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These trees have a reputation for killing birds… here’s how

Rather common in the islands of the Pacific and the Indian Ocean, the Pisonia grandis is a fairly ordinary tree, at first sight. But like the manchineel or the strangler fig, its reputation is sulphurous: it is called ” the bird catcher ” where the bird killer “… Well Named.

reproduce at any cost

This is ” the story of life, the eternal cycle “, as Disney reminds us: in nature, you have to reproduce to transmit your genetic heritage and give life to a new generation of individuals of your species.

In the animal kingdom, there are all kinds of tactics to achieve one’s ends and mate: violent fights between males, endless courtship displays, emissions of olfactory markers in strategic places… And in the plant world , also, different methods exist.

It is precisely one of them that causes the death… of seabirds that get a little too close of certain trees of the genus Pisoniawhose Pisonia grandisor Pu’atea in Tahitian.

Pisonia grandis seed

Seed of Pisonia grandis.

This grown tree indeed produces seeds that will have to be scattered to the right and left to plant themselves in the ground, and hope to see a new tree grow in this place.

To disperse not far from the point of origin, the tree that produces seeds can rely on the wind. But for spread over longer distancesand notably cross the seas to neighboring islands, Pisonia grandis has found powerful allies: seabirds.

But unlike other plants which produce fruits eaten by birds to see their seeds then disseminated via their droppings, Pisonia grandis use another technique. Its seeds produce a extremely sticky substance and small hooks that cling to birds and insects when they approach.

Problem: These seeds are so sticky that if too many cling to a bird’s plumage, it can get stuck on the ground. unable to fly under the weight of these stowaways. Trapped in this way, the birds are at the mercy of predators or die on the spot after several days, for lack of food.

As a result, the carcasses of dead birds are piled up at the foot of this tree, when they don’t cover its branches in the ” ghoulish christmas tree “. Enough to forge a terrible reputation in Pisonia grandis

Deaths that harm the tree itself

Contrary to what one can sometimes read about him, Pisonia grandis get no benefit from these dead birdsand the deadly entanglement caused by its seeds is absolutely not intentional or necessary as part of any natural process to promote its reproduction.

Indeed, in a study published in 2004 in the Journal of Tropical Ecology, the scientist Alan E. Burger was interested in the germination of the seeds of this tree according to the mode of dispersion. And his conclusions are formal: dispersal between islands is by living seabirdsbut not on floating bird carcasses.

Indeed, spent five days in sea water, the seeds die and therefore cannot germinate. On the other hand, they survive a few soaks in water, from time to time, when the living bird makes a few stops to rest or feed.

Moreover, germination is not improved by the enrichment that the soil could have benefited from after the decomposition of the dead bird on the spot. “ Pisonia therefore does not benefit from deadly entanglements “, underlines the scientist.

On the contrary : Pisonia grandis has every interest in keeping the birds that carry its seeds alive, so that they carry this precious genetic heritage as far as possible and thus extend the area of ​​occurrence of the species. For Alan E. Burger, this evolution which made the seeds of Pisonia so stickier is a regrettable evolutionary oddity.

Relative mortality

Fortunately, the number of birds killed by these entanglements remains low. This mortality therefore has little impact on large populations of seabirds. which nest in these trees or on the ground, under their foliage. The proof, many species nest in this tree and are doing well.

And that’s good, because seabirds don’t need additional threats. A (too) large part of 300 species of seabirds listed are now endangered.

The fault of various factors, such as overfishing which deprives them of food or causes their death when they approach too close to fishing boats, climate change, the deterioration of the coasts where they nest, plastic and marine pollution, etc. .

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