Focus on Science: 'Poisonous' Birds -- Look, But Don't Touch

Although rare in the avian world, several species of birds smell or taste bad, and a few are even poisonous – most notably, the Blue-capped Ifrit and five species of pitohui in New Guinea. These approximately Robin-sized songbirds contain nerve-paralyzing batrachotoxins that are among the most toxic natural substances known. The compounds do not serve to kill prey, but to repel or kill external parasites (e.g., lice, mites) and predators (e.g., snakes, hawks, arboreal mammals, and humans).

The Hooded Pitohui (Pitohui dichrous; bottom bird on Science magazine 30 October 1992 cover above) is the best-studied of these unusual species. A single feather from the breast or back of this bird from a certain geographic locality, if placed on one’s tongue, “would cause a burning, tingling sensation that would last for several hours or overnight. Merely handling these birds...caused [the researchers] to sneeze, experience watery eyes and runny noses, and generally respond as if [they] were having allergic reactions.”

Toxins are most concentrated in skin and feathers, much less so in heart and liver, and least in skeletal muscle. Skin and feathers are more toxic on the back and breast than elsewhere. Since the outside of the bird is where parasites or predators are most likely to encounter toxins, these differences are consistent with their role in providing chemical defense.

Pitohuis do not manufacture the toxins from scratch, but obtain them from their food, most likely certain small beetles. The presence of toxins in internal organs shows that the birds do not merely apply them topically to skin and feathers. Furthermore, since these compounds would normally poison muscles and the liver, their presence in these organs raises the question of how the birds remain unaffected by them. The birds may have evolved biochemical means to resist the toxins internally, but these have yet to be explored.

Levels of toxins vary markedly among individuals and geographic localities, chiefly those in the skin and feathers. The causes of such variation are unknown, but may be related to the mechanisms by which the birds acquire toxins in their diet and incorporate them into their feathers.

The skin of pitohuis has the same tissue structure as that of other passerine birds and does not appear to have any obvious modifications for storing or secreting toxins. Avian skin in general secretes fatty substances and continually sheds cells from the surface. Thus, pitohui skin may be pre-adapted for confining toxins and ridding them from the body. The poisons could be taken up and stored temporarily in the lipids produced by the epidermal cells.

If you go to New Guinea to see these rare birds, look, but don’t touch!

Peter Stettenheim -– Peter is a retired ornithologist with particular interests in the functional anatomy and evolution of birds. He lives in Plainfield, NH.

JOHN P. DUMBACHER, GOPINATHAN MENON, AND JOHN W. DALY. 2009. Skin as a toxin storage organ in the endemic New Guinean genus Pitohui. Auk 126: 520-530.