3 January 2012

Hooded pitohui – the toxic songbird

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This golden-breasted songbird has a deadly secret: its feathers contain one of the most toxic natural substances known to man.
hooded pitohui

The hooded pitohui, the toxic songbird. Credit: John Dumbacher

melyrid beetles

The colourful melyrid beetle, from which the hooded pitohui might have gotten its poison. Credit: John Dumbacher

hooded pitohui

The distinct colouring of the hooded pitohui. Credit: John Dumbacher

Topics: zoology, biology, ornithology

“We Boiled 10 Parokeets tonight for Dash who has had 10 Welps – purposely to try … the Poisoning effect of their hearts on animals. Yesterday We Were told that seven Cats had been Killed Last Summer by Eating as Many Parokeets.”

Traipsing along the Mississippi River in the early 1800s, ornithologist, naturalist and painter John James Audubon decided to test a handful of Carolina parakeets on his hunting dog, Dash. An inexplicably reckless move by the French-born American because, if his journal was anything to go by, Dash had proven herself to be an invaluable hunting companion, and if the native Americans were anything to go by, those exquisitely colourful parakeets were deadly toxic.

Historians have since failed to confirm if the boiled parrots were Dash’s final meal, but Audubon never mentioned her again.

At the time, Carolina parakeets littered the south-eastern United States, nesting in tree hollows, ransacking local orchids and decorating ladies’ hats. Deforestation and overhunting quickly rendered them extinct by 1918. Yet a few species of toxic birds remain, most notably in the New Guinea bush where, in 1989, American scientist John Dumbacher stumbled on the hooded pitohui’s extraordinary secret.

“It was all very accidental. I was in Papua New Guinea with a team of folks studying Raggiana birds of paradise. We had many mist nets scattered in the forest for catching the birds of paradise, but we caught many other birds as well. One day, several hooded pitohuis were in a net,” said Dumbacher, now chairman and assistant curator of the Department of Ornithology and Mammalogy at the California Academy of Sciences.

“These are large birds that can cut your hands, and as I struggled to free them, they bit and scratched my hands. These little scratches really stung, so I just put my fingers in my mouth to clean the cut, and after a minute or so my lips and tongue began to tingle and burn. After this happened to one of our volunteers, we put the stories together and wondered whether it was possible if the bird was the source of the tingling. The next time we caught a pitohui, we tasted a feather, and there was the tingling burning sensation – and the toxin. When we asked the local guides, they all seemed to know about this.”

Dumbacher investigated further by applying a feather directly to his tongue and found the sensation could last for hours. Carting some pitohui feathers back to the U.S., he showed chemist John Daly at the National Institute of Health, who in 1992 identified the presence of batrachotoxins, extremely potent neurotoxic steroidal alkaloids that in high doses can lead to paralysis, cardiac arrest and death. Gram for gram, it is one of the most toxic natural substances known, and had been identified by Daly years earlier in the poison dart frogs of South America. That year the poisonous pitohui found itself on the cover of Science.

In 2004, Dumbacher and his colleague reported in the Proceedings of the National Academy of Sciences a further discovery made by New Guinea villagers that the pitohuis got their batrachotoxins by feeding on small, colourful melyrid beetles. “We found the same toxins in these beetles, and we found the beetles in the birds’ stomachs.” It might seem like the perfect defence mechanism – eat a few beetles and everyone will leave you alone – but consistently toxic species of birds are exceptionally rare, Dumbacher attributing that to two big hurdles in the way of their evolution. “These toxins would poison most other birds, so first you would have to evolve some resistance to the toxin yourself, and only afterward could it be of some use in defence,” he said.

“Also, most birds have other defences – such as flying – to get away from predators,” he added. “If you were a brightly coloured, defenceless frog sitting in the leaf litter, there might be more pressure to come up with alternative defences.” Just why the free-flying pitohuis evolved their toxicity, we still don’t know.

According to Dumbacher, other species of toxic birds are rare, but not non-existent. “There are a few other suggestions that birds might carry toxins, but most of these remain unstudied or relatively poorly studied. Perhaps the best cases are the European quail, Coturnix coturnix, and the Spur-winged Goose, Plectropterus gambensis, which are both well documented to be toxic,” he said. “Nonetheless, the goose is only mentioned in passing at the end of a very different paper, and the quail is mostly known from the medical literature and the toxin has not been definitively identified. There is much work to do in this area.”

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