Blog by Alexis Osborne, Gough Island Field Assistant
House mice (Mus musculus) were introduced to Gough Island in the 19th century. Originally mice were thought to have little impact on Gough Island, or indeed anywhere else where they had been introduced. It is only in recent years that we have got to experience and understand the impact that the mice are having on Gough Island’s seabirds, and the MacGillivray’s prion (Pachyptila macgillivrayi) is no different.
MacGillivray’s prions were only discovered on Gough Island a few years ago when it was determined that they bred at a different time to the Broad-billed prion (Pachyptila vittata), and hence that they were a different species! We now know that the large majority of the world’s MacGillivray’s prions breed on Gough Island, with a small population breeding on St. Paul Island in the Southern Indian Ocean. This means that the impacts of mouse predation are not just felt locally but impact the global population of MacGillivray’s prion, and the species conservation status. The species is listed as Endangered – in other words, it is facing a very high risk of extinction.
The MacGillivray’s prions breeding in Prion Cave – our study colony on Gough Island – have been experiencing extremely high rates of chick mortality for the past few years:
Breeding Season - Chick mortality (% of chicks lost)
2013/14 - 82%
2014/15 - 100%
2015/16 - 98%
2017/18 - 100%
And this breeding season (2018/19) was no different.
2018/19 Breeding Season
Chris, Michelle and I started monitoring the MacGillivray’s prions in Prion Cave as the first eggs were laid late in November to early December 2018; it was not long before we noticed a high failure rate. Of the 50 eggs monitored this season, approximately 60% of those successfully hatched. Once hatched the chicks are not out of danger, mice have been known to predate on MacGillivray’s prion chicks and that is what we saw this year. Towards the end of the season only one chick was still alive.
This one chick was fighting the odds, so we installed an infra-red camera by the nest to monitor its progress which revealed that the chick was harassed by mice on several occasions. The camera footage also revealed when the chick left the nest. Following this we did a thorough search for a carcass or any sign of death, but thankfully none were found. We can safely say that this chick survived.
We are thrilled to share that this chick fledged; however, sadly this survivor is in the minority. These results show once more that mice on Gough Island are still causing havoc to seabirds and with this consistently high rate of chick mortality, the population of MacGillivray’s prions on Gough Island is in immediate danger. We cannot stress enough the importance of the much-anticipated operational phase of the Gough Island Restoration Programme that is on track for 2020. We are looking forward to the day when Gough Island is declared mouse free!
If you can help support the restoration of Gough Island and prevent the extinction of species like the MacGillivray’s prion, please donate via our website today.
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Acknowledgement
The Gough Island Restoration Programme is being carried out by the RSPB in partnership with Tristan da Cunha , BirdLife South Africa and the Department of Environmental Affairs in South Africa and Island Conservation . The programme is part-funded by the RSPB, the UK Government, the National Fish and Wildlife Foundation and other generous individuals and organisations.