Justification of Red List Category
This species is classified as Vulnerable owing to its very small breeding range, in which it is susceptible to human impacts and stochastic events. Confirmation that introduced predators are causing a decline would result in an uplisting to Critically Endangered.
In 1986 the population was estimated at 1,000,000 pairs, which extrapolates to a world population of at least 3,000,000 individuals. There is no firm evidence of decline.
Breeding sub-populations have remained unchanged between the 1980s (Brooke 1987) and 2009 (Hodum 2009).
Pterodroma externa breeds on Alejandro Selkirk Island in the Juan Fernández Islands, Chile. It is very numerous, but could be declining (Schlatter 1984), although there is no firm evidence of this (Brooke 2004). It is a transequatorial migrant, dispersing over the tropical and subtropical waters of the east Pacific, north to Hawaii (U.S.A.), and is regularly seen off the west coast of Mexico, with vagrants recorded to 42°N in the central Pacific (Howell 2012) and in New Zealand and east Australia (Carboneras 1992, Patterson 1996). Further information could indicate declines that would warrant uplisting.
The species is highly pelagic and rarely approaches land, except at breeding colonies. It nests in burrows on slopes in Dicksonia externa fern-forest, stands of low fern and adjacent grasslands and along open ridges at elevations of 600-1,150 m (J. C. Torres-Mura in litt. 1999, Hodum 2009, Reyes-Arriagada et al. 2012). One egg is laid in mid-December to early January (Hodum 2009) and chicks hatch in February-March (Hodum and Wainstein 2003). It is dependent on subsurface predators, especially Yellowfin Tuna Thunnus albacares, to drive prey to the surface (Au and Pitman 1986, Ballance et al. 1997).
Invasive mammals such as cats, rats, goats and dogs are thought to threaten this species. Feral cat Felis catus predation has been documented and is thought to be causing population decline, with a preliminary estimate of 2-3% annual adult mortality from cat predation (Hodum 2009). Additionally, feral cats prey on near-fledging chicks when they are on the surface exercising their flight muscles, although predation rates have not been quantified (Hodum 2009). Brown rats Rattus norvegicus also prey on chicks, though the incidence appears low and restricted to certain habitat types (J. C. Torres-Mura in litt. 1999, P. Hodum in litt. 2007). There is also limited evidence of predation on chicks by House Mice Mus musculus, but this does not appear to be widespread (Hodum 2009). Dogs also depredate the species. Goat impacts include habitat alteration through native plant consumption and also, at times, direct collapses of burrows (Hulm 1995, J. C. Torres-Mura in litt. 1999, P. Hodum in litt. 2007, J. C. Torres-Mura in litt. 2007, Hodum 2009). Despite attempts to reduce the goat population, it remains high (P. Hodum pers. obs.). Cows may also have a similar effect.
Flash flooding in 2002 caused severe localised habitat loss with an estimated 30,000 burrows destroyed (Hodum and Wainstein 2003). In 1995, a fire destroyed habitat at the edge of the principal colony and directly killed thousands of birds (J. C. Torres-Mura in litt. 1999, P. Hodum in litt. 2007).
Light pollution also represents a threat. On overcast nights, especially those with poor visibility due to rain and/or drizzle, lights in the village attract birds, occasionally causing collisions with structures in the village (Hodum 2009). The village is inhabited from September to May, during the period of lobster harvesting (P. Hodum in litt. 2007).The species is potentially threatened by climate change because it has a geographically bounded distribution: its altitudinal distribution, currently a range of approximately 500 m, falls entirely within 2,000 m of the highest mountain top within its range (1,649 m). Shifts in oceanic currents are ongoing, but their impacts are unclear for this species.
Conservation Actions Underway The Juan Fernández Islands were designated a National Park in 1935 (protected from 1967) and a Biosphere Reserve in 1977 (Stattersfield et al. 1998, J. C. Torres-Mura in litt. 1999). The Chilean government began a habitat restoration programme in 1997 (J. C. Torres-Mura in litt. 1999), and the islands have been nominated for World Heritage listing (Hulm 1995). Reserve rangers have been trained in fighting fires, but there is only one ranger on Alejandro Selkirk (J. C. Torres-Mura in litt. 1999). Sheep were removed from Alejandro Selkirk in 1983 (Carboneras 1992). A goat eradication programme was unsuccessful. Population monitoring plots were established between 2003-2006 by Oikonos Ecosystem Knowledge (Hodum 2009). Oikonos also maintains an ongoing small-scale community conservation education programme in the islands, including on Alejandro Selkirk. Incipient invasive plant species are also being systematically eliminated from the island by Oikonos in collaboration with the Corporación Nacional Forestal (CONAF), the Chilean agency that administers the park.
Conservation Actions Proposed Establish population monitoring plots (M. de L. Brooke in litt. 1999). Improve sustainable management of yellowfin tuna stocks. Eradicate introduced fauna (goats, feral cats, rats, house mice, etc) from Alejandro Selkirk. Eradicate invasive and incipient invasive plant species from the island. Monitor effects of flash floods on colonies. Improve community awareness of the status of the species and the need for eradication programs. Improve management of cattle herds to ensure that it is excluded from potential breeding habitat.
43 cm. Large, grey-and-white petrel. Black cap extends to below eyes while white of throat may extend up behind eyes, enhancing capped appearance. Grey upperparts and upperwing, with black M across wings. Base of grey tail can show whitish horseshoe. White underparts. White underwing with narrow black trailing edge, black tip, narrow black edge to leading edge distal to carpal joint and then short, bolder, black bar extending from joint towards centre of wing. Similar spp. Larger than overlapping Stejneger's Petrel P. longirostris, and has different underwing pattern. Hawaiian Petrel P. sandwichensis has darker cap and more extensive black on underwing.
Text account compilers
Temple, H., Clay, R.P., Calvert, R., Hermes, C., Fjagesund, T., Lascelles, B., Martin, R., Moreno, R., Stuart, A.
Torres-Mura, J., Morgan, K., Hodum, P., Brooke, M.
BirdLife International (2020) Species factsheet: Pterodroma externa. Downloaded from http://www.birdlife.org on 22/10/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 22/10/2020.