Hiding in plain sight

Prions are among the most difficult seabirds to identify – and none more so than MacGillivray’s Prion. Largely overlooked for 100 years after it was described, it has variously been treated as a subspecies of broad-billed prion, a subspecies of Salvin’s Prion, lumped with either of those species, or treated as a full species in its own right.

Figure 1. Spot the difference! From left to right: Salvin’s Prion (Pachyptila salvini), MacGillivray’s Prion (P. macgillivrayi), Broad-billed Prion (P. vittata) © Jean-Claude Stahl, Te Papa.

In addition to uncertainty over its taxonomic distinctiveness, MacGillivray’s Prion was left out of most field guides and handbooks because it was thought to be incredibly rare – confined when breeding to less than a hectare of habitat on an inaccessible rock stack off the coast of St Paul Island in the southern Indian Ocean.

Figure 2. La Roche Quille, off the coast of St Paul Island, southern Indian Ocean. For several decades this was thought to be the only remaining breeding site for MacGillivray’s Prion © Karine Delord, CNRS-IPEV.

Conservationists and ecologists started to pay more attention to MacGillivray’s Prion about a decade ago, with the unexpected discovery of a large population breeding among the hundreds of thousands of Broad-billed Prions on Gough Island in the South Atlantic. Genetic analyses revealed that MacGillivray’s Prion was most closely related to Broad-billed Prion; as the two breed at the same site (3 months apart) without significant introgression, MacGillivray’s Prion is now recognised as a distinct species.

Figure 3. A MacGillivray’s Prion feeding its chick on Gough Island, South Atlantic Ocean © Michelle Risi.

However, despite the total known population jumping from a few hundred pairs to a few hundred thousand pairs, MacGillivray’s Prion remained critically endangered. In some years introduced house mice killed every chick in monitored nests on Gough Island.

Ship rats were eradicated from 814 ha St Paul Island in 1997, allowing prions to recolonise from La Roche Quille (150 metres offshore) and to become more accessible to researchers. In December 2017, Karine Delord and colleagues attached geolocator loggers to the legs of breeding MacGillivray’s Prions on St Paul Island to investigate where they ranged to throughout their annual cycle. When they published their findings in 2022, they reported that three birds headed west to near South Africa in the nonbreeding season, while the other four spent the southern winter in the Tasman Sea, far to the east. This was a great surprise to Australasian ornithologists, as MacGillivray’s Prions were not recognised as occurring in either Australia or New Zealand.

Figure 4. A MacGillivray’s Prion with a geolocator tag attached to its leg, St Paul Island, southern Indian Ocean © Christophe Barbraud, CNRS-IPEV.

As MacGillivray’s Prions can be confused with other prion species even in the hand, we decided to search for them among the research collections of Museum of New Zealand Te Papa Tongarewa, and the South Australia Museum. Tissue samples were taken from specimens that had bill measurements that lay between those of broad-billed prion and Salvin’s prion, and the mtDNA haplotypes and nuclear DNA microsatellites of these specimens were compared with reference samples from all three species.

A total of five MacGillivray’s Prions were discovered in the two museum collections – one that had washed ashore in New Zealand in 1954, and four that were found on Australian beaches between 1930 and 1979.

While we achieved our initial goal of confirming the occurrence of MacGillivray’s Prions in both Australia and New Zealand, the bill measurements and DNA analyses revealed a most unexpected result. We had set out to confirm the findings of the French geolocator tracking study of MacGillivray’s Prions from St Paul Island (about 3,000 km south-west of Australia). However, the mtDNA haplotypes and relatively long bills of the Australasian specimens revealed that all five had originated from Gough Island, more than 10,000 km away!