Justification of Red List Category
Further research is needed to understand the relative abundance of this newly-recognised species, which has a highly restricted distribution. Nevertheless, although the actual population size and trend remain unknown, it has been demonstrated that it suffered a high chick mortality rate in 2013-2014 and 2014-2015 as a result of chick predation by mice. Because it has a highly restricted breeding range on two islands, and the population is inferred to be declining as a result of invasive species, MacGillivary's Prion is listed as Endangered.
The total population size is unknown, however an estimate of between 100,000 and 1 million individuals for the taxon on Gough Island (P. Ryan pers. comm. 2016) represents the vast majority of the global population, with only an estimated 150-200 breeding pairs (c.300-400 mature individuals) present on Roche Quille, Île Saint-Paul (Jiguet et al. 2007).
MacGillivray’s Prion are currently suffering a very high chick mortality rate as a result of nest predation by mice (Dilley et al. 2015). These levels are comparable to those for Atlantic Petrel Pterodroma incerta, another burrowing petrel species that is virtually confined to Gough and whose breeding population (although still relatively large) is declining as a result of nest predation by mice.
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). 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 and Research Actions Proposed
Invasive species control is required and a species conservation plan developed. The eradication of mice from Gough Island is planned and is the action that will have the greatest benefit to the conservation status of MacGillivary's Prion.
Text account compilers
Martin, R., Moreno, R., Hermes, C., Stuart, A., Fjagesund, T.
BirdLife International (2021) Species factsheet: Pachyptila macgillivrayi. Downloaded from http://www.birdlife.org on 25/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 25/10/2021.