Yellow-billed Stork

Breeding is seasonal and appears to be stimulated by the peak of long heavy rainfall 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 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. 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. In turn, the female keeps her bill horizontal with the male's or inclined downward at approximately 45 degrees. Average copulation time in this species has been calculated as 15.7 seconds. The male and female build the nest together either in high trees on dry land away from predators, or in small trees over water. Nest building takes up to 10 days. The nest may be 80–100 cm in diameter and 20–30 cm thick. The female typically lays 2-4 eggs (usually 3) on alternate days and average clutch size has been recorded as 2.5. The male and female share duties to incubate the eggs, which takes up to 30 days. As in many other stork species, 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. 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, 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. 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.

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. It does not generally migrate far, at least not out of its breeding range; but usually makes 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 too high or too low 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. Its preferred habitats include wetlands, shallow lakes and mudflats, usually 10–40 cm deep 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.

Not globally threatened.