Over a decade ago, an alien-like species of the praying mantis was released around the world to act as the ultimate pest control. The insects, which normally feed on small frogs and sometimes snakes, is now a pivotal part of a first-of-its-kind study regarding the praying mantis killing birds.

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Researchers throughout the US and Switzerland have documented over 147 cases of the formidable hunters silently blending into their surroundings and killing birds near feeders. Currently, 70 percent of the case studies have been taken from the U.S. where the insect has been choosing hummingbirds as its primary target.

According to the Daily Mail, praying mantises are opportunistic hunters often choosing hummingbird feeders as a prime hunting spot. Experts are concerned that if the attacks continue at this pace, the small bird population could be severely diminished.

“Our study shows the threat mantises pose to some bird populations,” Dr. Martin Nyffeler wrote in his study. “Great caution is advised when releasing mantises for pest control.”

When a praying mantis kills a hummingbird, it is anything but a quick and painless death, according to IFL Science. When the mantis catches a small bird, it will chew on the neck of the bird for over an hour until the flesh is gone. Hours after the bird is dead, the mantis is still working on eating its prey.

While the findings of this study may come as a shock to many, experts have been conducting these studies since 1982. The concern now is that the mantis is starting to attack other species of birds.

“The fact that eating of birds is so widespread in praying mantises, both taxonomically as well as geographically speaking, is a spectacular discovery,” Dr. Nyffeler said in a recent interview.

The Ruby-throated Hummingbird has been facing the brunt of the killings, per reports from Popular Science. In a video obtained by them, a Ruby-throated Hummingbird was eating from a bird feeder when a praying mantis wrapped its powerful front legs around it.

Before there was a declared winner in this bout of nature, the man shooting the video flicked the mantis away and the bird scurried off.

It’s safe to say that praying mantises won’t be taking the world over anytime soon, but they could do some real damage to the bird ecosystem.