Justification of Red List Category
MacGillivary's Prion is projected to undergo extremely rapid declines over the next three generations owing to chick predation by invasive mice. It is therefore listed as Critically Endangered.
Jones et al. (2021) estimate the population on Gough in 2020 to be 175,000 pairs (95% CI 24,000-1,210,000 pairs [48,000-2,420,000 mature individuals). An additional 150-200 breeding pairs (c.300-400 mature individuals) were estimated to be present on Roche Quille, Île Saint-Paul (Jiguet et al. 2007). Following the successful eradication of the Black Rat (Rattus rattus) and European Rabbit (Oryctolagus cuniculus), surveys showed an increase in numbers with a rough estimate of 740 breeding pairs (Barbraud et al. 2021). However, mice are still present on Saint-Paul, so the long term prospects of this population remain unknown (C. W. Jones in litt. 2022). The population size is therefore estimated to be in the range of 48,300-2,421,480 mature individuals.
This species is inferred to be declining as it is currently suffering a very high chick mortality rate as a result of nest predation by mice (Dilley et al. 2015). These declines are projected to occur at a rate of 9% per annum over the next 36 years (Jones et al. 2021), a rate equivalent to 95.7% over three generation lengths (33.45 years [Bird et al. 2020]). A mouse eradication attempt on Gough Island occurred in June-August 2021, and although this killed a large proportion of the island's mice, some mice did survive the attempt (S. Oppel in litt. 2022). Although it is unknown how long it will take mice to reach carrying capacity, it is likely there will be no long-term improvement in breeding success that could counteract the current projected declines.
Pachyptila macgillivrayi was described initially from Île Saint-Paul and Amsterdam in the French Southern Territories. It is now extinct on Amsterdam Island and confined to Roche Quille, a rocky stack a short distance off the eastern side of Île Saint-Paul, where the population is estimated at 150-200 pairs (Jiguet et al. 2007). However, MacGillivray’s Prion is, at present, considered to also include the recently recognised Thinner-billed Prion form found breeding asynchronously to Broad-billed Prion P. vittata on Gough Island (St Helena, to UK) (Ryan et al. 2014, Dilley et al. 2015).
On Gough, MacGillivray’s Prion breeds in the summer while Broad-billed Prion P. vittata breeds in late winter (Dilley et al. 2015).
Introduced House Mice Mus musculus on Gough Island are dramatically impacting reproductive success and causing very rapid population declines (Dilley et al. 2015). Estimates of breeding success were only 7% across the 2013/14 and 2014/15 breeding season and chick mortality in studied nests in the latter year was 100% (Dilley et al. 2015). A mouse eradication attempt on Gough Island occurred in June-August 2021, and although this killed a large proportion of the island's mice, some mice did survive the attempt, and they remain a significant threat (S. Oppel in litt. 2022). The impact of introduced rats is thought to have caused the extinction of the species on the main Île Saint-Paul. However, a subsequent rat eradication programme has resulted in the species occupying burrows on the island once again (Jiguet et al. 2007). The occasional vagrant occurrence of several species of falcon on Île Saint-Paul has resulted in periodic high predation rates on the species, which as an additive source of adult mortality has the potential to drive significant declines where the population is depleted, though following the rat eradication, the impact of this threat should be greatly reduced in the future (Jiguet et al. 2007).
Conservation Actions Underway
A mouse eradication attempt on Gough Island occurred in June-August 2021, and although this killed a large proportion of the island's mice, some mice did survive the attempt and therefore any improvements in breeding success are likely to be temporary (S. Oppel in litt. 2022).
Conservation and Research Actions Proposed
Invasive species control is required. The eradication of mice from Gough Island is the action that will have the greatest benefit to the conservation status of MacGillivary's Prion. A species conservation plan should be developed. The population size and trend should be monitored alongside and after the mouse eradication programme.
Text account compilers
Clark, J., Rotton, H.
Fjagesund, T., Hermes, C., Jones, C., Martin, R., Moreno, R., Oppel, S. & Stuart, A.
BirdLife International (2023) Species factsheet: Pachyptila macgillivrayi. Downloaded from http://www.birdlife.org on 31/03/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 31/03/2023.