The olive ibis (''Bostrychia olivacea'') is a species of
ibis
The ibises () (collective plural ibis; classical plurals ibides and ibes) are a group of long-legged wading birds in the family Threskiornithidae, that inhabit wetlands, forests and plains. "Ibis" derives from the Latin and Ancient Greek word f ...
native to dense tropical forests in central Africa. Between 65 and 75 cm in length, it is a small ibis with olive plumage displaying a iridescent sheen. Four subspecies are recognized.
Taxonomy and systematics
Historically, the olive ibis was included in various genera such as ''
Geronticus
__NOTOC__
The small bird genus ''Geronticus'' belongs to the ibis subfamily (Threskiornithinae). Its name is derived from the Greek ''gérontos'' (γέρωντος, "old man") in reference to the bald head of these dark-plumaged birds; in English ...
'', ''
Comatibis'' and ''
Harpiprion'' before finally being classified among the ''
Bostrychia
''Bostrychia'' is a genus of ibises in the family Threskiornithidae. Member species are found in many countries throughout Africa.
It contains the following five species:
Conservation
Four of these species are evaluated as Least Concern stat ...
''.
[Salvadori T., 1903. On the ibis Olivacea of Dubus. ''Ibis'' 45: 178-188.] It is also related to the
Madagascar crested ibis
The Madagascar ibis (''Lophotibis cristata''), also known as the Madagascar crested ibis, white-winged ibis or crested wood ibis, is a medium-sized (approximately 50 cm long), brown-plumaged ibis. It has bare red orbital skin, yellow Beak, b ...
''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
São Tomé is the capital and largest city of the Central African island country of São Tomé and Príncipe. Its name is Portuguese for " Saint Thomas". Founded in the 15th century, it is one of Africa's oldest colonial cities.
History
Álva ...
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.
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. 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
subpopulation
In statistics, a population is a Set (mathematics), set of similar items or events which is of interest for some question or experiment. A statistical population can be a group of existing objects (e.g. the set of all stars within the Milky Way g ...
s.
[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
Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
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 Loral may refer to:
* Loral Corporation, contractor founded in 1948 in New York by William Lorenz and Leon Alpert as Loral Electronics Corporation
* Loral Space & Communications, satellite communications company
* Loral GZ-22, a non-rigid airship
* ...
regions is bluish black and the bill is coral red. The
tail coverts
A covert feather or tectrix on a bird is one of a set of feathers, called coverts (or ''tectrices''), which, as the name implies, cover other feathers. The coverts help to smooth airflow over the wings and tail. Ear coverts
The ear coverts are s ...
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
Cameroon (; french: Cameroun, ff, Kamerun), officially the Republic of Cameroon (french: République du Cameroun, links=no), is a country in west-central Africa. It is bordered by Nigeria to the west and north; Chad to the northeast; the C ...
, the
RCongo,
DRCongo,
Cote d’Ivoire
Ivory Coast, also known as Côte d'Ivoire, officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital is Yamoussoukro, in the centre of the country, while its largest city and economic centre is ...
,
Gabon
Gabon (; ; snq, Ngabu), officially the Gabonese Republic (french: République gabonaise), is a country on the west coast of Central Africa. Located on the equator, it is bordered by Equatorial Guinea to the northwest, Cameroon to the north ...
,
Ghana
Ghana (; tw, Gaana, ee, Gana), officially the Republic of Ghana, is a country in West Africa. It abuts the Gulf of Guinea and the Atlantic Ocean to the south, sharing borders with Ivory Coast in the west, Burkina Faso in the north, and To ...
,
Kenya
)
, national_anthem = "Ee Mungu Nguvu Yetu"()
, image_map =
, map_caption =
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, capital = Nairobi
, coordinates =
, largest_city = Nairobi
, ...
,
Liberia
Liberia (), officially the Republic of Liberia, is a country on the West African coast. It is bordered by Sierra Leone to Liberia–Sierra Leone border, its northwest, Guinea to its north, Ivory Coast to its east, and the Atlantic Ocean ...
,
Nigeria
Nigeria ( ), , ig, Naìjíríyà, yo, Nàìjíríà, pcm, Naijá , ff, Naajeeriya, kcg, Naijeriya officially the Federal Republic of Nigeria, is a country in West Africa. It is situated between the Sahel to the north and the Gulf o ...
,
Sierra Leone
Sierra Leone,)]. officially the Republic of Sierra Leone, is a country on the southwest coast of West Africa. It is bordered by Liberia to the southeast and Guinea surrounds the northern half of the nation. Covering a total area of , Sierra ...
and
Tanzania
Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...
, besides
Príncipe
Príncipe is the smaller, northern major island of the country of São Tomé and Príncipe lying off the west coast of Africa in the Gulf of Guinea. It has an area of (including offshore islets) and a population of 7,324 at the 2012 Census; and
São Tomé
São Tomé is the capital and largest city of the Central African island country of São Tomé and Príncipe. Its name is Portuguese for " Saint Thomas". Founded in the 15th century, it is one of Africa's oldest colonial cities.
History
Álva ...
islands.
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 Upper Guinea is a geographical term used in several contexts:
# Upper Guinea (french: Haute-Guinée) is one of the four geographic regions of the Republic of Guinea, being east of Futa Jalon, north of Forest Guinea, and bordering Mali. The popula ...
in countries such as Sierra Leone, Liberia and Cote d'Ivoire; and the latter found in
Lower Guinea Lower Guinea may refer to:
* Maritime Guinea, the coastal region of the republic of Guinea.
* in biogeography, a region of coastal tropical forests stretching along the Gulf of Guinea, from Ghana through Benin, Togo, Nigeria, and Cameroon. It is se ...
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
Mount Kenya (Kikuyu: ''Kĩrĩnyaga'', Kamba, ''Ki Nyaa'') is the highest mountain in Kenya and the second-highest in Africa, after Kilimanjaro. The highest peaks of the mountain are Batian (), Nelion () and Point Lenana (). Mount Kenya is locat ...
and in the
Aberdares
The Aberdare Range (formerly the Sattima Range, Kikuyu: ''Nyandarua'') is a 160 km (100 mile) long mountain range of upland, north of Kenya's capital Nairobi with an average elevation of . It straddles across the counties of Nyandarua, Nye ...
of Kenya,
Kilimanjaro
Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world: above sea level and a ...
in Tanzania, on
Mount Meru
Mount Meru (Sanskrit/Pali: मेरु), also known as Sumeru, Sineru or Mahāmeru, is the sacred five-peaked mountain of Hindu, Jain, and Buddhist cosmology and is considered to be the centre of all the physical, metaphysical and spiritu ...
and on
Mount Elgon;
but is apparently absent from lowland forests in east Africa such as the
Kakamega tropical rainforest in Kenya. However, its call was once reportedly heard in the
Bwamba lowlands of Uganda. 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
Illustration of an adult bird of the nominate subspecies – a native of the ">Upper Guinean forests
The olive ibis feeds on insects such as beetles,
grubs and snails. It also reportedly feeds on
myriapods
Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial.
The fossil record of myriapods reaches back into the late Silurian, a ...
, 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
''Hagenia'' is a monotypic genus of flowering plant with the sole species ''Hagenia abyssinica'', native to the high-elevation Afromontane regions of central and eastern Africa. It also has a disjunct distribution in the high mountains of East ...
'' 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
This ibis breeds solitarily.
It was first recorded breeding in 1910 on
Mount Kenya
Mount Kenya (Kikuyu: ''Kĩrĩnyaga'', Kamba, ''Ki Nyaa'') is the highest mountain in Kenya and the second-highest in Africa, after Kilimanjaro. The highest peaks of the mountain are Batian (), Nelion () and Point Lenana (). Mount Kenya is locat ...
at 9,000 feet.
Later breeding records of this ibis have been made adjacent to water in
East Africa
East Africa, Eastern Africa, or East of Africa, is the eastern subregion of the African continent. In the United Nations Statistics Division scheme of geographic regions, 10-11-(16*) territories make up Eastern Africa:
Due to the historical ...
. One nest was discovered at the headwaters of the Pesi River in the northern
Aberdares
The Aberdare Range (formerly the Sattima Range, Kikuyu: ''Nyandarua'') is a 160 km (100 mile) long mountain range of upland, north of Kenya's capital Nairobi with an average elevation of . It straddles across the counties of Nyandarua, Nye ...
in mixed ''Juniperus/Podocarpus'' forest, and another at a streamlet near the Thiririka River in the southeastern Aberdares in mixed ''Podocarpus/Ocotea'' forest.
[Parker I. S. C., 1982. Two observations of nesting in the Eastern race of the green ibis ''Bostrychia olivacea akleyorum''. ''Scopus'' 6: 20.] 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 (Chapin, 1932). 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.
[Archibald G. W., Lantis S. D. H., Lantis L. R., Munetchika I., 1980. Endangered ibises Threskiornithae: their future in the wild and in captivity. ''International Year Zoobook'' 20: 6-17.] 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.
Threats and survival
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
The olive baboon (''Papio anubis''), also called the Anubis baboon, is a member of the family Cercopithecidae Old World monkeys. The species is the most wide-ranging of all baboons, being native to 25 countries throughout Africa, extending fro ...
'' 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.
Relationship to humans
This ibis has reportedly never been kept in captivity,
nor have live specimens ever been imported into Europe. However, numerous non-living specimens have been kept in museums around the world; such as in
Brussels
Brussels (french: Bruxelles or ; nl, Brussel ), officially the Brussels-Capital Region (All text and all but one graphic show the English name as Brussels-Capital Region.) (french: link=no, Région de Bruxelles-Capitale; nl, link=no, Bruss ...
,
Paris
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
,
Philadelphia
Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Sinc ...
,
Leyden
Leiden (; in English and archaic Dutch also Leyden) is a city and municipality in the province of South Holland, Netherlands. The municipality of Leiden has a population of 119,713, but the city forms one densely connected agglomeration wit ...
and
Lisbon
Lisbon (; pt, Lisboa ) is the capital and largest city of Portugal, with an estimated population of 544,851 within its administrative limits in an area of 100.05 km2. Grande Lisboa, Lisbon's urban area extends beyond the city's administr ...
.
Specimens of the ''akleyorum'' subspecies have also appeared in the
Nairobi Museum and the
British Museum
The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is among the largest and most comprehensive in existence. It docum ...
.
Status
The olive ibis is evaluated as
Least Concern
A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. T ...
by the
IUCN
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...
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
Príncipe is the smaller, northern major island of the country of São Tomé and Príncipe lying off the west coast of Africa in the Gulf of Guinea. It has an area of (including offshore islets) and a population of 7,324 at the 2012 Census; 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.
[Dodman T., 2007. Estimating the size and status of waterbird populations in Africa. ''Ostrich'' 78:475-480.]
References
{{Taxonbar, from=Q662226
olive ibis
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 a iridescent sheen. Four subspecies are recogn ...
Birds of Sub-Saharan Africa
Birds of the Gulf of Guinea
olive ibis
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 a iridescent sheen. Four subspecies are recogn ...
olive ibis
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 a iridescent sheen. Four subspecies are recogn ...
Taxonomy articles created by Polbot