Killer mice threaten giant birds

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Killer mice threaten giant birds



By John Yeld



Gough Island in the South Atlantic Ocean, a World Heritage Site, is probably the world's most important seabird breeding area. But scientists have found that the common house mouse, introduced to the island by sealers during the 18th and 19th centuries, is eating alive defenceless albatross and petrel chicks while they sit on their nests.


Richard Cuthbert, the first professional ornithologist to spend a year on Gough Island, is still kicking himself.

He can't quite believe that his year on the British-owned island deep in the South Atlantic Ocean was almost over before he stumbled on the nightmare answer to a problem that had been puzzling him for months.

Initially, Cuthbert, who is on board the SA Agulhas on his way back to Gough Island, couldn't understand the extremely poor breeding success rate or distribution of several species on the island, which is probably the world's most important seabird breeding area.
Dubbed 'super mice' because of their size

Then, he made the horrifying discovery that the common house mice on Gough - dubbed "super mice" because of their size - are eating alive defenceless chicks sitting on their nests, including young Tristan Albatrosses, an endangered species.

Because all Gough's seabird species have evolved over thousands of years in the absence of natural predators at their breeding sites, these chicks have no defence mechanism against the mice, and are literally eaten as they sit on their nests waiting for their parents to return to feed them.

Two students working on the island to confirm Cuthbert's gruesome finding and gauge the scale of the problem have managed to get video footage of mice attacking the defenceless chicks.

"It sounds incredulous, implausible, that a mouse could attack a chick," says Cuthbert.

"But these (albatross) chicks are really big spherical balls of fat covered in down, and because they're so big and fat, they can't defend themselves."
'It's a big, big conservation concern'

Although there had been some earlier ornithological work on Gough Island, done on a voluntary basis by the meteorological team, Cuthbert and his field assistant, Eric Sommer, were the first people to spend a whole year there dedicated to doing bird research, during 2000/1.

Cuthbert explains that rats, brought by sailors, have had a major impact on seabird populations around the world - and particularly on islands. "At the same time, we've known that mice are on the island, introduced by the sealers.

"But mice aren't a problem - at least, that's what we've been led to believe. So this was something that we nearly missed, and I only twigged after about 10 months down there that, hell, something strange is happening here."

Summer-breeding birds, like the Atlantic Yellow-nosed Albatross, the Sooty Albatross and the Great Shearwaters, appeared to be doing okay, Cuthbert recalls.

"But the winter breeders - particularly Atlantic Petrels and the Tristan Albatross - were getting hammered."

Still, he didn't think of mice as the culprits.

"Mice normally eat insects and seeds. I think it's been recorded that Storm Petrel eggs have been taken by mice, and possibly their chicks as well."

But a Storm Petrel weighs only 25 or 30g, while a Tristan Albatross chick weighs as much as 10 or 12kg, Cuthbert points out.

"So a mouse weighing only 50 or 60g attacking something that is over 10kg is unprecedented really."

How did he stumble on this grizzly phenomenon?

He explains that Atlantic Petrels breed in burrows, and that he and Sommer had a number of "study burrows" that they checked every four or five days.

"And we certainly noticed that the healthy chicks that we found there were, five days later, just skin-and-bone, or dead.

"And then in one case I saw a live chick with wounds around its rump, and later that day it was dead with mice feeding on it.

"At that time I didn't really think more of it, because it was so 'out of left field' that mice could be doing this."

Then, later in the year, the two researchers made a total count of Tristan Albatross chicks. "And their breeding success was appalling."

Ross Wanless and Andrea Angel, the two students from the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town, have confirmed that Atlantic Petrel and Tristan Albatross chicks are being killed by mice.

Given the current rates of mortalities, are any bird species actually at risk of extinction because of mice?

For example, the Tristan Albatross is of particular concern because it is now extinct at its original breeding ground on Tristan da Cunha, and Gough Island is home to some 99.8% of the remaining population.

"The Tristan Albatross is at risk anyway because of long-line fishing, and it's doubly at risk because of the mice," responds Cuthbert.

"And for the Atlantic Petrels, while we've never had any record of them getting caught by fishing boats or long-line boats, the level of breeding success they're having at the moment is not sustainable."

The one advantage that albatrosses and petrels have is that they are very long-lived, Cuthbert adds.

"It's a big, big conservation concern, but in the next five years or so - probably nothing will go extinct as a direct result of the mice, I would imagine."



The facts
They may be ordinary house mice, but the creatures creating havoc with Gough Island's bird populations are known as "super mice" - and for good reason.

They're the largest of any house mouse population anywhere in the world, in terms of body size.

"They're about twice the size of a normal house mouse in Britain," confirms ornithologist Richard Cuthbert.

This is because of a scientific rule that mammals get progressively bigger as the latitude gets higher - that is, moving towards the poles and as the climate gets colder.

"Even taking that into account, they are super-sized, and obviously super-charged in terms of their effect for attacking albatrosses," says Cuthbert.

But he points out that the fact that the mice are so big is probably "a bit of a false premise, really".

"Because even if they were only half the size they are, they could probably still attack the albatross chicks."



This article was originally published on page 17 of The Cape Argus http://capeargus.co.za/index.php?fArticleId=2218522 on September 09, 2004