Conservationists have launched an ambitious project to save albatrosses and other threatened seabirds on a remote island off the coast of South Africa from a plague of killer mice who eat the chicks alive. At least 18 out of the 27 species of bird on Marion Island are at risk of local extinction.
The mice were accidentally introduced to Marion Island, more than 2,000 kilometres south-east of Cape Town, in the early 1800s by sealers who brought them ashore on boats.
Over the last few decades, as the climate has become warmer and drier, mouse numbers on Marion Island — which is about twice the size of Paris — have exploded. In the southern hemisphere winter, as plant and insect food on the island runs low, the mice have turned to feeding on the flesh of seabird chicks with increasing frequency.
Birds that nest in burrows, like petrels and prions, are especially vulnerable.
“They eat the eggs and chicks, and the parent has no defence mechanism against this behaviour,” said Guy Preston, vice chairman of the Mouse-Free Marion Project, a South African non-profit. “For the mice they become ‘sitting ducks’.”
At least 18 out of the 27 species of bird that live and breed on the island — which is also home to colonies of king penguins — risk local extinction if the mice aren’t dealt with. These include birds like the Wandering albatross, whose worldwide population is vulnerable due to declining numbers.
‘Scalping’ albatross chicks
The conservationists are working towards dropping poisoned mouse bait across the island in mid-2023, during winter in the southern hemisphere. Mice stop breeding in winter, and most of the island’s resident birds are away foraging for food.
Both South Africa’s department of forestry, fisheries and the environment, and conservation group BirdLife South Africa are backing the project.
Evidence of the devastation wrought by the mice emerged in 2003. Researchers began to find bloody wounds on the chicks of Wandering albatrosses, one of four species that nest on the island. Similar head and neck wounds have more recently been recorded on the chicks of Grey-headed, Sooty and Light-mantled albatrosses. Scientists refer to the head wounds as “scalping”. Many of the chicks go on to die from their injuries.
There was also evidence that mice burrowing into the base of Wandering albatross nest cups for warmth caused some nests to collapse, killing the chicks.
Other islands have had similar problems. On Gough Island, in the Tristan da Cunha archipelago 4,000 kilometres to the north-west of Marion, mice destroy up to two million out of the three million eggs and chicks produced each year by seabirds there, said Preston.
The decision to eradicate Marion’s mice hasn’t been taken lightly.
“No-one wishes to see any animal suffer,” Preston told RFI. “One has to weigh the deaths of the mice with the deaths of tens – perhaps hundreds – of millions of birds if the mice are not killed. And those deaths of the birds can only be excruciatingly painful, as they are slowly eaten alive.”
The operation won’t be simple. Flying conditions for the helicopters deployed to drop the mouse bait during the island’s winter — marked by gales, storms, rain and snow — will be hazardous. There is also a risk that scavenging birds on the island, including Kelp gulls and Lesser sheathbills, could be affected by eating dead mice.
Those behind the project say that most mice will retreat into hiding places before dying. John Cooper, information officer for the Agreement on the Conservation of Albatrosses and Petrels, a multilateral initiative, has visited Marion Island 31 times in the past few decades.
He told RFI that once the rodent problem is solved this could improve Marion Island’s chances of gaining World Heritage Site status (a previous attempt by South Africa to get it listed was withdrawn due in part to the mouse problem).
“Removing mice will greatly help the island’s plants, invertebrates and birds,” he said.
“With success Marion’s international status will go up.”