The Plumbeous Ibis

Oct 07, 2018 Features / Columnists, Interesting Creatures in Guyana

The plumbeous ibis Theristicus caerulescens, also formerly called the blue ibis, is a large distinctive ibis species endemic to parts of central South America.
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 plumbeous ibis is the most morphologically similar to the sympatric buff-necked ibis Theresticus caudatus, but differs in colouration and neck structure.
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.

Ibis

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.
The plumbeous ibis has a relatively narrow range that extends through parts of central South America. Although this species appears to be relatively common, it is patchily distributed throughout its global range; being relatively abundant in some regions but less so in others.
This ibis’s diet comprises aquatic invertebrates; snails, especially apple snails of the genus Pomacea; mussels; crabs; frogs; fish and eels. It has also been reported to eat snakes, lizards, skinks and terrestrial invertebrates; which is plausible given its occasional occurrence in dry grasslands. Despite this ibis’s apparently generalist diet, it is found to specialise on molluscs in the Paraguay-Parana basin.
The plumbeous ibis has multiple nest predators. Although brooding parent birds are strongly aggressive toward intruders and can successfully deter small predators such as opossums, jays and vultures; they apparently lose to larger predators such as capuchin monkeys and great black hawks. For example, large groups of capuchins have been observed to attack plumbeous ibis nests, whereby some assailants violently force the parent bird out of the nest while others steal the eggs. Brooding adults of this species also show marked aggression toward human intruders. If approached by a human, this ibis screams loudly; whereby it lowers its body, bristles its feathers and jabs its bill at the intruder. Further, juveniles have been noted to defend themselves with forward lunges of the bill. These defensive behaviours in this ibis reflect the bird’s high predation risk and hence the adaptive value of such behaviours for survival.
Like many other sympatric wading birds, it is likely threatened by habitat loss through burning and logging of nesting trees, construction and operation of hydroelectric dams in the Pantanal, and expansion of mining and agriculture. This bird is however not persecuted directly.
The plumbeous ibis’s apparent association with humans may render it particularly vulnerable to the effects of pesticides from agriculture. Additionally, pesticides together with physical disturbance to soil through agricultural machinery may reduce local availability of macroinvertebrate prey to this species. One study in south Brazil has shown the plumbeous ibis to be more abundant in flooded rice fields during the post-harvest season compared to pre-harvest. This may be due to the cessation of use of agrochemicals and machinery after harvest, leading to greater macroinvertebrate richness and density; so that there is greater abundance of macroinvertebrate prey for the ibis to feed on. Nevertheless, this bird together with many other species still appears to be more abundant in natural wetlands than in flooded farmland. Therefore, a proposed conservation strategy for this species in the face of agricultural expansion that may also benefit other sympatric wading birds has been to retain expanses of natural wetland within the rice cultivation ecosystem.
This species is not typically hunted because it is difficult to kill and skin owing to its marked physical strength; and also emits an unpleasantly musty odour if caught. It has however been kept in captivity several times.
s ibis has been evaluated by the IUCN as Least Concern since 2004 because this species does not reach the thresholds for Vulnerable under the range size and population trend criteria. The world population has been estimated at 50,000 – 100,000 individuals. However, further information on the plumbeous ibis’s biology is required to more successfully conserve this species, and hence safeguard it against large potential future population declines. [Source: Wikipedia]