Atlantic petrel  (Pterodroma incerta)

The Atlantic petrel is one of the largest gadfly petrels (Pterodroma spp.), recognised by its striking white breast and abdomen that contrast with the uniformly black-brown plumage of the rest of the body (4). Particularly sharp colour demarcation exists from the brown upper breast to the white lower breast and belly (2). Sexes are alike (3).

The Atlantic petrel is a seabird predominantly restricted to the South Atlantic, ranging from the east coast of South America to the west coast of Africa (2). Breeding occurs only on Tristan da Cunha and Gough Island. On Tristan, the best estimate of the population is 100 to 200 pairs in 1972-1974, whilst on Gough Island, it is estimated that around 1.8 million pairs may exist (2).

This bird nests in colonies on islands, but is otherwise pelagic (2). Nesting occurs in burrows dug in peaty soils in fern-bush vegetation from 50 to 300 m above sea level on Gough, and at higher elevations on Tristan (2).

Atlantic petrels are winter breeders, returning to their colonies from February to March in order to mate (3). Single eggs are laid from June to July, with young hatching in August and September, and fledging in December and January (3) (5). The birds are exclusively nocturnal whilst on land (5). Chicks grow at a very slow rate, suggesting poor at-sea feeding conditions (5). The age of first breeding, breeding frequency and longevity are not yet known for this species (2).

Squid forms the bulk of the diet, although some crustaceans and fish will also be taken (2), and the Atlantic petrel possesses physical and behavioural adaptations geared towards catching such prey. The species' towering flight style gives it a commanding field of view, useful in detecting widely dispersed prey (3). Furthermore, few other gadfly petrels in the region have such an elongated bill as this species, which is equipped with a strongly hooked tip, thought to be an adaptation to snatching live, slippery squid from the sea surface (3).

Although still common, the Atlantic petrel qualifies as Vulnerable because it has a very small breeding range, with the vast majority of the population on just one island (2). Such a limited range places the bird at risk from chance events such as natural disasters or the introduction of mammalian predators (2) (6). On Gough, the only introduced potential predator is the house mouse (Mus musculus) and, if this species' impact on offspring survival proves to be as severe as recent evidence suggests, the Atlantic petrel may need to be up-listed to Endangered (2). Indeed, mouse predation is thought to be responsible for a significant proportion of the 700,000 Atlantic petrel chicks (about 60% of the island's population) that are reported to have died before fledging in 2000 and 2001 (7). The large population of native Southern Skua (Catharacta antarctica) are also known to feed on seabirds including the Atlantic petrel (2). One reason for the bird's near extinction on Tristan Island, where it was once common, is that it was formerly one of the few sources of food for the islanders during winter (6). However, the bird is now scarce on the island and is very unlikely to be exploited today (2). On Tristan, rats are also thought likely to have affected breeding success (2).

A cat eradication programme on Tristan in the 1970s was highly successful, eliminating this potential predator and helping to protect the island's endangered native birds (2). Fortunately, Gough Island is well protected, with the island being a nature reserve and World Heritage Site, uninhabited apart from the staff that run a meteorological station (2). In 2000/1 the first ever quantitative population survey was undertaken on Gough, coupled with research into breeding ecology and reproductive output of this species (2). Further research on reproductive success and mouse predation was begun in 2003 (2). The Gough mice are known to be endangering a number of threatened bird species on the island, and further research into this issue is a high priority. If mice are shown to pose a significant threat, future conservation efforts may focus on their eradication from the island. Gough Island is widely recognised as having one of the most diverse bird colonies in the world, including four endangered species, and every effort continues to be made to protect it and the rich biodiversity it hosts (8).

For further information on the Atlantic petrel see:

BirdLife International:
http://www.birdlife.org/datazone/species

Cuthbert, R. (2004) Breeding biology of the Atlantic Petrel, Pterodroma incerta, and a population estimate of this and other burrowing petrels on Gough Island, South Atlantic Ocean. EMU, 104(3): 221 – 228.

Authenticated (15/05/07) by Ross Wanless and Andrea Angel, Percy Fitzpatrick Institute of African Ornithology. 
http://www.fitzpatrick.uct.ac.za