White-vented storm-petrel  (Oceanites gracilis)

Classified as Data Deficient (DD) on the IUCN Red List (1).

Despite being numerous on one of the most studied island groups in the world, only around a dozen nests of the white-vented storm-petrel have ever been discovered (2). It was described in 1859 as the most slender of storm petrels and accordingly given the specific name gracilis. It is predominately brown to dark grey in colour, tending to be darker on its upperparts and paler on its throat and chest. The square-ended tail is black, except for a white bar that merges with a white-tipped rump to form a conspicuous white crescent. It has brown eyes, a black bill and black feet, which extend well beyond the tail in flight. Two subspecies, distinct in range and morphology, are recognised, with Oceanites gracilis galapagoensis being slightly larger and having a whiter abdomen than O. g. gracilis (3).

Occurs along the cold water Humboldt Current off the west coast of South America (3). The only nests ever found are located on Isla Chungungo, Chile, but the white vented storm petrel is thought to also breed on small rocky islets from Chile north to the Galápagos. It is suspected that there must be a breeding population of several thousand O. g. galapagoensis on the Galápagos (2).

White-vented storm-petrels rarely venture more than 100 kilometres from the shore (3) and are most abundant in cool, upwelled waters (4).

Given that until 2003 only one white-vented storm-petrel nest had ever been found, it is not surprising that very little is known about the biology of this bird. Whereas the first nest discovered in 1979 comprised scraps of vegetation underneath low plants (3), the roughly 11 nests found most recently were located in rocky crevices (2). It is believed, on the basis of examinations of dead birds from the Galápagos, that eggs are laid during the Austral winter from April to August (3).

In common with the feeding behaviour of several other species of storm petrel, the white-vented storm petrel flutters over the sea surface, appearing to “walk-on-water”, in search of plankton and scraps of fish killed by larger predators. This unusual technique is thought to be the origin of the name petrel, derived from the biblical account of St Peter walking on water (3) (5).

With the location of the breeding sites of thousands, if not hundreds of thousands, of white-vented storm-petrel still unknown, it is difficult to accurately assess the threats to this species or indeed even what the true population numbers are (6). On the tiny Isla Chungungo (which is unlikely to support undiscovered nest sites), rats, fire, Humboldt penguins and short-tailed snakes have all been cited as being potentially responsible for the absence of a larger petrel population (2) (6).

The white-vented storm-petrel is classified, with good reason, as Data Deficient on the IUCN Red List (1). Until significant breeding sites are discovered, the species’ population size, trends and threats which together inform its conservation status, will remain unknown (6). Consequently, in addition to a continued census of Isla Chungungo and further evaluation of the threats there, the central aim of proposed conservation measures is to locate the breeding colonies which have eluded biologists thus far (2).

View information on this species at the UNEP World Conservation Monitoring Centre.

For further information on the white-vented storm-petrel see:

• BirdLife International: www.birdlife.org