Phenomenal seabird breeding success on Gough Island: vindication of our restoration strategy

In 2021 a large consortium of conservation organisations led by RSPB and Tristan da Cunha tried to eradicate invasive non-native house mice from the World Heritage Site Gough Island – a rugged volcanic island in the middle of the South Atlantic that is home to millions of seabirds. Over the past decades the mice had started eating seabird eggs and chicks (and latterly adult birds too), and several species were decreasing due to mouse predation. To restore the island and its seabird populations, every single mouse on the island had to be removed.

A helicopter spreads rodent bait to eradicate invasive house mice from Gough Island in July 2021 (M. Risi)

However, despite heroic efforts under challenging weather conditions, the mouse eradication operation was not successful. Three months after the operational team had departed from the island, a single mouse was discovered by the remaining monitoring team (December 2021). They searched the island for more mice and documented a rapid increase in mouse detections at the end of the southern summer 2022.

At any one time there are seabirds breeding on Gough. Some species breed during the summer, others during the winter, and the largest species – the Tristan albatross – needs almost 12 months to raise chicks. Although we were not successful in eradicating the mice from Gough Island, our efforts led to an enormous reduction of the number of mice on Gough during this past breeding season and so we were hopeful this would provide the seabirds with some respite from predation. As a result, we were very interested to find out how the seabirds fared this year. Would their breeding success be as good as we had always envisioned in the (near)absence of mice?

A rare sight - MacGillivray's prion chick with its flight feathers following their respite from mouse predation (R. Daling)

Results from the winter breeders came through first and these were staggering: the Critically Endangered MacGillivray’s Prion increased breeding success from an average of 6% with mice (including many years of 0% success) to 82% in 2022, whilst the Endangered Atlantic petrel had a 63% breeding success – more than double the previous year’s rate and well above average. Gough Island is the global stronghold for both species.

Results for the Critically Endangered Tristan albatross came through in October, with a massive increase from an average of 32% to 76% breeding success. Almost every Tristan albatross in the world breeds on Gough Island.

Breeding success of Tristan albatrosses on Gough Island from 2004 to 2022. The horizontal dashed line is the typical breeding success on predator-free islands that would be sufficient for an albatross population to maintain itself. In 2022 the Tristan albatrosses on Gough exceeded this threshold for the first time since records began.

Our monitoring team on Gough has just sent through the numbers for the last species, the Near Threatened grey petrel (another species for which Gough is significant) and the results are similarly impressive: grey petrel breeding success rose from 30% to 75%.

All in all, the seabirds on Gough had a phenomenal breeding season this year and there was no indication of any mouse predation. Although we have long expected it to be the case, we now have evidence that removing mice will have huge benefits for this World Heritage Site.

Over 1000 Tristan albatross chicks fledged for the first time this century (L. Dorman)

Mice are omnivores and will primarily eat seeds, plants, and invertebrates. When mice become very abundant there is intense competition for food, and plant and invertebrate food sources can become depleted. Out of desperation hungry mice will then explore alternative food sources – and on Gough Island they started eating seabirds. In 2022 the low numbers of mice (and hence low competition) meant they had plenty of other food to eat, and the seabirds were able to raise many chicks.

Unfortunately, we do not believe that this situation will persist. We expect mice will become so abundant that they deplete their typical food sources and then start eating seabirds once again. We do not know when this will happen, but as long as mice remain on Gough Island the future for seabirds is not secure. This year has shown us what seabirds can achieve when their chicks are not eaten by mice – and this gives us a determination to return to Gough in the future and remove the mice forever.