Millions of the world´s most unique birds rely on remote and undisturbed nature of Gough Island, Tristan de Cunha in the South Atlantic ocean, but are threatened by mice that eat their eggs and chicks alive. Mice accidentaly escaped from one of the harbouring ships in the 19th century and have grown not only in numbers, but also in size.
Ten out of all seabird species found on Gough Island are losing an estimated 1 739 000 eggs/chicks annually and two endemic landbirds are threatened, too.[1] Some bird species that breed nearly nowhere else in the world such as Gough Bunting or Tristan Albatross are likely to be driven to extinction, if invasive mice predation continues. [2]
Gough Bunting, a beautiful small bird of olive color endemic to Gough, is one of the land birds having a hard time defending itself against mice attacks. The density of territorial pairs of this songbird roughly halved between 1990 and 2007 with only 400-500 pairs that remained on Gough.[3] Buntings nest on the ground and mice can easily reach them. Moreover, mice probably compete with buntings for other sources of food and push birds further from the shoreline into higher altitudes that are less favorable for their prosperity.[4]
Mice predation on bird´s chicks is rare and case of Gough Island is unprecedented. Researchers did not believe, that a mouse that weigh 30g could feed on a chick that is 300 times heavier (albatross chick), until it has been confirmed.[5] Lack of mammalian competitors and food alternatives such as seeds and insect during winter time provide possible explanation for strange preferential diet of mice.
Gough Island, World Heritage Site and Important Bird Area is uninhabited apart from research team and staff who runs small meteorological station. Terrestrial mammals were absent on the island prior to the introduction of house mice and birds have evolved with no native predators and thus are an easy prey.
Years of close and meticulous monitoring of the situation on Gough had led RSPB Wildlife Charity and Tristan island Council to the decision to undertake mice eradication in 2020. Poisoned bait will be dropped from helicopters during winter season. Most of the birds are at sea, other food sources are limited and mice do not reproduce during that time. Populations of critically endangered Gough Bunting and vulnerable Gough Moorhen stay on the island whole year round and will be at risk to non-target poisoning. They will be captured prior to eradication and held in captivity for several months until its end.[6]
The decision raised some controversial voices. Rebecca Ram, scientific consultant for Animal Aid critisized the fact, that no effort has been made to evaluate more human options, such as use of immunocontraceptives or rodent proof construction for bird´s nests. The chosen approach is going to leave mice dying slowly, according to Ram. [7] Despite the good results of several trials undertaken on the island by RSPB, there is no guarantee, that the eradication will wipe out mice completely. The terrain is mountainous and there are numerous caves on Gough, only fraction of them identified. Poison might also affect other animals on the island. Uncertain if all mice would accept the bait or not, the organisation expressed confidence in proposed solution.[8]
[1,2,4] The impacts of introduced House Mice on the breeding success of nesting seabirds on Gough Island: Anthony Caravaggi, Richard J. Cuthbert, Peter G. Ryan, John Cooper, Alexander L. Bond, 2018 https://onlinelibrary.wiley.com/doi/10.1111/ibi.12664
[3] https://www.goughisland.com/get-in-touch
[5] Gough Island Restoration Programme; RSPB, Birdlife International, Tristan da Cunha https://www.rspb.org.uk/globalassets/downloads/documents/conservation-projects/gough-island-restoration-programme.pdf
[7] https://www.independent.co.uk/environment/mice-cull-poison-rspb-birds-albatross-bunting-gough-island-a9013576.html
[8] Preparations for the eradication of mice from Gough Island: results of bait acceptance trials above ground and around cave systems R. J. Cuthbert, P. Visser, H. Louw, K. Rexer-Huber, G. Parker, and P. G. Ryan: http://www.issg.org/pdf/publications/Island_Invasives/pdfHQprint/1Cuthbert.pdf
[6] https://acap.aq/en/latest-news/3317-feeling-strong-the-gough-island-restoration-programme-needs-20-bait-loaders-to-help-eradicate-mice-next-year
[0] Introduced house mice: A significant predator of threatened and endemic birds on Gough Island, South Atlantic ocean?, Richard Cuthbert, Geoff Hilton https://www.researchgate.net/publication/223587704_Introduced_house_mice_Mus_musculus_A_significant_predator_of_threatened_and_endemic_birds_on_Gough_Island_South_Atlantic_Ocean