Bec-ouvert africain

Identification

The African openbill is a stork measuring 80–94 cm (31–37 in) and weighing 1–1.3 kg (2.2–2.9 lb). The species does not exhibit any major sexual dimorphism, although the males are slightly larger than the females and have a longer and heavier bill. The adult bill is between 135–196 mm (5.3–7.7 in) in length, brownish in color and has a gap between the upper and lower mandible. This gap is non-existent in juvenile birds and develops as the bird ages. It is because the two mandibles of the beak only meet at the tip and not in the middle that this stork genus received the name of “openbill”.<p>The adult plumage is overall black with iridescent green feathers on their backs, coverts and abdomens. The juvenile plumage is usually less vibrant and browner in color with the feathers of the underparts having pale tips. The legs and feet are black. The eyes are grey with grey lores and no distinct coloration of the eye-ring.<p>The gap in the bill of the African openbill was first hypothesized to serve as a nutcracker, crushing the shells of the snails this stork feeds on. Scientists later demonstrated that the bill does not serve this function. Rather the opening between the two mandibles facilitates grasping the shells of the snails. The convergent tips of the mandibles prevent the slipping forward of a spherical object being carried by the storks. The curve in the lower mandible allows the stork to have the perfect angle of attack to force its way under the operculum and release the molluscs from their shells. The bill of the African openbill storks closely evolved with the bird’s specialized diet, allowing for the perfect handling of snails and other molluscs.<p>Many of the social displays of the African openbill stork are accompanied by bill clatters as is common in most of the Ciconiidae species. This bird also croaks and honks loudly.<p>These storks are smaller in stature than most of the other species in their family, which allows them to be much more agile in the air. This agility can be observed during their descents from high altitudes as they often perform steep dives and remarkable acrobatic manoeuvres. During their migration flights, the African openbill storks will use sustained flapping flight with an average wing flap rate of 200–210 flaps/minute. They will also often soar on the thermals when commuting to and from distant feeding areas. This allows them to travel long distances to obtain food without expending too much energy. In flight, these birds have their necks and head extended and their long, broad wings are spread out and perfect for soaring and gliding on the air currents.<p>The adult African openbill does not have many predators, although the species is preyed upon by the tawny eagle (Aquila rapax). The eggs of the African openbill are preyed on by the African harrier-hawk (Polyboroides typus), the African marsh-harrier (Circus ranivorus) and monitor lizards (genus Varanus).<p>Adult African openbills have dark shiny feathers on their wings and chest that serve in parade and other types of signaling between individuals of the species. These special feathers do not follow the typical structure of feathers. Usually, feathers have a rachis, barbs and barbules, but none of that is found in these parade feathers. Instead, the feathers have a single ribbon-shaped body and a few long, fine stems that disperse laterally from the flat part of the ribbon. This structure serves to create the iridescent colors observed on the feathers. The bottom part of the feathers presents the normal structure of feathers and serves for insulation purposes. It does not produce any color. <p>The coloration is perceived on the outside of the ribbon structure and therefore the iridescence is produced by the external portion of the cortex. This cortex consists of melanin nodules repeated periodically along planes inside the ribbon. These planes are parallel to each other. Scientists believe the dark green appearance of these feathers to be due to the low reflectance levels of the melanin nodules. <p>Two color-producing processes have been proposed to explain why these feathers have a green coloration: <p>The African openbill stork is considered out of danger worldwide as it is abundant in various portions of its range. There are no current estimate for the total population numbers, but they appear to be quite stable. Although they are not currently threatened, they remain a large bird with low reproductive success meaning that human activities affecting their habitat could have a big impact on the species.

Taxonomie

Nom scientifique

Anastomus lamelligerus

Autorité

Temminck, 1823

Ordre

Ciconiiformes

Famille

Ciconiidae

Genre

Anastomus

Répartition et habitat

African openbill storks live in every habitat where suitable mollusc prey can be found. Their preferred habitats remain long stretches of freshwater. They can sometimes be seen far from humid habitats, but only exceptionally. The place you are most likely to see them are on large freshwater wetlands with nearby tall trees or shrubs for them to nest in. <p>Some African openbill population are trans-equatorial migrants. Timing their migration to arrive in West Africa for the start of the dry period. They migrate in flocks and most of the reason and organisation behind these movements remain unknown. Some populations migrate while others are residents, migration in African openbill highly depends on the weather conditions.

Migration

Complete migrants

Comportement

The African openbill uses the same comfort movements as seen in other storks such as preening, scratching and stretching. In addition, they rub the crown of their heads on the feathers of their backs in a circular, rolling motion. Other comfort movements include adopting the spread wing posture while sunning. While in the spread wing posture, the African openbill stork will sometimes excrete on its. This is known as “urohidrosis” and serves for thermoregulatory purposes.<p>Few social interactions are observed when the storks are foraging. Flocks are even often seen foraging together. Sometimes hostile confrontations can occur between birds. In those situations, the dominant bird gives the Forward Threat display which lasts for a few seconds. The subordinate bird will respond to that display by adopting the Upright Display. If this doesn’t terminate the threat from the dominant bird, the subordinate will stop its display and fly away.<p>The bird erects the feathers on its neck, breast and upper back, as well as the ones on its crown which gives the appearance of a square head. The tail is usually slightly cocked, and the tip of the wings are lifted.<p>The bird finally orients its bill at its opponent and advances. The bird will occasionally hold it bill slightly ajar and shake its head as it approaches the subordinate bird.<p>As the dominant bird is approaching, the subordinate will turn away and take evasive steps moving either sideways or backwards.<p>The birds alternate their bill grabs in order to land damaging blows only rarely on the other bird.<p>After grabbing, the bill is often snapped loudly.<p>The wings are held open and sometimes flapped to help stay balanced and the bird’s tail is cocked.<p>Both sexes perform this display in succession as a greeting when one of the birds’ lands at the nest after a period of absence. As the breeding season progresses, this display is observed less.<p>The bird then leans forward while arching its neck and peering intently at the disturbing object. Sometimes the bird will also per 90° away from the disturbance.<p>Each cycle of greeting lasts between 5 and 7 seconds.<p>The bill is pointed slightly upwards and gaped open, the wingtips are slightly lifted, and the tail is cocked.<p>The birds then move their head and neck slowly downwards until their bill almost touches the ground. A series of vocalizations with a high-pitch rasping quality are given through this downwards movement at 1-2 second intervals.<p>Sometime the birds will also perform a loose shake of the head, before, during or after the greeting.<p><p>Sometimes both mates will perform this display together after completing the Up-Down display<p>The bird then oscillates left to right, grabbing sticks at the end of each oscillation.<p>When performed by both mates, the male and female cross their necks as they reach for sticks, with the male usually crossing over the female due to its larger stature.<p>The male might open and close its bill during this display, but the clattering sounds actually comes from the male’s bill striking the female’s.<p>The female opens her wings slightly and her bill between 45° and 90° downwards.<p>The male shakes its head violently from side to side while slapping the bill of the female at a rate of ~4 times per second. <p><p>The comfort movements of the nestlings are exactly the same as those of the parents. They include the typical preening, scratching and stretching as well as the spread-wing sunning and urohidrosis for thermoregulation.<p>For the first 55 days of their life, the nestlings depend on their parents for everything. They are confined to the nest and rely on their parents for food and water. For the first few weeks of their life, they remain dependant on their parents for protection from predators.<p>The main display that nestlings exhibit towards their parents is called the Begging Display. During this display, the nestlings drop down as the parent approach with their body almost horizontal. They then open their wings and pump them up and down rhythmically while giving weak and sporadic “yes” nods and occasional jerky “no” nods. They accompany these movements of nasal chittering vocalizations that are timed with the upwards motion of the head.<p>African openbill nestling only perform one display towards other individuals, which is named the Nestling Defense display. It is used when any individual unknown to the nestling approaches too close to the nest. This can be other adults from the same colony or even birds of different species. Overall, the display resembles the Begging Display with a few exceptions. During this display, the feathers of the head, neck, breast and upper back are strongly erect, the wings are held farther out from the sides and the nestlings makes high pitched screams and gobbling sounds as they try to grab at their opponent.

Conservation

Tendance de la population

stable