This species was only described in 2001 (Imber et al in Emu) and remains one of the most enigmatic seabirds on the planet, as it is extremely similar to White-necked Petrel. Whilst some authorities suggest it should be treated as a subspecies of White-necked Petrel (eg Birdlife International at the time of writing: August 2014), there are multiple other instances in the Pacific of similar looking seabirds, where size is the main difference, with Tahiti and Beck's Petrel probably being the most well-known example of this.
In April 2014, I was fortunate to work with Rodney Russ (Managing Director of Heritage Expeditions) on the Spirit of Enderby as we travelled from New Zealand to the Solomon Islands via the Kermadecs, Tonga, Fiji and Vanuatu and one of the targets of this expedition was to look for Vanuatu Petrel. Our itinerary included Vanua Lava (in the northern part of the Vanuatu archipelago) which is where this species is known to breed and we spent a couple of days in the vicinity of this rarely visited island.
We saw multiple birds and spent a very memorable evening in the zodiacs off the island, as the birds congregated before heading inshore to their burrows.
Vanuatu Petrel (above and below) - photographed offshore from Vanua Lava island in the northern part of the Vanuatu archipelago April 2014. Note the dirty belly on the bird above, presumably freshly out of its burrow !!!
Separating Vanuatu and White-necked Petrels
White-necked and Vanuatu Petrels are clearly very similar and identifying a vagrant Vanuatu Petrel would certainly be a challenge. Having now had an opportunity to watch and photograph both species, I believe there are a few pointers which may help in separating the two, although location is clearly helpful !!!
Jizz and flight
To my eye, Vanuatu Petrels have a more languid, floppy flight and at distance their flight somehow vaguely reminds me of large shearwaters such as Streaked Shearwater. This is probably due to the fact that there is a subtle difference in the jizz between White-necked and Vanuatu Petrels and the proportions are somewhat different (eg see the paper published by Shirihai and Bretagnolle in the BOC journal).
As I have been fortunate enough to have photographed both White-necked and Vanuatu Petrels off their breeding grounds (White-necked Petrel: Macauley Island; Vanuatu Petrel; Vanua Lava Island), I have been able to look at multiple images of both, confident that I was definitely looking at photos of correctly identified birds.
Although the extent of white on the underside of the primaries is often cited as 'the way' to tell these birds apart (albeit that even this doesn't seem to be 100% reliable), photos seem to suggest that there may be a few other subtle differences:
Above - Vanuatu Petrels off Vanua Lava - above
Above - underwings of Vanuatu Petrel (left) and White-necked Petrel (right) with the former photographed off Vanua Lava and the latter off Macauley Island. It is appears that the top of the carpal bar is closer to the middle of the underwing on a Vanuatu Petrel than on a White-necked Petrel, although whether this feature is consistent remains to be seen. As can be seen, the amount of white on the underwing primary shafts is pretty much identical.
Above - White-necked Petrels off Macauley Island - note the slight difference in the shape of the cap compared with Vanuatu Petrel with the white neck collar getting wider on the sides of the face.
Above - Vanuatu Petrel (left) and two White-necked Petrels (centre and right). The spurs do vary on both species although to some extent this undoubtedly depends on the light.