In shocking new video footage, a giant tortoise creeps toward a baby bird perched on a log, slowly and steadily cornering the chick before chomping down on its tiny skull.

The footage ends after the lifeless bird tumbles to the ground, but the researcher who captured the video reported that the tortoise swallowed the chick whole moments later. The chilling video is the first documented case of "deliberate hunting" in any tortoise species, the researchers wrote in a report published Monday (Aug. 23) in the journal Current Biology.

This dramatic, albeit slow-motion, hunt took place on Frégate Island, part of the Seychelles archipelago in the Indian Ocean off the East African coast. The footage was recorded by Anna Zora, the island’s deputy conservation and sustainability manager, who had been surveying seabird populations in a woodland when she spotted a female Seychelles giant tortoise (Aldabrachelys gigantea) exhibiting some very strange behavior.

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"There was just something odd about the way it was behaving," said senior study author Justin Gerlach, the director of biology studies at the University of Cambridge’s Peterhouse College in the U.K.

Typically, giant tortoises meander about, munching on plants as they go, and they only really walk in a purposeful manner when engaging in aggressive behavior, Gerlach said. For instance, male tortoises might make a beeline toward each other when fighting over mates. The Seychelles tortoise in Zora's video walked with similar deliberation, very unlike how a tortoise would move if engaging in casual eating behavior, Gerlach said.

Zora observed the tortoise's minutes-long approach to a juvenile lesser noddy tern (Anous tenuirostris) that was sitting on a log nearby; the flightless chick had likely fallen from a nest in the trees above. The tortoise clambered onto the log and marched toward the tern, opening its jaws wide and retracting its tongue — as is "typical for aggressive tortoise behavior," the study authors noted. The small bird pecked at the approaching tortoise in vain, while stumbling backward on the log and flapping its wings.

"It does exactly the wrong thing," Gerlach said of the bird. "If it had hopped off the log, it could have got away easily." But because terns nest in trees, the chick likely viewed the ground as a dangerous place, and so it stayed put on the log in spite of the approaching reptile, he said.

The tern's decision proved fatal. After about 90 seconds of pursuit along the log, the tortoise clamped its beak around the bird's head, killing it instantly. "From first approach to the death of the chick, the interaction took seven minutes in total," the authors wrote in their report.

After recording this grisly interaction, Zora emailed Gerlach, who has studied giant tortoises since 1996. When Zora wrote that she’d seen a tortoise hunting a bird, Gerlach was doubtful. "I thought, 'Yeah, that doesn't sound likely. There's some sort of misunderstanding here,'" Gerlach told Live Science. But upon seeing the footage for himself, he was amazed.

"It's clearly trying to injure the bird. And then it goes, it goes all the way and kills it," he said. He and Zora suspect that the tortoise had experience hunting down terns on logs, given that it spotted and approached the chick from some distance away, apparently knowing that it wouldn't fly off as an adult tern would.

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Although they primarily stick to a plant-based diet, tortoises occasionally snack on the flesh of dead animals, which are likely a useful source of protein, the authors wrote in their report. Although tortoises' serrated beaks aren't adapted for biting or chewing flesh, if they manage to swallow an animal whole they can still digest the meat, Gerlach said. Tortoises have also been observed munching on bones and snail shells, which provide the animals with calcium, the authors wrote.

In terms of hunting behavior, there are some published reports of tortoises squashing small birds or crabs beneath the edge of their upper shells, but it's unclear whether the tortoises use this as a deliberate hunting strategy or if they're just clumsy, the authors wrote. Anecdotally, there have been several accounts of tortoises seemingly hunting small birds, but until now none had been caught on camera, Gerlach said. The newly documented evidence of a tortoise hunting a bird hints that the animals might hunt other small creatures, perhaps using a variety of strategies, he said.

Gerlach plans to investigate the behavior further to determine how many Seychelles tortoises hunt birds, and how often. He said he wonders whether this behavior may become more common on Frégate as birds recolonize the island, as the island has undergone extensive habitat restoration in recent years. "We may be looking at the redevelopment of behaviors that used to exist in the past," when thriving bird and tortoise populations both inhabited the island, "or we may be looking at the evolution of a totally new behavior," brought on by the recent surge of seabirds, he said.

Whatever the case, the behavior is "so alien to the way we think of tortoises" and suggests that their behavior may be more complex than once thought, Gerlach said.

Originally published on Live Science.