In the tropical waters of the Pacific and Indian oceans, a tree appears to be trapping and killing some of the birds landing on it, for no apparent reason. An ecologist decided to investigate whether these murders were accidental or premeditated.
Some tree and bird species cooperate to serve each other's mutual interests. The former provides refuge to the latter, who in turn disperse their host's genetic material. However, when it comes to the Pisonia grandis, this relationship is quite different. The seeds of this endemic tree found in the Indian and Pacific Oceans are covered with a substance that leads to the death of many birds, which end up getting stuck to its branches.
Many tree species produce seeds covered in a special mucus and small hooks that allow them to adhere to anything that rubs against them. This technique allows them, for example, to attach to the feathers of birds that will then carry and disperse them. The problem with the Pisonia is that its vegetable glue is far too powerful...
When a bird lands on the tree, its feathers are quickly covered with seeds, which also stick to the branches, foliage and everything around them. The smaller birds, therefore, get stuck, entangled even, and are unable to move. If a predator doesn't end their slow agony, they end up starving.
Like sinister Christmas decorations, their decaying bodies adorn the branches and base of pisonias. The black Noddy (Anous minutus), a marine bird, is one of their most common victims.
In search of the motive of the crime
To understand the reasons behind this terrible massacre, ornithologist and marine ecologist Alan Burger of the University of Victoria, Canada, visited Cousin Island in the Seychelles archipelago in 2005. His goal was to discover if there were evolutionary reasons behind this behaviour, through various experiments.
The first of these experiments aimed to analyze the potential benefits of decomposing nutrient-rich cadavers in seed development. He was able to show that seeds that grow in the vicinity of dead birds do not seem to benefit more than those that grow at a greater distance from the tree. The birds have even proved more useful alive, fertilizing the soil with their faeces.
In a second experiment, Burger immersed the seeds in seawater, thinking that dead birds could be some kind of bridge that allows the tree's genetic material to travel to other islands. However, the seeds died after 5 days, invalidating this hypothesis. In no way could they have held on long enough to reach another fertile land.
Finally, the ecologist drowned the seeds in seawater for a period of four weeks. These seeds eventually sprouted, which suggests that their best method of dissemination was to travel aboard live birds, by clinging to their feathers. The fact that some of these birds fall victim to the glue is, therefore, accidental, collateral damage with no specific benefit.
Although it is tempting to think that everything in nature happens for a reason, some phenomena are just the results of failed attempts, or simply side effects of a radical survival technique. Either way, you've been warned: don't touch the Pisonia.