- Andrew Callender
Breeding birds & multiplying mice: what next for Gough?
The spectacular breeding success of seabirds on Gough Island over the past year has gone some way in easing the desolation so many of us felt when evidence of a surviving House Mouse surfaced 14 months ago. The numbers are worth repeating: 1,186 Tristan Albatross chicks from 1,570 breeding pairs were recorded in the 2022 season, equating to a breeding success of 75.5%, and more than twice the average of 30.2% over 2001-2021; Grey Petrels, one of the most heavily impacted winter breeders, were at 78.6% in 2022, compared to an average of 30.3% from 2014-2021; and Atlantic Yellow-nosed Albatrosses returned 77.9% in 2021 against an average of 53.5% across 2010-2020.* But perhaps most uplifting was that of the MacGillivray’s Prion with a phenomenal 82% breeding success compared to near or complete failure in six of the last seven years, and an average of just 6% over the seven years 2014-2020.
Yet the mice have also been doing what mice do: breeding and multiplying. We have not recorded any resumed attacks on seabirds. But at some point, the predatory behaviour of the mice is expected to return; we don’t know when and there is little precedent from other islands (mainly because there are seldom available resources to monitor failure). We, of course, hope that a resumption of predation will take considerable time to reappear, and we will need to be circumspect in interpreting numbers that will undoubtedly show variability. The second year of ‘post-eradication attempt’ Atlantic Petrel data is a case in point. Like the Grey Petrel, the Endangered Atlantic Petrels are winter breeders. The second year of data showed a slightly lower figure for breeding success, of 56.4% in 2022 compared to 62.7% in 2021, though this is still nearly double the 2014-2020 longer-term average of 32.3% (see Figure 1).
Was that modest decline in 2022 vs 2021 the first sign of a renewed mouse impact? Perhaps but probably not: hatching success which includes (in our recording methodology) the first, most vulnerable days of a chick’s life, was actually higher in 2022 than 2021. We don’t know exactly why but there is bound to be natural variability, and the sample sizes are invariably modest – even to sample 20 nests takes an extraordinary amount of work.
Looking forward, next will be results from Prion Cave and the 54 monitored nests of the MacGillivray’s Prions – a species that is the standard bearer of all the smaller burrow nesting species (at least for this author). We know that several nests have already failed – both at the egg and chick stages – though we don’t know why. The next few weeks are crucial. Predation is likely to be felt by the smaller species (storm-petrels, prions, diving-petrels) first, even if we have so few windows into their nesting attempts, so this is a key dataset. We expect to have a figure for chick survival in late March or early April.
Just as important will be the results of the Tristan Albatross breeding that will be assessed towards the end of the year. Given the length of time that their chicks take to fledge, breeding adults that successfully fledge a chick take the following year off to recuperate. For some time now, we have recognised that on Gough many Tristan Albatrosses have been attempting to breed every year, given that often a breeding attempt has failed so early on in a season due to mice, and thus relatively less energy has been expended, meaning another attempt in a consecutive year is feasible. This artificially inflated frequency of breeding attempt, and thus number of breeding pairs each year, has been masking the true rate of decline of the species as measured by total population. The actual jeopardy for the species was highlighted in RSPB-led research that concluded that “Low reproductive output for long-lived species may lead to a cryptic population decrease, which can be obscured from readily available counts of breeding pairs by changes in the population structure.” Given the high breeding success in 2022, it is quite possible that the number of pairs attempting to breed on the island this year will be lower than the 1,570 pairs last year – but it will be their breeding success that will be key, for higher productivity is a prerequisite if the projected population decline is to be reversed.
The data for all the seabird breeding results is as ever collected by the Overwintering Team on island. The team has also been tasked with continuing to collect evidence that may help understand why the eradication attempt was not successful, building on the information gathered in those early days in 2022 of surviving mice detections. All this data, and indeed everything available as to how the Gough Island Restoration Programme carried out the 2021 eradication attempt, has been given over to the Independent Review panel. Their findings and recommendations are due over the (northern) summer and will then be shared with New Zealand’s Island Eradication Advisory Group. We reiterate our commitment to share the recommendations more widely, hopefully for the benefit of other island eradications, and, of course to give us the best chance of returning to Gough to finish what we started.
At the very least, the seabird breeding successes that we witnessed in 2022 gives us the evidence of what will happen on a rodent-free Gough. That remains our goal.
*N.B. Some figures have increased slightly since our last report on breeding success.
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.