BirdLife International now recognises the split of Pachyptila salvini into P. salvini and P. macgillivrayi.
Prior to this taxonomic change, Salvin’s Prion Pachyptila salvini (http://www.birdlife.org/datazone/speciesfactsheet.php?id=3915) was listed as Least Concern, on the basis that it has a very large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality or population size, and a small number of locations or severe fragmentation). The population trend also appears to be stable and extremely large, and hence the species does not approach the thresholds for Vulnerable under the population trend (>30% decline over ten years or three generations) and/or size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure).
Following Dilley et al. (2015) and Ryan et al. (2014), BirdLife now recognises MacGillivray’s Prion P. macgillivrayi as a separate species from P. salvini.
P. salvini is found breeding at the Prince Edward Islands (South Africa), Crozet Islands, Amsterdam Islands and St Paul Island (French Southern Territories), while P. macgillivrayi (as defined following this taxonomic change) is found only on Gough Island (St Helena, to UK). It is possible that P. macgillivrayi is the same taxon as the thin-billed form of the prion known from Amsterdam (where now extinct) and St. Paul (where a few hundred birds are now confined to one rat-free islet) in the Indian Ocean, but genetic analyses and direct comparisons of skins are still required to resolve this (Ryan et al. 2014).
It is suggested that P. salvini should be still listed as Least Concern, on the basis of a very large range and extremely large and stable population.
Further research is needed to understand the distribution and relative abundance of Broad-billed Prion P. vittata and P. macgillivrayi on Gough Island (Dilley et al. 2015). Swales (1965) estimated that at least 10 million pairs of prions bred on Gough in the 1950s, and although their numbers have decreased since then, due to predation by introduced house mice Mus musculus (Cuthbert et al. 2013), Cuthbert (2004) estimated roughly 1.5–2.0 million pairs of prions at the island in 2000/2001, making Gough still the most important breeding site for P. vittata worldwide (Brooke 2004). Although the population size of P. macgillivrayi is not known, it is thought likely to comprise between 100,000 and 1 million mature individuals (Peter Ryan pers. comm. 2016).
On Gough, P. macgillivrayi breeds in the summer while P. vittata breeds in late winter (Dilley et al. 2015). P. macgillivrayi suffered a high chick mortality rate of 82% in 2013-2014 and 100% in 2014-2015, while P. vittata also had a chick mortality rate of 100% in 2014 (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. As a consequence of that decline and its small occupied breeding range, Pterodroma incerta is currently listed as Endangered under criterion B2ab(v).
While the actual population size and trend of Pachyptila macgillivrayi remain unknown, pending further research, it seems reasonable to infer that it is also declining, which – combined with its small occupied breeding range on one island – implies that it probably also qualifies as Endangered under criterion B2ab(v).
Comments are invited on these proposed categories and further information would be welcomed.
Brooke, M. (2004). Albatrosses and petrels across the world. Oxford: Oxford University Press, 520 pp.
Cuthbert, R..& Hilton, G. (2004). Introduced house mice Mus musculus: a significant predator of threatened and endemic birds on Gough Island, south Atlantic Ocean? Biological Conservation, 117, 483–489.
Cuthbert, R.J., Louw, H., Lurling, J., Parker, G., Rexer-Huber, K.,Sommer, E.,Visser, P.& Ryan, P.G. (2013). Low burrow occupancy and breeding success of burrowing petrels at Gough Island: a consequence of mouse predation. Bird Conservation International, 23, 113–124.
Dilley, B.J., Davies, D., Bond, A.L. & Ryan, P.G. (2015) Effects of mouse predation on burrowing petrel chicks at Gough Island. Antarctic Science 27(6), 543–553.
Ryan, P.G., Bourgeois, K., Dromzée, S and Dilley, B.J. (2014) The occurrence of two bill morphs of prions Pachyptila vittata on Gough Island. Polar Biol. DOI 10.1007/s00300-014-1473-2.
Swales, M.K. (1965). The seabirds of Gough Island. Ibis, 107, 17–42.