Yellow-billed Stork

The yellow-billed stork, 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.


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.


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.  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. This species breeds especially in Kenya and Tanzania.


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. 


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”.


Source : Wikipedia