Justification of Red List Category
This species has been listed as Endangered because it has a very small occupied breeding range, and there is now evidence that chick predation by introduced mice is causing very low breeding success and is likely to be causing the population to decline. It has not been recorded from Tristan de Cunha for 35 years, and, were it to be confirmed as locally extinct there, it may qualify for uplisting to Critically Endangered.
On Gough Island, Cuthbert (2004) estimated 1.8 million breeding pairs in 2001, suggesting a world population of around 5 million birds. The population size was re-estimated by Rexer-Huber et al. (2014), taking into account the occupancy rates of suitable burrows in 2010 and 2012. This new estimate indicates a population size of 860,000 pairs, varying between 630,000 and 1,100,000 pairs. Because occupancy rates varied interannually, the species’s total breeding area could only be estimated crudely, and burrow densities were assumed to be similar throughout the habitat. This estimate should, therefore, be treated with caution.
Stochastic population models computing a breeding success of 24.7% and parameters estimated for other gadfly petrels (adult survival = 93%, immature survival = 79% and recruitment age = 6 years) suggest a population decline of 0.7% per annum (Wanless et al. 2012). This would equate to a decline of 28% over three generations (c. 47 years). However, given that breeding success seems to be less than 20% (Cuthbert and Hilton 2004, Wanless et al. 2012, Dilley et al. 2015) and that survival rates did not take into account predation by Subantarctic Skuas Catharacta antarctica and thus were probably over-estimated, the population might be decreasing at a faster rate. Therefore, declines are placed here in the range 30-49% over three generations.
Pterodroma incerta breeds only on Gough Island, Tristan da Cunha (St Helena to UK). It is absent from Nightingale Island where there is no suitable habitat, and probably also from Inaccessible Island, although it is possible that a small number of birds could breed there (P. G. Ryan in litt. 1999). The species's occurrence on Tristan da Cunha is unclear; in 1972-1974 the population there was estimated to be 100-200 pairs, and there may only be a few scattered pairs remaining. On Gough Island, the first quantitative population estimate indicates a total of around 1.8 million pairs (Cuthbert 2004), which is considerably larger than the earlier estimate of at least 20,000 pairs (P. G. Ryan in litt. 1999). However, a recent estimate taking into account the occupancy rates of burrows which are suitable for the species halves this number and suggests a population size of 860,000 pairs, varying between 630,000 and 1,100,000 pairs (Rexer-Huber et al. 2014), and several studies revealed very low breeding success of less than 20% (Cuthbert and Hilton 2004, Wanless et al. 2007, Dilley et al. 2015). At sea, it is practically restricted to the South Atlantic, occurring off the east coast of South America to the west coast of Africa (Enticott 1991, P. G. Ryan in litt. 1999), occasionally rounding the Cape of Good Hope into the Indian Ocean (Hobbs in litt. 2009).
This species feeds mainly on squid with some fish and crustaceans (Klages and Cooper 1997). It nests in burrows dug in peaty soils in fern-bush vegetation from 50-300 m on Gough Island and formerly, at higher elevations on Tristan da Cunha. Nothing is known of age of first breeding, breeding frequency or survival (P. G. Ryan in litt. 1999). Heavily affected by Hurricane Catarina in March 2004, 354 birds were recorded blown inland, all in heavy moult and near starvation (Bugoni et al. 2007). Larger numbers of females among these were thought to be due to difference in at-sea distribution or in body condition (Bugoni et al. 2007). Evidence suggests moult is completed during the exodus period and before egg-laying, which occurs from 15 June to 21 July (Cuthbert 2004).
This species is primarily threatened by a reduction in reproductive success due to depredation of nests by invasive House Mice. On Gough Island, where 90% of the global population is found, chick failure was 87% in 2014-15, with chicks attacked by mice within 2 hours of hatching and dead within 3-7 hours (Dilley et al. 2015). On Tristan da Cunha, rats and mice are present and have likely contributed to the extirpation of the Atlantic Petrel from the island (Richardson 1984, Wanless et al. 2012), with human exploitation representing another potential driver of decline on this island (Richardson 1984). It is very unlikely to be exploited today (P. G. Ryan in litt. 1999, A. Bond in litt. 2016). The large population of native Southern Skua Stercorarius antarcticus feed on seabirds, including P. incerta (Richardson 1984); however, this is unlikely to be significant except in the situation that the population becomes greatly depleted.Significant mortality of moulting adults, largely females, due to Hurricane Catarina indicates that increased storm intensity and frequency is likely to cause unpredictable adult mortality. Night strikes, as a result of being attracted to lights, are a further threat. This has been ameliorated at the Gough Island meteorological station, but may still pose a problem on ships at sea (Glass and Ryan 2013). It is, however, not thought to be causing significant declines in the population.
Conservation Actions Underway
On Tristan da Cunha, a programme to eradicate cats was successful in the 1970s. However, searches on Tristan da Cunha have failed to locate any birds (Wanless et al. 2007). Gough Island is a nature reserve and World Heritage Site (Cooper and Ryan 1994). The first ever quantitative population survey was undertaken on Gough Island in 2000/2001, together with research into the species's breeding ecology and reproductive output (Cuthbert 2004, Cuthbert and Hilton 2004). Between 2003 and 2007, and again between 2010 and 2014, further research on reproductive output and mouse predation was conducted (Wanless et al. 2007, Cuthbert et al. 2013, Rexer-Huber et al. 2014, Dilley et al. 2015). Burrow density and occupancy as well as breeding success are monitored annually on Gough Island (A. Bond in litt.)
Conservation Actions Proposed
Eradicate mice from Gough Island. Minimise the risk of further introduced species establishing on Gough Island, particularly any rat Rattus species (P. G. Ryan in litt. 1999). Confirm the status of the population on Tristan da Cunha and assess whether birds breed on Inaccessible Island. Measure demographic rates on Gough Island to better understand population trends.
43 cm. Large, stocky, dark brown-and-white petrel. Uniformly dark chocolate-brown above and on upper breast. Head can appear grey in worn plumage. Sharp demarcation from brown upper breast to white lower breast and belly. Brown vent, undertail-coverts and tail. Uniform brown underwing. Similar spp. Soft-plumaged Petrel P. mollis has patterned underwing. Trindade Petrel P. arminjoniana has white wing flashes. Hints May flap at top of glides unlike most Pterodroma spp. Concentrates around subtropical convergence.
Text account compilers
Stattersfield, A., Stuart, A., Fjagesund, T., Temple, H., Hermes, C., Clay, R.P., Martin, R., McClellan, R., Ekstrom, J., Moreno, R., Shutes, S.
Bourgeois, K., Wanless, R., Bond, A., Ryan, P.G., Bugoni, L., Hilton, G., Cuthbert, R.
BirdLife International (2019) Species factsheet: Pterodroma incerta. Downloaded from http://www.birdlife.org on 23/07/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 23/07/2019.