Trilobites: The Green-Feathered Terror That Slaughtered Bats in Spain

The greater noctule is the largest bat in Europe. It is listed as “vulnerable” on the threatened species list, but for many years these winged mammals found shelter in the hollows of trees at María Luisa Park in Seville, Spain.

Then a few years ago, they started to turn up dead.

The corpses were savaged, with holes torn in their wings, scientists who study the colony discovered. Many of them were nursing pups; one corpse was a pregnant female. Two bats that managed to survive were so injured that they could not climb or fly back up to their nests. What was behind the attacks?

The culprit turned out to be another park inhabitant: beautiful, invasive rose-ringed parakeets who also make their homes in tree hollows.

When scientists first began studying the bats more than 15 years ago, they didn’t pay much attention to the birds. But now, there are thousands of them, and they are pushing the bats out of their holes, killing some, and taking over the trees where noctules once lived.

Researchers found that parakeets had taken over the hollows where the bats lived, and soon observed the birds attacking the winged mammals.CreditDailos Hernández-Brito

The first attack Mr. Hernández-Brito saw was against a pregnant female bat. He and collaborators then saw more than 30 attacks in the last two years and collected 20 bodies presented in the current study, though there likely have been many more deaths.

Today, the number of trees that noctules live in has dropped by 81 percent since the researchers first began keeping track, with the parakeets taking over the bats’ old homes. Although it is difficult to get exact numbers for the bat population, it appears to have roughly halved since the work began, bringing their numbers down to about 250. The bats are also living in tight quarters, with double the old number occupying a given hole.

The scientists helped the city government come up with an eradication plan to get rid of the parakeets in 2016, but at the last moment, officials canceled the plans. The parakeets are popular, and citizens argued there must be some other way besides killing them — perhaps artificial nest boxes for the bats. However, the bats do not take to such nests well, and the situation is growing urgent.

The parakeet population will surpass 3,000 this year. Dr. Carrete hopes that this paper will help provide evidence that if the noctules are to be saved, eradication is the only option.

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