The hooded pitohui (Pitohui dichrous) is a species of bird in the genus Pitohui found in New Guinea. It was long thought to be a whistler (Pachycephalidae) but is now known to be in the Old World oriole family (Oriolidae). Within the oriole family, this species is most closely related to the variable pitohuis in the genus Pitohui, and then the figbirds.

A medium-sized songbird with reddish-brown and black plumage, this species is one of the few known poisonous birds, containing a range of batrachotoxin compounds in its skin, feathers and other tissues. These toxins are thought to be derived from their diet and may function both to deter predators and to protect the bird from parasites. The close resemblance of this species to other unrelated birds also known as pitohuis which are also poisonous is an example of convergent evolution and Müllerian mimicry. Their appearance is also mimicked by unrelated non-poisonous species, a phenomenon known as Batesian mimicry. The toxic nature of this bird is well known to local hunters, who avoid it. It is one of the most poisonous species of pitohui, but the toxicity of individual birds can vary geographically.

The hooded pitohui is found in forests from sea level up to but is most common in hills and low mountains. A social bird, it lives in family groups and frequently joins and even leads mixed-species foraging flocks. Its diet is made up of fruits, seeds and invertebrates. This species is apparently a cooperative breeder, with family groups helping to protect the nest and feed the young. The hooded pitohui is common and is currently not at risk of extinction, with its numbers being stable.

Taxonomy and systematics

The hooded pitohui (Pitohui dichrous) Bonaparte placed it in the genus Rectes which had been erected in the same year by Ludwig Reichenbach as an alternative name for the genus Pitohui, which had been described by René Lesson in 1831. No explanation was given for the preference of the newer name over the established older one, but it was common to prefer Latin names over non-Latin names, and to provide Latin names to those without. Eventually however the principle of priority, which favours the first formal name given to a taxon, was applied, and Rectes was suppressed as the junior synonym of Pitohui. A 2008 examination of the genus, however, found it to be polyphyletic (meaning that the genus contained unrelated species), with some purported members of the genus not actually falling within the whistlers. The hooded pitohui and the closely related variable pitohui were both found to be related to the Old World orioles (Oriolidae). A 2010 study by the same team confirmed that the hooded pitohui and variable pitohui were orioles and indeed were sister species, and that together with the figbirds they formed a well defined basal clade within the family. As the variable pitohui was the type species for the genus Pitohui, the hooded pitohui was retained in that genus and the four remaining species were moved to other genera.

The hooded pitohui is monotypic, lacking any subspecies. Birds in the south east of New Guinea are sometimes separated into a proposed subspecies, P. d. monticola, but the differences are very slight and the supposed subspecies are generally regarded as inseparable.

Pitohui, the common name for the group and the genus name, is a Papuan term for rubbish bird, a reference to its inedibility. The specific name dichrous is from the Ancient Greek word , meaning . Alternate names for the hooded pitohui include the black-headed pitohui and lesser pitohui.

Physiology and description

thumb|The plumage of the hooded pitohui is dichromatic, black and reddish brown.

The hooded pitohui is long and weighs . The adult has a black , head, chin, throat and upper breast and a black tail. The rest of the plumage is a reddish brown. The bill and legs are black, and the irises are either reddish brown, dark brown or black. Both sexes look alike. Juvenile birds look like adults, except that the of the tail and of the wing are tinged with brown. other than some reports of coturnism caused by consuming quail (although toxicity in quails is unusual), and the first bird discovered with toxins in the skin. The same toxin had previously been found only in Central and South American poison dart frogs from the genera Dendrobates, Oophaga and Phyllobates (family Dendrobatidae). The batrachotoxin family of compounds are among the most toxic compounds by weight in nature, The presence of the toxins in muscle, heart and liver shows that hooded pitohuis have a form of insensitivity to batrachotoxins. A bird has been estimated to have up to 20 μg of toxins in its skin and up to 3 μg in its feathers. The presence of the toxins in the internal organs as well as the skins and feathers rules out the possibility that the toxins are applied topically from an unknown source by the birds.

Ecology

The function of the toxins to the hooded pitohui has been the source of debate and research since its discovery. The initial suggestion was that the toxins acted as a chemical deterrent to predators. and others noted that the levels of batrachotoxins were three orders of magnitude lower than in the poison dart frogs that do use it in this way.

Another explanation for the purpose of the toxins is to mitigate the effects of parasites. A comparative study of the tick loads of wild birds in New Guinea would seem to support the idea, as hooded pitohuis had considerably fewer ticks than almost all the 30 genera examined. The batrachotoxins do not seem to have an effect on internal parasites such as Haemoproteus or the malaria-causing Plasmodium.

thumb|alt=a brown snake with bars on body in foliage|Brown tree snakes are bird predators that have been shown to be vulnerable to the poisons found in hooded pitohui.

A number of authors have noted that the two explanations, as a chemical defence against predators and as a chemical defence against ectoparasites, are not mutually exclusive, and evidence for both explanations exists.

There have also been experiments to test pitohui batrachotoxins on potential predators. They have been shown to irritate the buccal membranes of brown tree snakes and green tree pythons, both of which are avian predators in New Guinea. The unpalatability of the species is also known to local hunters, who otherwise hunt songbirds of the same size. They feed at all levels of the forest, from the forest floor to the canopy,

Breeding

Little is known about the breeding biology of the hooded pitohui and its relatives due to the difficulties of studying the species high in the canopy of New Guinea. Nests with eggs of the hooded pitohui have been found from October through to February. The incubation period is not known, but the species is thought to be a cooperative breeder, as more than two birds in a group have been observed defending the nest from intruders and feeding the young. Young birds, which are covered in white down as nestlings before developing their adult plumage, have been observed being fed acorn-shaped red berries and insects. Young birds will make a threat display when approached in the nest, rising up and erecting their head feathers. As chicks develop directly into adult plumage, it has been suggested that this display may be signalling its identity as a toxic species, even though young birds have not developed toxicity at that age. In spite of this, and reports of toxicity in birds going back to classic antiquity, before the discovery that the hooded pitohui was toxic, toxicity was not a trait that scientists attributed to birds. The discovery of toxicity in birds, triggered by this species, sparked interest in the subject and a re-examination of older accounts of unpalatability and toxicity in birds, although the field is still understudied.

Notes and references

  • Xeno-canto: audio recordings of the hooded pitohui.