MacGillivray’s Prions: good 2022-23 breeding success but evidence of mouse predation recorded

For decades the seabirds on Gough Island had been struggling with introduced House Mice eating their eggs and chicks, leading to population declines for several species from small MacGillivray’s Prions to huge Tristan Albatrosses. In 2021, the RSPB – on behalf of Tristan da Cunha and in collaboration with many partners – attempted to restore this globally important seabird breeding site by eradicating the mice in an ambitious operation. Unfortunately, that operation was not successful.

However, the operation lowered the mouse population substantially and, as a consequence, in the first full breeding season following the eradication attempt we observed very high breeding success across our monitored seabird species, with prions, petrels and albatrosses raising thousands of chicks.

One of the many Tristan Albatrosses that survived to fledge last year (L. Dorman)

The chicks that have fledged in 2022 and 2023 will need to survive several years at sea before they return to Gough and join the breeding population. When they return, they will work hard to raise (at best) a single chick each season – the effort is such that for the Tristan Albatross a chick will only be reared every other year. The mice, by contrast, will be breeding by the time they’re eight weeks old and a typical litter will contain at least half a dozen young which will be weaned within a month. As such, when we discovered surviving mice in December 2021, we knew that their population would likely recover very quickly. To us, the question has been when (rather than ‘if’) would the mice start to eat seabirds once more?

The seabirds most vulnerable to mouse predation are the smaller species that nest underground, but these are also the hardest species to monitor. Prion Cave offers us a window into this underground world, and the MacGillivray’s Prions that nest there function as a bellwether for the interactions between mouse and bird on Gough. During the 2022-2023 breeding season, our team installed camera traps at a number of MacGillivray’s Prion nests within the cave to monitor for signs of the resumption of predatory behaviour by mice.

Sadly, on watching through the Prion Cave footage, one camera captured precisely this. A mouse was filmed approaching a small prion chick that was maybe two weeks old and unguarded by its parents (though even had they been present they have no defence mechanisms against mammalian predators and the outcome is not likely to have been different). The chick was then attacked by the mouse – when the mouse leaves, a dark patch can be seen on the prion’s back reminiscent of the open wounds to which we had become accustomed over the past few years. The chick later died. This is the first evidence of a mouse on Gough Island predating a seabird chick since the eradication attempt.

A MacGillivray's prion chick (left) is agitated by the House Mouse that is climbing onto its back (RSPB)

The MacGillivray’s Prions still managed to raise many chicks – of the 54 monitored nests, chicks fledged successfully from 34 to give a breeding success of 62.9%; this figure is in line with average productivity rates observed from similar prion species on other islands that are free of introduced mammalian predators. Moreover, it continues to represent a step change in population recruitment for MacGillivray’s Prions when compared to the average of 6% from 2014-2020 (including several years of complete breeding failure).

Many MacGillivray's Prions survived to fledge including this one, nicknamed Ray by the team (R. Goodwill)

Breeding success for MacGillivray's Prion 2014-2022

Our Overwintering Team found this eggshell that raised their suspicions about mice predation (RSPB)

We do not know how many, if any, of the other losses were attributable to mice, as there was no direct evidence of attack beyond this single chick – though one eggshell was also found that caused us to be suspicious of mouse activity. The fact that around 40% of the remaining failures occurred around the time of hatching is undoubtedly disconcerting but we cannot draw any further conclusions at this stage. Future monitoring will show how rapidly the predatory behaviour of the mice spreads – we will, as ever, keep you all updated.

We know we must return and attempt once more to restore Gough Island by removing the mice – this new evidence doesn’t harden our resolve to do so because that resolve has never wavered.

Acknowledgement The Gough Island Restoration Programme is being carried out by the RSPB in partnership with Tristan da Cunha, BirdLife South Africa, the Department of Forestry, Fisheries and the Environment (South Africa), the Royal Zoological Society of Scotland, Island Conservation,Conservación de Islas,Manaaki Whenua - Landcare Research and BirdLife International. The programme is part-funded by the RSPB, the UK Government, the National Fish and Wildlife Foundation and other generous individuals and organisations.