The olive ibis (Bostrychia olivacea) is a species of ibis native to dense tropical forests in central Africa. Between 65 and 75 cm in length, it is a small ibis with olive plumage displaying an iridescent sheen. Four subspecies are recognized.
Taxonomy and systematics Historically, the olive ibis was included in various genera such as Geronticus, Comatibis and Harpiprion before finally being classified among the Bostrychia.Salvadori T., 1903. On the ibis Olivacea of Dubus. Ibis 45: 178-188. It is also related to the Madagascar crested ibis Lophotibis cristata and Geronticus ibises.Hancock J. A., Kushlan J. A., Kahl M. P., 1992. Storks, Ibises and Spoonbills of the World. Academic Press.
Four subspecies of this ibis are currently recognised: Bostrychia olivacea olivacea, B. o. cupreipennis, B. o. akleyorum and B. o. rothschildi. The dwarf olive ibis Bostrychia bocagei of São Tomé was also formerly considered to be a subspecies of the olive ibis, but is now classified as a separate species because of its considerably smaller body size and different morphology.Collar N. J., Stuart S. N., 1985. Threatened birds of Africa and related islands: the ICBP/IUCN Red Data Book. (Third edition, part 1). Cambridge, U.K.: International Council for Bird Preservation, and International Union for Conservation of Nature and Natural Resources.
The various subspecies inhabit different regions of Africa and despite differences in size and colouration; they are still regarded as belonging to the same species.Amadon D., 1963. Avian systematics and evolution in the Gulf of Guinea. Bulletin of the American Museum of Natural History 100: 393-452. These individual subspecies probably formed by geographic isolation through expansive grasslands or seas that separate each subspecies' forest habitat, hence preventing passage of individuals between the subpopulations.Chapin J. P., 1923. The Olive Ibis of Dubus and its representative on Sao Thome. American Museum Novitates 84: 1-9. This probably also led to the formation of the dwarf olive ibis species of São Tomé.
Description
Appearance This relatively small ibis measures 65–75 cm in length, depending on subspecies.Matheu E., del Hoyo J. & Kirwan G. M., 2016. Olive Ibis (Bostrychia olivacea). In: del Hoyo J., Elliott A., Sargatal J., Christie D. A. & de Juana E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved fromhttp://www.hbw.com/node/52763 on 1 August 2016). 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.Brown L. H., Urban E. K., Newman K., 1982. The Birds of Africa Vol. I. Academic Press. 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.Chapin J. P., 1921. A note on the genus Lampribis in East and Central Africa. American Museum of Natural History 33: 609-610. 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.Bannerman D. A., 1919. Exhibition of a rare ibis Lampribis olivacea and description of a new species Lampribis rothschildi. Bulletin of British Ornithologists' Club 40: 6-7.
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.Sclater W. L., Moreau R. E., 1933. Taxonomic and field notes on some birds of North Eastern Tanganyika Territory. – Part V. Ibis 3: 399-440 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.Meinertzhagen R., 1937. Some notes on the birds of Kenya Colony, with especial reference to Mount Kenya. Ibis 1: 731-760.
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.Chapman F. M., 1912. A new ibis from Mt. Kenia, British East Africa. Bulletin of the American Museum of Natural History 31: 235-242. 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.
Vocalisations The call is a distinctive, loud, squawky "gar-wa", repeated several times and uttered in flight high above the trees. Reportedly, it only vocalizes in flight and then only during twilight. The rothschildi subspecies instead utters a call described as "HAAN-ha HAAN-ha". Other calls have been transcribed as "aka-a", "ka" and "kau". When threatened by an intruder, it repeatedly utters the "ga" element at the threat. Vocalisations at the nest have not been documented because the sound of rushing stream water may mask these noises. This ibis remains silent during the day when feeding in its typical habitat, so is difficult to detect at this time.
The call of the olive ibis also sounds harsh but measured, unlike the clamorous, three-syllable shrieking of the hadada ibis. The call of the spot-breasted ibis lacks the honking timbre of that of the olive ibis and the olive ibis does not emphasise the second note of its call as does the spot-breasted ibis. The pattern of the call is therefore a more reliable feature than superficial appearance by which to distinguish the olive ibis and spot-breasted ibis in the field.
Distribution and habitat 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, Côte d'Ivoire, Gabon, Ghana, Kenya, Liberia, Nigeria, Sierra Leone and Tanzania, besides Príncipe and São Tomé islands.Ash J. S., 1990. Additions to the avifauna of Nigeria, with notes on distributional changes and breeding. Malimbus 11: 104-116. Alongside its dense forest habitat, it has also been recorded in regenerating forest in Gabon and cliff holes.
The different subspecies inhabit different geographical regions in Africa. Bostrychia olivacea olivacea and B. o. cupreipennis occur in the west. The former is found in Upper Guinea in countries such as Sierra Leone, Liberia and Cote d'Ivoire; and the latter found in Lower Guinea in countries slightly further south such as Cameroon, Gabon, RCongo and DRCongo. B. o. cupreipennis is also found more toward central Africa than olivacea, which is more common on the western coast. B. o. rothschildi lives on Príncipe island and São Tomé.
B. o. akleyorum occurs in the montane forests of eastern Africa at altitudes of 1,800-12,000 m above sea level. It is found in the eastern DRCongo, on Mount Kenya and in the Aberdares of Kenya, Kilimanjaro in Tanzania, on Mount Meru and on Mount Elgon; but is apparently absent from lowland forests in east Africa such as the Kakamega tropical rainforest in Kenya.Brooks T. M., Pimm S. L., Oyugi J. O., 1999. Time lag between deforestation and bird extinction in tropical forest fragments. Conservation Biology 13: 1140-1150. However, its call was once reportedly heard in the Bwamba lowlands of Uganda.Turner D. A., 1998. East African Rare Birds Committee. Scopus 20: 84. It is readily distinguished from the similar hadada ibis by elevation of habitat since the latter occurs at altitudes in this region of less than 6,000 feet. The olive ibis may be the only resident African species to inhabit only lowland tropical forests in West Africa and only higher-altitude montane forests in the East.
The olive ibis is probably largely sedentary, although short migrations appear to happen in Liberia in extremely dry weather; during which individuals move from northern Grand Gedeh county to regions further south such as Glaro, Sapo and Sinoe.
Behaviour and ecology
Feeding and roosting
The olive ibis feeds on insects such as beetles, grubs and snails. It also reportedly feeds on myriapods,Jackson F. J., 1938. "The Birds of Kenya Colony and the Uganda Protectorate", vol. I. Gurney and Jackson, London 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.Bannerman D. A., 1953. The Birds of West and Equatorial Africa, vol. 1. Oliver and Boyd, London. 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.
Breeding