Secrets of the world’s poisonous birds

Editor’s note: On Sunday, October 24, our founding editor and publisher Eldon Greij passed away following complications due to surgery. If you’re a longtime reader of BirdWatching/Birder’s World, you know that Eldon wrote our column “Those Amazing Birds” in every issue. Here we present his last column, which we published in the November/December 2021 issue. 

Sometimes, scientific discoveries result from ingenious experiments. And other times, they’re serendipitous. This story is the latter.

Ornithologist Jack Dumbacher, who since 2003 has been the curator of ornithology and mammalogy at the California Academy of Sciences, has interest in molecular ecology. One morning in 1990, while on expedition in Papua New Guinea, Dumbacher caught several songbirds in a mist net. As he removed a Hooded Pitohui from the net, his finger was cut. He felt tingling and then burning. He put the finger in his mouth to ease the pain and then his tongue started to tingle and burn for hours. Later, the scientist in him took over, and he put a pitohui feather in his mouth. Yes, the pain returned. He had just discovered the first toxic (poisonous) bird known to science.

Pitohui (Pit-oo-eey) feathers were given to John W. Daly at the National Institutes of Health, who discovered that they contained a chemical from a class of compounds known as batrachotoxins (BTXs). Daly had identified and named the toxin in the poison dart frogs of Colombia in the 1960s. (Bátrachos is Greek for frog.) BTXs are neurotoxic steroidal alkaloids and the most toxic compounds by weight in all of nature. They are 250 times more toxic than strychnine.

How the poison works

BTX functions by interrupting the flow of sodium ions through channels in nerve and muscle membranes, which, at higher concentrations, result in paralysis, followed by cardiac arrest and death. An adult golden poison dart frog, about 4.5 cm long, can hold 1,100 micrograms (1 microgram equals 1 millionth of a gram) of BTX. Because 0.05 micrograms will kill a mouse, one frog could potentially kill 22,000 mice or, if you’re curious, at least 10 humans.

I need to digress. Daly was the preeminent scientist in the world working with toxins in nature and was well known for his work with toxins of frogs of South and Central America. He identified 800 toxic alkaloids from frog skin, where the toxin was stored in special skin glands. Indigenous Colombians rubbed the tips of their darts on the backs of the frogs, and the amount of toxin obtained would kill any of their vertebrate prey.

The implications of Dumbacher’s find were astounding: The complex molecule BTX was found in frogs of the New World and birds in the Old World. Was that all? Or was it more common, and we were missing it?

By raising frogs from eggs in captivity, Daly learned that they could not develop toxicity on their own. In the early 1990s, Daly countered conventional thinking that frogs synthesized their toxins. Rather, he stated, the toxins were sequestered from arthropod prey. Safety concerns in Colombia prevented a search for specific arthropods at that time. Daly died in 2008.

Back to birds. Knowing that frogs got their toxins from prey, Dumbacher examined stomach contents of pitohuis, looking for common prey items. Rather quickly, he found parts of small Choresine beetles (family: Melyridae) that proved to be the source of BTXs for pitohuis. As an aside, the Melyridae beetles are a cosmopolitan family and are present within the ranges of poison dart frogs as well.

The amount of BTX in pitohuis is much lower than that found in the poison dart frogs. Whether it is enough to deter all predators is not clear, but at least the brown tree snake and green tree python are vulnerable to BTX. Biting lice and ticks are much reduced in the feathers of pitohuis compared to other songbirds in the region. Concentrations of BTX are highest in the feathers of the breast and belly, and specifically the feathers of the chest and abdomen, suggesting that toxins rub off on the eggs and nestlings for protection.

Pitohuis are medium-sized birds (8 to 9 inches long) with jay-like behavior. They are two-colored, with a brown or black head, back, chin to upper breast, wings, and tail, and they’re orange to red on the breast and abdomen with a large orange to red saddle across the upper back. The most toxic species, Hooded Pitohui, is also the most brightly colored and contrasted (red and black) — an aposematic trait shared with other toxic animals (think monarch butterfly and coral snake) to warn predators of their toxicity. Less toxic pitohuis are more brown and orange.

‘Rubbish birds’

In New Guinea, Dumbacher utilized local tribesmen as guides. When the guides saw the pitohuis, they called them “rubbish birds.” They’re useless, they said; you can’t even eat them. The local hunters, of course, knew they were toxic. The Papuan word for “rubbish bird” is pitohuis, which has become the general name of this group of birds and their assigned genus.

While Dumbacher’s research renewed the scientific world’s attention to poisonous birds, reports about toxicity in birds date back to 1831, when Alexander Wilson and Charles Bonaparte wrote in American Ornithology about Ruffed Grouse. When the birds fed on mountain laurel in winter, they reported, “their flesh becomes highly dangerous to eat of, partaking of the poisonous qualities of the plant.” In some cases, they said, grouse meat could be fatal to humans (although death was rare). Sickness from eating grouse became less common in the 20th century, presumably because winter hunting was outlawed. Other species can sometimes contain toxic substances from their diet, but they do not form a chemical defense based on toxins as seen in the pitohuis. Since then, research has shown that other birds that are or may be poisonous in certain circumstances include the European Quail, Africa’s Spur-winged Goose, hoopoes, Australia’s bronzewing pigeons, and Mexico’s Red Warbler.

In 2000, Dumbacher, Daly, and a fellow researcher published a follow-up paper to the pitohui research, announcing that other birds of New Guinea, the Blue-capped Ifrit and several shrike-thrushes, were discovered to sequester BTXs as well. The ifrit lives in montane rainforests, far upslope from pitohuis, meaning that unrelated species in separate habitats had evolved the ability to store toxic substances.

At one time, the genus Pitohui contained six species of forest-dwelling birds restricted to New Guinea and nearby islands. All are gregarious, flock forming, have a clutch of one or two eggs, practice cooperative breeding, and are toxic. Because they are all toxic, they are all called pitohuis. Genetic analysis, however, reveals that the birds are not closely related. This is a case of convergent evolution, whereby a similar trait appears in unrelated species.

In 2014, Dumbacher proposed a revision of the group of birds at the generic level. He suggested that only two species, Hooded Pitohui and Variable Pitohui, would remain in the genus Pitohui. Later, Variable Pitohui was split into three species, so the genus now has four species. The other birds formerly in the Pitohui genus are listed in three different genera in two different families, reflecting their individual characteristics.

Pitohuis commonly join mixed flocks for foraging, and they often associate with other black and brown birds. A flock of similar-looking dark birds including different species of toxic birds is an example of Mullerian mimicry, as they collectively increase the number of birds, teaching predators that they are inedible. Any similarly colored nontoxic birds in the flock would benefit from Batesian mimicry as predators would tend to view them as toxic and inedible, thus leaving them alone.

Dumbacher rocked the ornithological world when he announced the discovery of a poisonous bird using toxins for defense. And he unlocked a new avenue of research on the evolution of toxicity in birds. It’s yet another act of cleverness among our world’s amazing birds.

Read more columns by Eldon Greij