NT
Sooty Shearwater Ardenna grisea



Justification

Justification of Red List Category
This species is classified as Near Threatened because it is thought to have undergone a moderately rapid decline owing to the impact of fisheries, the harvesting of its young and possibly climate change.

Population justification
The global population totals at 4.4 million pairs, roughly equating to 19.0–23.6 million individuals (Newman et al. 2009, Waugh et al. 2013).

Trend justification
There are persistent signs of a current decline in the global population (Brooke 2004). In North America and in New Zealand, the population trends were found to be decreasing (Butcher and Niven 2007, Clucas et al. 2008, Scott et al. 2008). Though the rate of decline of the whole population has not been quantified, moderately rapid population declines are suspected.

Distribution and population

Ardenna grisea is an abundant shearwater, breeding on islands off New Zealand, Australia and Chile, and the Falkland Islands (Malvinas). In Australia, there are colonies on 17 islands, all of which contain less than 1,000 pairs. In southern Chile, some colonies number up to 200,000 pairs, with the largest colony of up to 4 million individuals on Isla Guafo (Reyes-Arriagada et al. 2007). In the Falklands (Malvinas), 10,000-20,000 pairs have been recorded. New Zealand supports more than 180 colonies. The global population totals at 4.4 million pairs, roughly equating to 19.0–23.6 million individuals (Newman et al. 2009, Waugh et al. 2013). In 1970-1971, the colonies on the Snares Islands were estimated to support 2,750,000 breeding pairs (del Hoyo et al. 1992, Heather and Robertson 1997). Although this is an extremely numerous species, there are persistent signs of a current decline (Brooke 2004). In New Zealand, the number of burrows in the largest colony on the Snares Islands declined by 37% between 1969-1971 and 1996-2000, and burrow occupancy may also have declined, indicating that an overall population decline may have occurred (Warham and Wilson 1982, Scofield and Christie 2002, Scott et al. 2008). Some colonies on mainland New Zealand have declined, and several colonies on offshore islands, especially in northern New Zealand, have not responded to predator control (Gaze 2000, Jones 2000, Waugh et al. 2013). However, other colonies in southern New Zealand have responded to sustained pest management with increasing numbers (Newman et al. 2009).
The species migrates to the northern hemisphere during the austral winter (Shaffer et al. 2006, Hedd et al. 2012), and in the California Current, Sooty Shearwater numbers have fallen by 90% in the last 20 years (Veit et al. 1996). It remains uncertain whether this has resulted from population declines or distributional shifts (Spear and Ainley 1999).

Ecology

The species nests on islands and headlands in large colonies. Burrows are dug for breeding under tussock grass, low scrub and on the Snares Islands under Olearia forest. Birds typically do not return to their natal colonies until the age of four. The species feeds on fish, crustacea and cephalopods, which are caught while diving. Short (1-3 days) and long (5-15 days) provisioning trips are made by parents; longer trips allow foraging along the Antarctic Polar Front, reducing competition close to breeding grounds and allowing vast colonies to persist (Weimerskirch 1998).

Threats

The species is at risk from incidental capture in longline, trawl and gill-net fisheries and suffers the additional effects of depletion of prey stocks (Uhlmann 2003). A large number of deaths occur as a result of interaction with fisheries both during the breeding season and the winter migration to the Northern Hemisphere (Uhlmann 2003). The species is also subject to direct persecution, and harvesting of young birds ('muttonbirding') currently accounts for the take of around a quarter of a million birds annually (del Hoyo et al. 1992, Heather and Robertson 1997, Newman et al. 2008, 2009), but is unlikely to account for the scale of the observed decline.

Past investigation into the biological impact of climatic trends led to predictions of large-scale shifts in foraging distribution during the boreal summer and/or dramatic reductions in abundance and survival rate (Ainley et al. 1995, Veit et al. 1996, 1997, Spear and Ainley 1999, Wahl and Tweit 2000, Oedekoven et al. 2001, Hyrenbach and Veit 2003). Climate change is already affecting the foraging distribution of this species along the Californian coast (Veit et al. 1997). Evidence shows that birds are still visiting many of the same foraging areas, but are taking new routes to avoid altered current systems (Veit et al. 2003). Declines at monitored breeding sites appear to be linked to changed patterns in large scale oceanic cycles which reduce prey availability (Clucas 2011).

Both Brown Rat Rattus norvegicus and House Rat R. rattus are present within the species breeding range, and although egg and chick predation by rats has been demonstrated, the extent of impact is unknown (Jones et al. 2008).

Conservation actions

Conservation Actions Underway
The species is monitored at some sites and has been extensively studied in parts of its range. Some breeding grounds are protected and have benefited from the eradication of introduced predators.

Conservation Actions Proposed
Continue monitoring key colonies and migration bottlenecks. Research the key threats driving declines and assess appropriate responses. Buffer important colonies against invasive species invasions.

Acknowledgements

Text account compilers
Symes, A., Bird, J., Hermes, C., Martin, R., Benstead, P., Fjagesund, T., McClellan, R., Moreno, R., Stuart, A.

Contributors
Taylor, G.A.


Recommended citation
BirdLife International (2019) Species factsheet: Ardenna grisea. Downloaded from http://www.birdlife.org on 19/09/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/09/2019.