, with the tail cocked at 45 degrees and all feathers erect.It approaches the opponent and points its bill at it, sometimes gaping. If the opponent does not capitulate, the attacker may grab at it with its bill and the two may briefly spar with their bills until one retreats in an erect stance with compressed plumage.

Hostility can also arise between opposite sexes when a female approaches a male on a potential nest site. Both sexes may display a similar aforementioned Forward Threat, but clatter their bills after grabbing with them at the other stork and extend their wings to maintain balance.Another hostile behaviour between sexes is the Snap Display,[whereby they snap horizontally with their bills while standing upright. This may occur during and immediately after pair formation, but subsides later in the breeding cycle as the male and female become familiar with each other and it eventually disappears.

Nestlings show remarkable behavioural transformations at 3 weeks of age. During the constant parental attendance before this time, the young show little fear or aggression in response to intruders (such as a human observer), but are found to merely crouch low and quietly in the nest. After this time, when both parents go foraging and leave the young in the nest, a nestling shows strong fear in response to an intruder. It either attempts to climb out of the nest to escape or acts aggressively toward the intruder.


Leave a comment