A "horrifying" video showing a giant tortoise devouring a baby bird has taken researchers by surprise.
The footage, filmed in Frégate Island, Seychelles, shows a Seychelles giant tortoise, which is widely considered to be a herbivore, slowly approaching a tern chick on a log.
It reaches out with its mouth open several times as it retreats along the log. As the chick stops at the end of the log, the tortoise closes its jaws on its head, killing the chick.
The video stops there, but Anna Zora, a conservation manager who filmed the encounter, described seeing the tortoise then pursue the chick at the bottom of the log before swallowing it whole.
The whole interaction, seen in July 2020, took seven minutes, researchers said, with the pursuit along the log taking about a minute and a half.
The video is the first to show "deliberate hunting" in any tortoise species, researchers wrote in their study published in Current Biology journal.
Dr Justin Gerlach, the University of Cambridge researcher who led the study, said: "This is completely unexpected behaviour and has never been seen before in wild tortoises.
“The giant tortoise pursued the tern chick along a log, finally killing the chick and eating it.
"It was a very slow encounter, with the tortoise moving at its normal, slow walking pace – the whole interaction took seven minutes and was quite horrifying.”
Ms Zora described the tortoise "moving in a strange way" when she decided to start filming.
In the past, there have been reports of opportunistic feeding by tortoises, such as on the bones of snail shells, and in captivity, the Asian forest tortoise was seen to eat frogs. There have also been reports of tortoises squashing birds and crabs, the study stated.
Ever since spotting the bird-eating tortoise, researchers said other tortoises in the area have been spotted making similar attacks.
They added that the way the tortoise cornered the chick on the log suggests it has had previous experience of hunting baby birds in this way.
Experts suggest this new hunting behaviour is driven by the combination of a tree-nesting tern colony and a resident giant tortoise population.
In most places, potential prey are too fast or agile to be caught by giant tortoises.
But habitat restoration on the island has enabled seabirds to thrive on the island, which in turn has led to more dropped fish and chicks in the ground under the birds' nests.
Dr Gerlach said: “These days Frégate island’s combination of tree-nesting terns and giant tortoise populations is unusual, but our observation highlights that when ecosystems are restored totally unexpected interactions between species may appear; things that probably happened commonly in the past but we’ve never seen before."