Toxic Birds: Defence against Parasites?

Recently, three New Guinean passerine birds, all belonging to the genus Pitohui, were discovered to be poisonous. The toxic substance, located primarily in the skin and feathers of the birds, was isolated and identified as homobatrachotoxin, a steroid alkaloid with the ability of depolarizing Na super(+) channels, known only from neotropical poison-dart frogs of the genus Phyllobates. The exceptional phenomenon of toxicity in Pitohui was hypothesized to serve as a chemical defence against predators, such as snakes, raptors and some arboreal marsupials. As an alternative to this predation explanation, we hypothesize here that the skin/feather toxicity in Pitohui functions as a defence against ectoparasites. Support for this parasite-defence hypothesis comes from recent studies, showing that batrachotoxin is poisonous for distantly related orders of insects. This suggests that batrachotoxin may well be effective against a broad spectrum of ectoparasitic arthropods.