Invasive mice are pushing the Tristan albatross to the brink of extinction

A Critically Endangered species of albatross may be in more danger than previously thought.

The Tristan albatross, which breeds only on remote islands in the south Atlantic, is under threat from humble house mice. Each year, thousands of chicks and eggs are consumed by the invasive rodents, which are believed to have been introduced by sailors in the nineteenth century.

Despite this, it appeared that albatross numbers were stable. But a new study taking a more comprehensive look suggests that the population is actually declining by around 1% a year, equating to a loss of roughly 2,000 birds between 2004 and 2021.

Dr Alex Bond is the Principal Curator and Curator in Charge of Birds at the Museum, and was one of the co-authors on the new paper which was published in the Journal of Applied Ecology.

He says, 'Albatross have a complex life cycle, with young birds spending years at sea after fledging before returning to their breeding colony.'

'This research shows that it is more complex than we previously realised to monitor such species, including the effects of change, threats and interventions on their population.'

The study feeds into ongoing efforts to conserve the seabirds of the south Atlantic and eliminate the mice which threaten them. 

How do mice threaten seabirds?

While the Tristan albatross has a range spanning across the Southern Hemisphere from South America to Australia, it only breeds on two small islands in the south Atlantic. Both Inaccessible and Gough Island are part of the Tristan da Cunha archipelago and together, in recognition of the unique biodiversity they harbour, are a UNESCO World Heritage Site.

The vast majority of Tristan albatrosses breed on Gough Island, with an estimated 1,500 breeding pairs raising one chick every two years.

The islands are relatively untouched by humans, apart from one significant exception. While Inaccessible Island remains undisturbed, Gough Island plays host to a large population of mice.

It is not known exactly when the rodents arrived, but without any natural predators their populations have exploded. The mice grow to twice the size of their mainland relatives and are estimated to consume 1.7 million eggs and chicks each year.

In 2021, the Royal Society of the Protection of Birds (RSPB), in partnership with the island's government, dropped poisoned rodent pellets across Gough Island in an effort to wipe out the mice. Unfortunately, follow-up work revealed that the eradication campaign had not been successful.

Despite this setback, conservationists hope that enough mice have been killed to give the birds breathing room while they work out how some of the rodents survived. This work comes alongside efforts to reduce other threats facing the albatrosses, such as longline fisheries in which the birds can be mutilated or drowned.

The new study aims to shed light on how these threats facing Tristan albatrosses are impacting their population, and how the birds could recover. 

How is the population of Tristan albatrosses changing?

Using counts of adults and chicks taken over a period of 40 years, the study was able to estimate the number of individual albatrosses at sea at any one time. This was then combined with counts on Gough Island to give an estimate of the overall population size.

The researchers found that while breeding pair numbers remained relatively constant between 2004 and 2021, this disguised an overall population decline from around 9,795 birds to 7,752. This is mostly thought to result of mice killing chicks.

At the same time, efforts to combat bycatch from long line fishing have increased the survival of adult albatross out at sea. This means that older albatrosses are remaining in the breeding population for longer, which in the short term is partially mitigating the decline in younger birds.

But despite this, the study found that if the situation continues as it currently is the population of the Tristan albatross will continue to fall. If the mice begin to attack adults as well as chicks, then it was predicted that the rate of decline would increase six-fold.

The only scenario in which populations recovered came from the successful eradication of mice. In this case, there could be an average of 10,000 Tristan albatross by 2050 - around the same number there were at the turn of the century.

Organisations committed to the survival of the Tristan albatross have called on nations around the world to implement the study's findings and invest further in the island's conservation.

John Cooper, the Information Officer of the Agreement on the Conservation of Albatrosses and Petrels, says, 'Besides the persisting problems of albatross bycatch in fisheries, this study gives us hope that some albatross populations can be restored with technically feasible management actions that can be implemented now if governments honour their commitments under the Convention of Migratory Species and financially support these efforts.'