South Africa

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South Africa, Our Burning Planet

Fatal mouse attacks on adult birds spark red alert at world’s biggest wandering albatross colony

One of eight wandering albatross adults found dead at a nesting site on the north coast of Marion Island this April, some showing clear signs of mouse-bite injuries. These are the first recorded fatal mouse attacks on adult albatrosses at this globally important breeding island. (Photo: Anton Wolfaardt)

By Leonie Joubert

16 Jun 2023

This autumn, conservationists recorded the first fatal mouse attacks on adult birds at the Prince Edward Islands Marine Protected Area, the world’s biggest wandering albatross breeding colony. But the slow-burning mouse crisis has been devastating smaller wildlife here for decades, something which a 2025 mouse eradication programme hopes to end.

The discovery was grim, rare, and troubling. The carcasses of two wandering albatross adults, fresh enough to give clues to the cause of death, turned up on the northern coastline of Marion Island in the sub-Antarctic this April. They were among eight adult albatross bodies that were all within walking distance of each other. They were all in various stages of decomposition and had died within weeks of each other.

“It’s very unusual to find albatross carcasses on land, because these birds spend most of their lives at sea,” says marine ecologist Dr Maëlle Connan, from the Institute for Coastal and Marine Research at South Africa’s Nelson Mandela University.

The two-week-old carcasses had deep wounds at the elbows. The surrounding blood pattern told that the injuries had been inflicted while the birds were still alive. The likely cause of death: secondary infection, or starvation as the crippled birds would have been unable to head out to sea to feed.

Marion Island is part of South Africa’s Prince Edward Islands marine protected area, and is home to a quarter of the world’s breeding wandering albatrosses. (Photo: Otto Whitehead)

When Connan and her fellow researchers saw these wounds, they knew immediately what they were looking at. These were typical of the injuries caused by mice, a non-native animal that is increasingly finding easy prey in nesting seabirds here.

This is a dramatic escalation of mouse attacks on seabirds on Marion Island since the first evidence of mauled chicks surfaced in 2003. It is a sign that the mice are learning new behaviour which conservationists fear could spread like a cultural fever among these opportunistic predators.

Yet, while the gruesome images of live wounded chicks and the corpses of adult birds make for headline-grabbing news, hungry mice have been gutting Marion’s miniature wildlife for decades.

Hungry mice, missing insects, starving sheathbills

The lesser sheathbill is to the sub-Antarctic scenery what the domestic chicken is to the farmyard: unremarkable next to the grandeur of a wandering albatross in courtship dance, or a haughty king penguin in its cobble-beach fiefdom. Yet this pint-sized snowy bird is often in the background, grubbing about for bugs and worms.

But the sheathbill’s main food source on Marion — insects such as a rare flightless moth, weevils, springtails and mites, among others — has collapsed. Researchers have counted a 90% drop in invertebrate numbers since the mid-1970s. Sheathbill declines have tracked the same trend: in 1976, birders counted 3,602 sheathbills; by the mid-90s, they were down by 20%, to 2,850 birds. A study in 2000 also showed that breeding sheathbills were leaner, and their clutch sizes smaller.

This is starkly different to wildlife abundance on neighbouring Prince Edward Island — the other half of the twin volcanic outcrops that make up the Prince Edward Islands Marine Protected Area — where sheathbill numbers have remained steady.

The sheathbill is one of the Marion Island’s only birds to live on the island year-round, and doesn’t feed at sea. This insect-eating bird is going hungry as mouse numbers gut their main food source. (Photo: Leonie Joubert)

A sheathbill takes a bath in a blubber rendering pot, a relic from sealing days when the house mouse first arrived here. (Photo: Leonie Joubert)

The Prince Edward Islands lie 2,200km southeast of Cape Town in the southern Indian Ocean and are home to half of the world’s breeding wandering albatrosses, with Marion the breeding site for 25% of the global population. Many other seabirds nest here too, including the sooty, grey-headed and light-mantled albatross, several species of burrowing petrels and many different penguins.

The non-native house mouse has been on Marion for two centuries, most likely coming ashore when the island was a stop-off point for seal-hunting vessels during the blubber harvesting years in the early 1800s.

Prince Edward, 20km away, somehow escaped this fate, and is still humming with insect life, according to Peter Ryan, emeritus professor with the Percy FitzPatrick Institute of African Ornithology, at the University of Cape Town.

“There are flightless moths wandering everywhere on Prince Edward, they crawl over you. But you barely see one on Marion,” he says.

Vegetation map of Marion and Prince Edward. (Image: Supplied)

The enigmatic azorella cushion plant, unique to the sub-Antarctic, is easily damaged when mice burrow into it for shelter or food. (Photo: Leonie Joubert)

Prince Edward’s vegetation is also vibrant, where the enigmatic cushion-shaped azorella can reach an enormous 10m across. On Marion, they’re much smaller, and often punctured with mouse tunnels. This uniquely sub-Antarctic plant is a crucial nursery for insect life and helps build the island’s soils and peat. They’re delicate, though, and easily upset by any form of disturbance. Mice, burrowing into them for shelter and food, can destroy cushions that are 30 to 80 years of age.

Read more in Daily Maverick: Good news for seabirds – the days of the Marion Island mice are numbered if an eradication project gets off the ground

For two centuries, Marion’s mice have become the new ecosystem engineer in this fragile system, but the rate that they’re changing things has rocketed in the past three decades. Marion has become noticeably warmer and drier since the 1970s, as global temperatures nudge higher, giving the mice a longer summer breeding season, and sparking a population explosion.

With more mouths to feed, they’ve gutted their main food source. Nesting seabirds have become the next obvious choice.

Mouse attacks started with seabird chicks, but adult deaths matter more

The first grizzly findings emerged in 2003, and scientists quickly saw a pattern: when mice come upon young, down-covered chicks in their nests, they first attack the birds’ wings or rumps. With older fledglings, whose feathers have toughened, the mice find easier pickings on the chicks’ heads, where the soft crown feathers still give easy access to the skin. Over the course of hours, a single mouse can gnaw the chick’s entire crown away.

The scalped, weakened chicks are left vulnerable to secondary, often fatal infections, or are easy pickings for predators such as petrels.

Video footage from 2015 by doctoral researcher Ben Dilley with the Percy FitzPatrick Institute confirmed the “unprecedented” escalation over two decades, and the precise details of how these mostly nocturnal attacks happen.

This April’s grim discovery is the first time anyone has documented mice attacks on adult wandering albatrosses, though, and the first fatal ones.

A grey-headed albatross, scalped by a mouse. A 2015 study confirmed the dramatic increase in mouse attacks on seabird chicks on Marion Island in the previous two decades. Mice have now taken to feeding on adult birds. (Photo: Ben Dilley)

Grey-headed albatross, scalped by a mouse. (Photo: Ben Dilley)

A grey-headed albatross, scalped by a mouse. (Photo: Ben Dilley)

Read more in Daily Maverick: Gough Island: Mission to stop mighty mice from literally eating threatened seabirds alive in South Atlantic

Conservationists such as Birdlife South Africa’s Dr Anton Wolfaardt worry that these attacks may follow the pattern seen on Hawaii’s Midway Atoll island chain in the Pacific, something that would have devastating consequences for Marion’s albatrosses and other breeding seabirds. In 2015, mice started attacking Laysan albatrosses, and the escalation in just one year showed how quickly they learned and transferred this behaviour.

“On Midway Atoll, in the first year there were 42 albatrosses killed, and a year later it had escalated to 242 killed adults, and many more wounded ones,” says Connan. “That’s a huge number.”

Even though these first adult albatross fatalities are small in number, this change in mouse behaviour is extremely concerning because of the implications for the bird population, according to Ryan.

“The difference between chicks and adults dying is massive from a demographic perspective,” he says.

Wandering albatrosses only start breeding at eight to 10 years, they only have one chick every two years, and it takes almost 12 months for chicks to grow large enough to fly.

Wandering albatrosses may be able to reach a ripe old age of about 60 — the data are a bit thin on this — but from the age of 30 their breeding potential drops.

“To have a sustainable albatross population, you need to have a high adult survival rate,” Ryan says. “Every one adult killed is the equivalent to many more juveniles dying.”

Mice to get the boot from Marion Island

In 2025, the South African Department of Forestry, Fisheries and the Environment plans a rodent eradication programme that aims to kill the mice on the island in a single winter-long blitz.

From early April until late August, helicopters flying for the Mouse-Free Marion project will sweep the island, spreading blood-thinning rodent-targeted poisoned bait. This is the method used in other similar eradication programmes on sub-Antarctic islands such as South Georgia, Macquarie and Campbell, all of which were successful and allowed for heavily impacted seabird colonies to recover.

The Mouse-Free Marion initiative will be the biggest of its kind, according to Birdlife South Africa, which will run it on behalf of the South African government.

Now, with the recent discovery of the adult albatross fatalities on Marion, it gives further impetus for the eradication of this invasive predator, according to Birdlife’s Wolfaardt, also the Mouse-Free Marion project manager.

“We have known for some time that mice are significant predators of seabird chicks. The death of each adult breeding wandering albatross is an even more serious loss for the population,” Wolfaardt says.

“These recent fatalities add to the growing body of evidence of the devastating impacts mice are inflicting on the ecology and seabird populations of Marion Island.”

As the climate here continues to warm and dry, it will keep driving the booming mouse population to find new food opportunities, with birds squarely in the crosshairs. DM

To read all about Daily Maverick's recent The Gathering: Earth Edition, click here.

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South Africa

Hungry mice, missing insects, starving sheathbills

Mouse attacks started with seabird chicks, but adult deaths matter more

Mice to get the boot from Marion Island