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Ross Wanless Ben Dilley a narrow margin Prions formerly were abundant at Amster- Paul Visser and Henk Louw found chicks in above, left A Broad-billed Prion, showing its dam and St Paul islands in the southern In- Prion Cave in January, several months after broad, blackish bill, incubating on a nest in peter ryan dian Ocean, but following the introduction Broad-billed Prion chicks fledge. In 2013, an outbuilding of the Gough Island weather of rats only a small number of birds survive, Ben was able to confirm that the narrow- station in mid-August. New prion discovered on Gough Island breeding on a stack off St Paul Island. How- billed birds in Prion Cave and Gonydale lay ever, newly-fledged prions have narrower eggs in late November, three months later above By comparison, a medium-billed prion, eabirds comprise barely three per cent Fregetta maoriana. To discover a new species the Percy FitzPatrick Institute. When they bills than adults, so I was concerned that this than the broad-billed birds. This marked dif- incubating in Prion Cave in mid-December has of the world’s birds and, given their of prion breeding in vast numbers on Gough returned to Cape Town, I was sceptical when factor was confounding the local differences ference in breeding season argues strongly a narrower, blue-edged bill. popularity with scientists and birders Island almost beggars belief. they told me there were two types of prions in bill size at Gough Island. But I was suffi- that the narrow-billed birds are a new species alike,S it is surprising when an entirely new Gough Island, in the central South Atlan- on Gough. ciently intrigued to make this a top priority of prion, rather than just an example of bill prions there had two breeding seasons, one species is discovered. There was one such tic Ocean, is renowned as one of the most Until then, there had been no hint that when I next had a chance to visit the island in polymorphism. in September and one in November, so there surprise in 2013 when a new storm petrel important seabird breeding islands in the anything other than Broad-billed Prions September 2013. Ironically, the new species appears to have may have been two species there before they Oceanites pincoyae was described from cen- world. It is home to virtually all Tristan Al- Pachyptila vittata occurred on the island. Assisted by Ben Dilley and other members been overlooked in part because prions are were extirpated by introduced predators. tral Chile (Auk 130: 180–191). batrosses and Atlantic Petrels, the largest Indeed, Gough is the most important breed- of the relief team, I caught adult prions and the most abundant seabird breeding on There is no evidence that the small-billed In most cases, new species have resulted single-island population of Sooty Albatross- ing site for this species worldwide, with an collected carcasses of freshly-killed birds at Gough. Broad-billed Prions breed around prions breed at the Tristan archipelago, some from species splits (for example, the descrip- es, and globally significant populations of at estimated 10 million pairs on the island in six sites in the south-eastern sector of the the weather station and along the east coast 380 kilo­metres away; all birds examined tion of Bryan’s Shearwater Puffinus bryani least 10 other seabird species. Unfortunately the 1950s. Despite the impacts of the mice, island. There shouldn’t be any fledglings at of the island, where all scientific expedi- there to date and all observations of the tim- from the Hawaiian islands in 2011) or the Gough has been in the news over the past Gough still supports between one and two that time of year because Broad-billed Pri- tions land. It seems that because prions are ing of breeding suggest that only Broad-billed rediscovery of species long thought ex- decade or so following the discovery that in- million pairs, making prions the most abun- ons breed in early spring at Gough, laying so abundant, ornithologists collected their Prions occur there. If the new prion is indeed tinct, such as the New Zealand Storm Petrel troduced house mice attack and kill seabird dant bird on the island. eggs mainly in August. At three sites birds specimens close to the landing sites and confined to Gough, it provides yet more mo- chicks on the island. The mice have slowly Karen and Sylvain collected skulls of had bills averaging 21 to 22 millimetres so failed to sample the narrow-billed birds tivation for eradicating mice from the island. been eroding the island’s vast burrowing pet- wide, typical of Broad-billed Prions, but at breeding nearby. PETER RYAN south Gough prions killed by skuas and discovered that america africa Glen rel populations, and have had an especially the bills of birds collected in Gonydale at the other three sites their bills averaged only This begs the question as to why the two (old base) Tristan (21.6) serious impact on Tristan Albatross chicks. 400 metres elevation were narrower than 18 to 19 millimetres wide, similar to Macgil- forms are segregated on the island, given that Reference: Ryan, P.G., Bourgeois, K., Gough The new prion was discovered by the those collected around the island’s weather livray’s Prion. all foraging occurs at sea. For now, we have Dromzée, S. & Dilley, B.J. 2014. ‘The occur- research team working to eradicate mice station, close to sea level. Interestingly, the difference was not linked no idea why this might be. We also need to rence of two bill morphs of prions Pachyptila Haularound from the island. Karen Bourgeois and Syl- Bill width is important in prion taxonomy to elevation. Narrow-billed birds predomi- assess the ranges of the two species on Gough vittata on Gough Island.’ Polar Biology, DOI Point vain Dromzée spent a year on the island in and links directly to their feeding ecology. nated in Prion Cave, which is close to the so that we can estimate their populations. 10.1007/s00300-014-1473-2 South Golden Peak Highway 2011/12 as field assistants for the RSPB and Broad-billed birds have more filtering lamel- weather station where prions often gather in Genetic data are required to determine the Gonydale (21.8) Hummocks(17.9) Admirals lae – akin to a whale’s baleen – than species spring. And north of the base, typical broad- relationships between the Gough birds and Green Hill above Prions are the most abundant seabirds with narrower bills, and their diets are more billed birds were found at 250 metres above other prions, especially those breeding in the WATCH & LISTEN Base (21.5) to the ‘new’ narrow- Gonydale breeding on Gough Island. The recent discov- specialised. While breeding, Broad-billed sea level. Indian Ocean. However, it may be that, like Seal Beach billed prion species Path (18.7) Prion ery that two species breed on the island three Prions on Gough feed mainly on two- to the recently discovered radiation of Band- Cave(19.2) interacting in the months apart came as a complete surprise. three-millimetre-long copepods and their ith hindsight, we should have rumped Storm Petrels Oceanodroma castro, depths of Prion Cavern Head Richmond Hill lamellae are perfectly spaced to filter these realised several years ago that species pairs have evolved independently Cave on Gough Rockhopper Point left The southern part of Gough Island, show- tiny crustaceans. something odd was going on. at Gough and islands in the Indian Ocean. Island www.youtube. ing where Broad-billed Prions (white circles) The birds from Gonydale had bills similar Broad-billedW Prions are supposed to have Temporal segregation in breeding seasons com/watch?v=aRduIhc1mxE, or scan N (22.0) and the smaller-billed birds (black circles) in size to those of the enigmatic Macgilli- blackish upper mandibles, yet the birds in reduces intraspecific competition, and thus the QR code to go directly to the South Point link. Courtesy Ben Dilley. 1 km occur. The numbers in parentheses show the vray’s Prion P. [salvini] macgillivrayi, a large- Prion Cave have variable amounts of blue on may facilitate speciation. Intriguingly, early average bill width in millimetres at each site. billed form of Salvin’s Prion. Macgillivray’s the sides of the upper mandible. And in 2009, visitors to St Paul Island reported that the

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