Pseudobulweria

ALL-DARK PSEUDOBULWERIA

On 21 January 2017, at c.15.10 h, we observed an all-dark Pseudobulweria resembling Fiji Petrel P. macgillivrayi, c. 1 km offshore at the northern end of Silur Bay, New Ireland (04°25’S, 153°06’E; Fig. 2). It had been a calm day until a light squall descended from the Hans Mayer Range giving a 10–15 knot wind from the south-east. Sauvage was travelling south following the coastline c. 1 km offshore. A fish-oil drip was running and we towed sardines from c.15.00 h. The bird was initially spotted in the wake by RLF. Flight was strong and purposeful as it progressed to within 150 m, before flying off in a more leisurely fashion to c. 0.5 km from shore, then heading back and crossing the wake c. 200 m away, before finally flying off north following the coast. The bird was in view for c.2 minutes and was also seen by ACW, KZ & M. Danzenbaker. Photographs were hurriedly taken as the bird crossed the wake, but the images are small and out of focus.

At first sight, head on, RLF thought that it was another Beck’s Petrel. However, its flight action was more hurried and purposeful than Beck’s Petrel, conveying the impression of a smaller bird. Overall dark plumage—eliminating Beck’s Petrel, unless melanistic—was first seen when the bird peeled off toward the coast. Compared to Beck’s Petrel, our bird was perhaps 10% smaller, with a more heavily built head and body, an apparently heavier bill, less pointed wings, and relatively short tail. The overall plumage was blackish, brownish black in the best light, with uniformly dark upperwings. A probable all-dark Pseudobulweria was briefly seen by RLF & ACW off Cape St. George, in southern New Ireland, at c.15.00 h on 22 January 2017, but it was further away than the first bird.

The main potential confusion genus for Pseudobulweria is Bulweria. The latter has long slim wings, elongated rear body, and all-dark plumage except for upperwing ulnar bars— typically strong in Bulwer’s Petrel B. bulwerii, dull in Jouanin’s Petrel B. fallax (Shirihai et al. 2009: 140). There are reports of Bulwer’s Petrel in the region, including one during our expedition, but the main potential confusion species is Jouanin’s Petrel, given its size and typically dull upperwing ulnar bars. Shirihai et al. (2009: 141) noted that experienced observers will differentiate Fiji Petrel (hence a Fiji-like Petrel) from Bulweria by their characteristic flight. Our experience includes>200 Jouanin’s Petrels off Oman (Flood 2016) and at least three different populations of Bulwer’s Petrel (e.g. Flood & Fisher 2011). Two of us have experience in distinguishing all-dark Mascarene Petrel P. aterrima from Bulwer’s Petrel (Flood et al. 2015).

The flight actions of our bird were more like a Pterodroma than a Bulweria. Flight was consistently strong and steady, with fairly stiff wingbeats, and the bird climbed to> 8 m on several occasions. Bulweria has especially low wing loading, consequently an effortless buoyant flight, gliding for long periods, meandering or with erratic changes of direction, has flexible wingbeats, and flies low over the water in relatively calm conditions such as on 21 January 2017. Given the same conditions, the flight action of Jouanin’s Petrel is slower than the smaller Bulwer’s Petrel, with floppier wingbeats (Flood 2016).

Shirihai (2008a: 13) reported three sightings of all-dark Pseudobulweria that resembled Fiji Petrel in the Bismarck Archipelago (Fig. 2). The main characteristics of our bird on 21 January 2017 match his birds. We are aware of two other sightings of all-dark Pseudobulweria in the same general area: 12 April 2007, at 01 o 42’S, 153 o 56’E, and 19 April 2008, at 05 o 38’S, 153 o 56’E (S. N. G. Howell, P. Fraser & I. Sinclair in lit. 2017).

It seems unlikely that the relatively tight cluster of all-dark Pseudobulweria sightings around New Britain and New Ireland simply reflects greater effort. Since 2006, several commercial and privately funded expeditions have sailed from New Caledonia / Fiji northward, without encountering all-dark Pseudobulweria in Vanuatu or the Solomons. In addition, evidence to date indicates that Fiji Petrel is very rare, with only eight observed during an intensive search off Gau, Fiji, where it probably breeds (Shirihai et al. 2009). Thus, if the birds are Fiji Petrels, then either the region is a key foraging zone for the Gau population (breeding c. 3,000 km away), or it holds a previously undiscovered population.

Alternatively, the Papua New Guinea all-dark Pseudobulweria may be an undescribed taxon. Indeed, following his work off Gau, Shirihai et al. (2009: 145) concluded that the Bismarck birds had a different shape and flight behaviour compared to Fiji Petrel.

Our sighting off Silur Bay, as with Beck’s Petrel (Bird et al. 2014), suggests that breeding might occur in the Hans Meyer Range. The possibility that all-dark Pseudobulweria breed in the region should be kept in mind when searching for the breeding grounds of Beck’s Petrel.