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Tristan

Gough

to leg rings onto prions breeding at Gough and Tristan in 2014/15. The records of light levels the loggers store allow us to estimate where the birds puzzling prions spend the year – provided we can catch them again the following breeding sea- CHRIS JONES ne of the more surprising sea- station. Subsequent observations con- is listed as Endangered because almost son to recover the loggers. This is easier Broad-billed Prions breed from August bird discoveries in the past firmed that these ‘medium-billed’ all chicks hatched on Gough are killed said than done at Gough, where there to November and moult from November decade, at least in this part of prions bred three months later than by introduced house mice. This unhap- are still several million prions, many of to February, whereas MacGillivray’s Pri- Othe world, was the revelation that two Broad-billed Prions at Gough, provid- py situation will change if the planned which share burrows. ons breed from November to February species of prions breed on Gough Island ing a plausible mechanism for two such eradication attempt succeeds in remov- In the end, the team recovered log- and moult from February to May. Per- (Ryan 2014, African Birdlife 2(4): 10–11). similar species to co-occur. ing mice from Gough Island. gers from eight MacGillivray’s Prions haps the most interesting finding of Until 2011, we thought that the Broad- The new species is very similar in Most prions are notoriously difficult and 13 Broad-billed Prions (nine from Chris’s study was to confirm that the billed Prion Pachyptila vittata was the terms of bill structure to the St Paul Pri- to identify at sea and so the recognition Gough and four from Tristan). They annual cycle of Broad-billed Prions on only prion that bred on the island, as is on P. macgillivrayi. This species, which of yet another species in the Slender- found that MacGillivray’s Prions tend Gough Island is two to three weeks earlier the case at the Tristan da Cunha archi- was once abundant at Amsterdam and billed–Antarctic–Broad-billed contin- to forage farther south and west than than the birds at Tristan. This is probably pelago, 380 kilometres farther north. St Paul islands in the south-central In- uum is bound to challenge birders in Broad-billed Prions. As might be ex- a way to reduce competition between the Sylvain Dromzée and Karen Bour- dian Ocean, has been hard hit by intro- the South Atlantic and Indian oceans. pected, both species were confined to two species where they breed together. andrew jenkins geois, who spent a year on Gough from duced rats and is now largely confined The conventional wisdom is that Broad- areas close to the breeding colonies Chris tracked only a small number of September 2011 to October 2012, no- to a small stack off St Paul. Its taxonomy billed Prions can be distinguished from during the summer breeding season, adult birds; increasing the sample size ticed that birds breeding in some parts has been much debated, being bounced smaller species by their blackish bill, but although a few Broad-billed Prions did will doubtless increase their known of the island had narrower bills than back and forth between being a large many of the birds breeding at Tristan range east as far as the waters south of range. And juveniles and immatures the birds breeding around the weather race of Salvin’s Prion P. s alv ini or a small and Gough have bluish sides to the up- Africa at this time (possibly after failed most probably disperse more widely race of Broad-billed Prion. per mandible. At present, probably the breeding attempts). than adults. But at this stage we have above Prion identification at sea is extremely Genetic evidence confirms that the only way to tell them apart is to get really Soon after breeding, both species no definite evidence of MacGillivray’s tricky. The bird on the left shows the steep, birds on Gough are closely related to good images of birds showing bill width, undergo a three- to four-month wing Prions entering southern African wa- bulbous forehead and large bill typical of a the St Paul birds (and the extinct popu- which might allow ‘typical’ individuals moult, when they spend roughly half ters (within 200 nautical miles of the peter ryan (4) Broad-billed Prion. The centre bird also has a lation on Amsterdam) and both are sis- to be identified. But with the overlap in the amount of time in flight than when continent). Claiming one based on ob- top Distribution of Broad-billed Prions (from blackish bill, contrasting with the blue lower ter to the Broad-billed Prion, whereas bill width between the largest MacGilli- they are not moulting. Most individu- servations at sea will be very difficult, Gough and Tristan, blue) and MacGillivray’s mandible – a supposed character of Broad- Salvin’s Prion is closer to Antarctic Pri- vray’s and smallest Broad-billed Prions, als of both species moult in the south- because south of Africa you not only Prions (from Gough, red) during their breeding, billed Prion. It appears to have a paler head on P. desolata. Indeed, a recent paper some individuals will be impossible to west Atlantic, between 30 and 50 °W, have to make the case that the bill is moult and non-breeding periods. Dark shading than the bird on the left, but this could just be by Juan Masello and colleagues (2019, identify. although a few Broad-billed Prions too narrow for a Broad-billed Prion but shows core ranges; pale shading reflects the a result of different lighting. However, when it Molecular Biology and Evolution 36: A new study by Chris Jones and col- moult south of Africa. After moult- also too broad for a Salvin’s Prion! To total ranges. turns towards the camera (on the right), the bill 1671–1685) suggests that Salvin’s Prion leagues (2020, Marine Biology 167: 45) ing, they range widely throughout the confirm the species for the southern can be seen to be narrower and the bill sides evolved as a result of hybridisation be- helps birders by giving some idea of South Atlantic, but only Broad-billed African region we’ll either have to track above Although Broad-billed Prions are and tip are actually blue-grey. I think that this is tween Antarctic and Broad-billed pri- where they are most likely to encounter Prions enter African waters. However, more birds or confirm the identity of a supposed to have blackish bills, this pair a MacGillivray’s Prion, but there remains some ons. The Gough birds are now treated the two species. Chris’s team deployed one MacGillivray’s Prion ventured beached bird using genetic markers. breeding on Inaccessible Island both show uncertainty. as part of MacGillivray’s Prion, which miniature geolocator loggers attached south of Africa. PETER RYAN pale sides to the bill.

12 african birdlife may/june 2020 news & views 13