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. It is also related to the Madagascar crested ibis Lophotibis cristata and Geronticus ibises. 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. 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 subpopulations. 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. 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.
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.
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