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.