Buller’s Shearwater, Puffinus bulleri

The Buller’s Shearwater (puffinus bulleri) is a Pacific species of seabird in the Procellariidae family; it is also known as the Grey-backed Shearwater or New Zealand Shearwater. It is a member of the black-billed wedge-tailed Thyellodroma group, among the bigger shearwaters of the proposed genus Ardenna; it creates a superspecies with the Wedge-tailed Shearwater.

The adult birds are 46 to 47 cm in length, with a 97 to 99 cm wingspan, and they have been recorded to weigh 342 to 435g. The upper side of Buller’s Shearwater is a blueish grey. A blackish stripe runs from the tertiary remiges to the primary wing coverts. The primary remiges are also blackish; the two black areas do not meet at the hand however; the area between them is a rather light gray, under a bright light it may appear almost white. With the bird facing upwards, the pattern gives the impression of a broken black “M”, with light gray interspersing areas.

The underside is a bright white; on the head the upper side’s grey extends down to eye height and the white cheeks may shine up conspicuously, just like the smaller shearwaters of Puffinus sensu stricto. The rectrices are blackish and the tail is wedge-shaped; the bill and iris are dark. Fledged juveniles already have an adult’s coloration; the nestlings are covered in grey down feathers.

Compared to other shearwaters, the species is unusually easy to identify at sea by its combination of considerable size and the distinctive M-shaped banding pattern on its upper side while flying, uniquely among its genus and more akin to some gadfly petrels (Pterodroma), the prions (Pachyptila) and their relative, the Blue Petrel (Halobaena caerulea). All of these are much smaller birds, perhaps two-thirds in length and wingspan and less than half in bulk of Buller’s Shearwater.

This species is pelagic just like the other Ardenna shearwaters; it’s a transequatorial migrant ranging across most of the Pacific Ocean outside of the breeding season. Though its seen in the sub arctic waters off Kamchatka and the Aleutian Islands; it’s not documented from the sub Antarctic Pacific; this apparent absence might simply be because the lack of study opportunities in the vast islandless region south of the Polynesian Triangle however. It’s fairly common well off the west coast of the United States during late summer and early autumn, and can generally be observed not far from land along the whole temperate and tropical coastlines of the Americas. It’s general absence from most of Melanesia and western Micronesia — where human settlement and sea traffic are considerable — is thus probably genuine; only isolated records, such as from the Marianas, Palau and Yap, exist from west and southwest of the Marshall Islands. A nomadic bird was also recorded in the Atlantic once, offshore New Jersey (USA).

The Buller’s Shearwater mainly feeds on fish, squid, and crustaceans such as the krill Nyctiphanes australis. It occasionally follows ships such as fishing trawlers, and may be part of a mixed species feeding flock. The food is caught mainly at a heads length below the surface at most, the bird either picking it up with the bill only, often out of flight, or briefly inserting its entire head, usually while swimming. It neither dives out of flight very often, nor in a plunge off the waters surface.

This bird is a colonial nester, breeding mostly on Tawhiti Rahi and Aorangi, the main islands of the Poor Knights group offshore northern New Zealand. This bird nests in burrows, crevices in rocks or under tree roots, preferring densely forested slopes. Buller’s Shearwater also can be found to breed in cracked-up rock on treeless stacks or cliffs however, and most of the other colonies — on the smaller Poor Knights islands between the main islands and off the southeast of Aorangi — are of such a nature. A pair was observed to breed on the Simmonds Islands in 1980, but this seems to have been an isolated incident.

Their breeding season starts in October and lasts for almost 6 months. A single egg is incubated for about 51 days, with the parents changing between incubation and feeding every 4 days or so. Time to fledging isn’t well known, but by analogy with Buller’s Shearwater’s relatives assumed to be around 100 days.

In the past, it was heavily used as a food source by the Maori, and on Aorangi it suffered massive predation by feral pigs. Its population had crashed to a low of just 100 to 200 pairs on Aorangi in the late 1930s. The pigs were removed from the island in 1936, and the shearwater population recovered, numbering 200,000 pairs again in the early 1980s to approach carrying capacity on the island around the end of the 20th century. However, at all times, the colonies at Tawhiti Rahi and on the smaller islets could supply birds for the resettlement of Aorangi, the Buller’s Shearwater was never consider threatened with extinction in the foreseeable future. Indeed, it’s a very abundant bird, with an estimated world population of 2.5 million birds. But as it’s not known to occur on any larger island in the region outside the Poor Knights Islands, it is classed as Vulnerable by the IUCN; a single localized disaster could wipe the species out.

Image Caption: Buller’s Shearwater (Puffinus bulleri). Credit: Aviceda/Wikipedia (CC BY 3.0)