Olive ibis

Appearance

This relatively small ibis measures 65–75 cm in length, depending on subspecies. Recorded wing lengths in the various subspecies are 330–334 mm in "olivacea", 309–355 mm in "cupreipennis", 343–372 mm in "akeleyorum", and 328 mm and 313 mm in males and females respectively of "rothschildi". The culmen in "olivacea" measures 95–96 mm. Single male specimens of the subspecies "akeleyorum", "cupreipennis" and "rothschildi" have reported culmen lengths of 108 mm, 85 mm and 95 mm respectively; with the corresponding lengths on females being 102 mm, 94 mm and 90 mm respectively.

"B. o. cupreipennis" and "rothschildi" are roughly equal in size, "olivacea" is slightly larger, and "akleyorum" is the largest of all subspecies and also has the longest bill. The number of available specimens of this species is however considered too small to confirm whether this pattern of size variation among the subspecies holds generally.

The adult plumage is dusky brown with green and bronze iridescent tinges. The head and neck are also brown, and pale stripes extend underneath the eye. The face around the orbital and loral regions is bluish black and the bill is coral red. The tail coverts and tail are dark blue. The back and rump are dusky greenish bronze. The secondary flight feathers and wing coverts stand out from the plumage through their pinkish green tinge. The colour of the legs and feet has been described as dull dark red, yellowish green or pinkish brown. The differing descriptions of soft part colourations probably refer to individuals in different stages of breeding.

Colouration differs slightly between the different subspecies. The plumage in the standard "olivacea" race is said to have a warm brown tone. The back in the "akleyorum" subspecies is greener than in the other subspecies, with wing coverts that are also green as opposed to coppery brown in "olivacea". "Cupreipennis" has more green on the neck and body than in the "olivacea" subspecies. The rump in "rothschildi" is said to be purplish blue with olive green reflections.

The olive ibis has a distinctive brown crest which is purple near the neck, clearly distinguishing it at close range from the similar hadada and spot-breasted ibises. The purple is more extensive on the crest of "rothschildi". The crest is not usually seen in flight because of the flyer's extended neck. The olive ibis and spot-breasted ibis are reportedly impossible to distinguish superficially in the field, but the "akeleyorum" subspecies appears heavier than the hadada ibis and has a thicker neck.

This ibis has a straggling but strong and powerful flight with a wingbeat that is apparently similar to that of night herons. It is very difficult to identify in the field and much more commonly seen when in flight at dawn and dusk. Flocks of the "akeleyorum" subspecies have been observed to fly high above the forests at about 800 ft, presumably en route to montane forests to roost. The natal young have uniformly brownish-black down, and their bare facial patches reportedly have the same pattern and spatial extent relative to the feathered tracts as those of adults. They however lack the long crest found in an adult.

Distribution

This species is patchily distributed in dense forests from the West to East coast of tropical Africa. The countries in which it natively occurs are Cameroon, the RCongo, DRCongo, Cote d’Ivoire, Gabon, Ghana, Kenya, Liberia, Nigeria, Sierra Leone and Tanzania, besides Príncipe and São Tomé islands. Alongside its dense forest habitat, it has also been recorded in regenerating forest in Gabon and cliff holes.

Status

The olive ibis is evaluated as Least Concern by the IUCN because the population does not apparently approach the threshold for Vulnerable under the range and population trend criteria.

Because this species is very secretive with largely inaccessible habitats, it has proved difficult to census. The world population has been conservatively estimated at 3,000-25,000 wild individuals. It is not globally threatened, but the putatively small world population is considered to be declining due to deforestation and hunting. However, the extent of its population decline is not considered large enough to render the world population Vulnerable. Because of the difficulty in identifying wild individuals, this species may be more common than documented; especially also considering its frequently heard vocalisations.

The "rothschildi" subspecies was believed to have become extinct on the island of Príncipe off the West African coast in 1901. However, this race was last reportedly sighted there in 1991; and a group of fewer than 10 wild individuals may remain on this island.

Reproduction

This ibis breeds solitarily. It was first recorded breeding in 1910 on Mount Kenya at 9,000 feet. Later breeding records of this ibis have been made adjacent to water in East Africa. One nest was discovered at the headwaters of the Pesi River in the northern Aberdares in mixed "Juniperus/Podocarpus" forest, and another at a streamlet near the Thiririka River in the southeastern Aberdares in mixed "Podocarpus/Ocotea" forest. In both cases, the nest measured 20–40 cm in diameter, was situated 2-4m above the surface of the water and precariously balanced in slender branches. Generally, the nest is loosely composed of dead branches supported on a tree limb and appears surprisingly unstable for a bird of its size. Additionally, both the male and female have been observed to silently construct the nest. Although only nests of "akleyorum" have been thus far discovered, those of the remaining subspecies are probably similar in structure and location. The only breeding record in West Africa comes from Liberia in June.

From the few known breeding records in Kenya, egg-laying appears to take place from June to August. The clutch typically comprises three eggs. The egg is pea green with cinnamon rufous stains and chesnut-brown marks. It typically weighs about 50g and measures 56-58 x 40-41mm.

Food

The olive ibis feeds on insects such as beetles, grubs and snails. It also reportedly feeds on myriapods, forest floor vegetation and snakes. It feeds singly, in pairs or in flocks usually comprising 5-12 individuals in densely forested areas along small stretches of fast flowing water, but also in mangroves, marshes, swamps and backwaters as well as larger rivers such as St. Paul's River in Liberia and Camma River in Gabon. Observations from the "Hagenia" forests of Kenya suggest that this ibis may preferentially forage on the forest floor with little or no undergrowth.

The olive ibis preferentially roosts in tops of large dead trees at night such as silk-cotton and probably uses the same ones daily. It apparently uses the same routes daily to fly between roosting and feeding grounds. When landing on a tree branch from flight, the ibis does not do so at right angles, but along the length of the branch and runs a short distance along the branch after landing.

Predators

This species is affected by hunting and habitat destruction through deforestation. Secondary threats probably include predation by forest birds and mammals such as the olive baboon "Papio anubis" and yellow baboon "P. cynocephalus". However, the extent of all threats is unknown because the population of this ibis is largely undocumented.

When disturbed, the olive ibis makes no sound as it flies away and hides itself in thickly leaved trees.