Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
Brooke (2004) estimated the global population to number > 23 million individuals, while national population estimates include: <1,000 individuals on migration in Taiwan; >1,000 individuals on migration in Japan and >1,000 individuals on migration in Russia (Brazil 2009).
Although the population trend is increasing in North America (based on BBS/CBC data: Butcher and Niven 2007), the global population is suspected to be in decline owing to ecosystem changes resulting from climate change (Brooke 2004).
This species breeds on Tasmania and off the coast of south Australia, with the bulk of the population in the south-east. It undergoes transequatorial migration, wintering north of Japan near the Aleutian Islands (U.S.A.), with some individuals moving north of the Bering Strait. The return migration route incorporates the central Pacific, with some individuals moving down the western coast of North America.
Breeding occurs mainly on coastal islands, typically in areas of grassland or other vegetation, but sometimes cliffs or bare ground (del Hoyo et al. 1992). Whilst breeding, the species alternates short foraging trips to local waters with long foraging trips (up to 17 days) to the Polar Frontal Zone. Short trips allow greater chick provisioning at the sacrifice of body condition, which is then recovered in richer sub-Antarctic waters. Diet includes fish (particularly mycotphids), crustaceans and squid (Weimerskirch and Cherel 1998). Feeding occurs in flocks of up to 20,000 birds, and it has been seen in association with cetaceans. It is a trans-equatorial migrant, wintering off Aleutian Islands, some moving north of Bering Strait (del Hoyo et al. 1992).
The species suffered substantial mortality from entanglement in gill nets set for salmon in the Northern Pacific, particularly off Japan with c. 40,000 birds captured per annum (DeGange and Day 1991, Uhlmann 2003). Between 1952 and 2001, bycatch by North Pacific driftnet fisheries accounted for the mortality of 4,600,000 – 21,200,000 Short-tailed Shearwaters (Uhlmann et al. 2005).
Light pollution represents a considerable threat to this species in parts of the range. A number much higher than anticipated, of (predominantly juvenile) shearwaters were found dead or injured as a result of being attracted to lights and grounded. Over a 15-year period of patrols on Phillip Island, Australia (1999-2013), 8,871 fledglings were found grounded, 39% of which were dead or dying. The incidence levels far exceed those of other Shearwater spp. (up to 8x), however less than 1% of fledglings produced annually are thought to be affected by mortality from attraction to artificial light (Rodriguez et al. 2014).Wreck (mass-mortality events) have been reported, e.g. at Bering Sea 1997 (Hyrenbach et al. 2001) and in Australia 2013 (Peter and Dooley 2014) and are potentially linked to ecosystem changes caused by climate change (Brooke 2004). Recent studies indicate that both young and adults suffer from the ingestion of marine plastic debris, with potentially lethal effects (Carey 2011). Competition for food by Pink Salmon Oncorhynchus gorbuscha may also be contributing to declines (Toge et al. 2011).
Text account compilers
Fjagesund, T., Butchart, S., Calvert, R., Hermes, C., Martin, R., Newton, P., Ekstrom, J., Stuart, A.
BirdLife International (2019) Species factsheet: Ardenna tenuirostris. Downloaded from http://www.birdlife.org on 15/12/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 15/12/2019.