A Seychelles giant tortoise, a species previously thought to be a strict herbivore, has been filmed chasing and eating a baby bird in a “horrifying and amazing” attack, with researchers stating it was the first documented example of deliberate hunting in the wild by the species.

The video, taken on Fregate Island in July 2020, shows a female giant tortoise slowly stalking a lesser noddy tern chick, snapping at it unsuccessfully before delivering a lethal bow by clamping its jaws directly around its head.

The video shows the tortoise climbing off its log to retrieve the now-dead chick before swallowing it whole.

The privately owned Fregate Island is managed for ecotourism and home to about 3,000 giant tortoises. Other tortoises in the area have been seen making similar attacks but no tortoise species has been seen actively pursuing prey in the wild before.

The Seychelles giant tortoise is a member of the species Aldabrachelys gigantea, which are “generally assumed to be strict herbivores”, according to a study published in Current Biology.

Dr Justin Gerlach, the director of studies at Peterhouse, Cambridge, and an affiliated researcher at the University of Cambridge’s Museum of Zoology, led the study. He said: “This is completely unexpected behaviour and has never been seen before in wild tortoises.”

Researchers think this new hunting behaviour has been driven by the unusual combination of a tree-nesting tern colony and a resident giant tortoise population on Seychelles’ Fregate island.

Previously all tortoises were thought to be vegetarian – although they have been spotted feeding opportunistically on carrion, and they eat bones and snail shells for calcium.

Extensive habitat restoration on the island has enabled seabirds to recolonise, and there is a colony of 265,000 noddy terns that leave the ground “littered with dropped fish and chicks that have fallen from their nests”.

In most places, potential prey are too fast or agile to be caught by giant tortoises. But scientists said the way the Seychelles giant tortoise approached the chick on the log suggested this type of interaction happens frequently. On the Galapagos and Seychelles islands, giant tortoises are the largest herbivores and eat up to 11% of the vegetation.

Gerlach said: “These days, Fregate 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.”

Gerlach said the video could be evidence of a “population of tortoises that is developing a new type of behaviour with evolutionary implications”, but added that the revival of seabird and tortoise populations may also have “recreated conditions for natural behaviours that people haven’t seen for hundreds of years”.

Naturalist Charles Darwin’s encounters with giant tortoises in the Galapagos Islands were key to the development of modern understandings of evolution. After visiting a number of islands in the chain, Darwin realised that each island had its own, distinct species of giant tortoise.

Gerlach added: “It’s clear that they enjoy eating terns. Compared to the ease of eating plants, they’re going to quite a lot of trouble.”