The plumbeous ibis has historically undergone frequent reclassification and has been placed in various genera such as Molydophanes, Geronticus, Theresticus and Harpiprion. It has also been referred to as Ibis caerulescens and Theresticus (Harpiprion) caerulescens. Nevertheless, this ibis has traditionally been considered as belonging to the monotypic genus Harpiprion and carrying the binomial name Harpiprion caerulescens, on the basis that this species was previously believed to have no close relatives.
However, phylogenetic analyses based on both nuclear and mitochondrial DNA markers have suggested that the plumbeous ibis is closely related to other ibis species in the genus Theresticus such as the buff-necked ibis; given the markedly low genetic distances between the plumbeous ibis and other Theresticus species. Hence, the current placement of the plumbeous ibis in the genus Theresticus appears justified.
This notably large ibis measures 71–77 cm in length, with a wing length of 397-450mm in males and 360-406mm in females. The culmen length is 145-167mm in males, and 126-147mm in females. The sexes appear similar, but the female is slightly smaller.
The plumbeous ibis is easily recognisable through a distinctive shaggy crest comprising long grey plumes that extend from the back of head 10 cm down the nape. A prominent, narrow, white feathered band also extends around the forehead, which has earned this species the German common name Stirnbandibis (headband ibis).
Overall, it is readily distinguishable in the field from other ibises by its solid colouration and thick neck.
The adult plumage is largely grey; but can be bluish grey, greenish grey or brownish grey. The dorsal feathers are mottled greyish-brown, so that the plumage appears lighter below than above. The flight and tail feathers are dark brown or black, superimposed with a glossy greenish bronze. This ibis has also been reported to show various other subtle tinges over its body, depending on the light.
The black, downward-curved bill is serrated between the mandibles. The bare skin of the lores and throat is black or dark grey. The legs are a pinkish orange, darkening in the breeding season in preparation for courtship. The iris is deep orange, but has been noted to be slightly paler around the pupil. In some individuals, the iris has also been reported to be brownish yellow.
The chicks have light grey downy plumage that lacks the more sophisticated tinges in adults. Fledged young are also distinguishable from adults by a dark iris, grey legs, a less developed nuchal crest, a more extensive white band on the forehead and red bare skin patches. The white band on the forehead begins to develop in the young after 18 days of age and up close appears a brighter white than that of adults.
Through its strong musculature, this ibis is sturdy and direct in flight; stretching out its neck and wings and steadily beating its wings with intermittent gliding.
The vocalisations of this species are diverse and markedly loud. The basic call is a high-pitched, rapid pi-pi-pi-pi which has a somewhat metallic timbre and can be uttered when the bird stands on the ground during daytime. This call has been said to resemble the sound of a banjo with brass strings so big that it could be heard a mile and a half away.
This ibis also makes a slow but energetic trumpeting flight call that has been described as kree kreee kree or k k kuh kuh KEE KEE KEE KEE KEE KEE KEE keh kuh kuh. While making flight calls, it glides through the air with motionless downward curved wings. It also utters a nasal, high-pitched, rapid series of cackles when perched on branches or at the nest site described as tututututu; sometimes interspersed with elements of ti-ti and often in duet between mates large distances apart.
Another contact call is made between foraging mates and has been described as a cackling kuk-kuk…kuk-kuk…kuk kuk. Overall, the broad range of loud distinctive calls of this species indicates the importance of long-range communication between individuals, especially mates.
This ibis commonly occurs near human habitation and other manmade features; including dams and seasonally-flooded rice fields.
Individuals are usually sedentary and can remain in a specific area all year round. They have however been reported to migrate on a local scale. Since no subspecies have been noted for this species, the world population is probably kept genetically homogenous by short migrations of individuals between the two geographically separated subpopulations. However, it remains unknown whether this species has regular migration patterns.
Unlike other ibis species, the plumbeous ibis does not form large intraspecific flocks. It is mostly seen singly or in pairs, but is also more rarely seen in groups of up to six individuals. The larger groups appear to consist of two mates accompanied by their fledged juveniles. [Source: Wikipedia]
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