Plants in the genus Pisonia don’t appear to be particularly menacing. The trees lack the thorns of acacia, the poisonous fruit of the manchineel tree and the botanical jaws of Venus flytraps. But Pisonia trees, which are found from Hawaii and New Zealand all the way to India, have a dark secret. Search among their roots and you’re likely to find gardens of tiny, delicate bones.
This is because Pisonia, or “birdcatcher trees” as they’re more commonly known, produce sticky seedpods that entrap insects. Of course, many bird species eat insects, and if they’re not careful, the birds can become stuck in the tree’s clutches while trying to snag a free lunch. Too many seeds can weigh the birds down and prevent them from flying. These birds starve to death on the ground, if the crabs or other predators don’t get them first.
In some cases, the birds die without ever escaping the tree's branches, which means their mummified corpses can be found hanging here and there like macabre Christmas tree ornaments. What's more, owls and other birds of prey have themselves been known to get covered in sticky seeds trying to snack on the immobilized birds. It's what you might call a viscous cycle.
Perhaps the tree's diabolical yet fascinating tendency to murder birds is why BBC America dedicated a segment to them in the documentary series "Planet Earth II."
But when Alan Burger, an ecologist at the University of Victoria, learned of these trees two decades ago, he wondered what benefit the trees might be receiving. After all, plenty of plants utilize sticky, hooked or barbed seeds to trick birds and mammals into carrying their progeny to new territories — but what good is a winged courier if it can’t fly?
It turned out that no one had really studied these trees in detail, so Burger started doing some digging. People who were familiar with Pisonia usually offered two competing, and untested, hypotheses. Either the trees were enjoying a boost in nutrients provided by carcasses decomposing on top of their roots, or perhaps the tree's seeds required a starter kit of corpse-enabled fertilizer to sprout. Neither of these seemed quite right, but Burger wanted to keep an open mind, so he traveled to the archipelago of Seychelles in the Indian Ocean in May 1999. Once there, he spent 10 months conducting experiments on Pisonia seeds. (His findings were published in the Journal of Tropical Ecology in 2005.)
“The results from my experiments showed quite convincingly that the Pisonia derived no obvious benefit from fatally entangling birds,” Burger says. In other words, seeds given a chance to sprout next to dead birds did not show any evidence of enhanced germination or elevated seedling survival compared with seeds without carcasses.
What’s more, Burger found that the amount of nutrients provided to the trees’ roots from entangled birds would be dwarfed by those added by living birds. This is because the trees also serve as nesting sites for seabirds, and seabirds are sort of like a fertilizer delivery service. The birds will eat insects for a quick snack, but most of their food is more substantial. They fly out to sea, snatch fish out of the water and then bring those nutrients back to the islands, where they deposit them as guano. (Translation: bird poop.)
So why the sticky seeds? Burger first tackled this question by immersing Pisonia seeds in seawater for long periods of time. This was meant to reflect what might happen if a bird weighed down by seeds died at sea and was swept to a distant shore. Unfortunately, he found that birdcatcher seeds died after as little as five days in seawater — so the floating carcass hypothesis was out.
But another experiment proved more successful. Because many dispersing seabirds spend a majority of travel time in the air, only diving into the ocean to grab an occasional fish, Burger dipped Pisonia seeds into seawater intermittently for a period of four weeks. This time, the seeds survived.
“Having the birds alive seems to be the key to dispersal, but an unfortunate consequence of having extremely sticky seeds, and producing many seeds in a cluster, is that some birds get fatally entangled,” Burger says.
This is how evolution works. So long as enough birds survive to carry Pisonia’s seeds to new lands, the trees will continue to produce sticky seeds. Killing birds isn’t the goal, then, but collateral damage.
The weird thing is, seabirds absolutely love Pisonia trees, says Beth Flint, a wildlife biologist for the U.S. Fish and Wildlife Service. “It’s rare to see a Pisonia tree that doesn’t have seabirds in it,” she says. “At least in places where there are seabirds left.”
Flint manages habitats within the Pacific Remote Islands Marine National Monument, and as such, she and her colleagues actually work to encourage Pisonia groves, which create prime nesting habitats for red-footed boobies, frigatebirds and black noddies. White terns have even been seen laying their eggs directly in the tiny divots left behind when Pisonia branches break off in windstorms.
By the way, the trees’ brittle, easily broken branches serve another purpose. Every branch that hits the ground has the potential to sprout roots and form its own tree. Burger and Flint both noted that this is the tree’s primary reproduction method once it has become established on an island. All those sticky seeds seem to be more for long-distance conquest.
In fact, Flint says that if you were to travel to Lisianski Island, nearly 1,000 miles northwest of Hawaii, you could see Pisonia's invasion in action. Thirty years ago, she says, this uninhabited island boasted only grasses and short shrubs. But then in the mid-1990s, a scientist hiked to the middle of the island to investigate some bird burrows and discovered a single Pisonia tree sprouting up from the brush. Today, that one tree has branched out into a dense thicket of clones nearly 100 feet wide. It's so strikingly different from the surrounding vegetation, you can even see it using Google Earth.
“And now that there’s that Pisonia patch, the birds love it,” Flint says. “There are lots of boobies and terns nesting in it.”
Of course, what Google Earth can’t show you is that beneath those branches, you’ll probably also find some sun-bleached bones.