VU
Aleutian Tern Onychoprion aleuticus



Justification

Justification of Red List Category
This tern of the northern Pacific Ocean coasts has undergone very rapid population declines at its Alaskan breeding colonies. Trends in Russia are less clear, but it is likely that overall the species is undergoing rapid declines over three generations, and it has therefore been uplisted to Vulnerable. Precise drivers of declines are unclear but likely include habitat modification, predation, egg harvesting and human disturbance.

Population justification
Previous published population estimates for Alaska ranged from 9,000 to 12,000 birds, and estimates for Russia ranged from 7,200 to 13,000, although they were based on data that are more than 20 years old (H. Renner in litt. 2013). Wetlands International estimated a total population of 17,000-20,000 individuals, including 9,500 birds in Alaska (Wetlands International 2014), based on data from the U.S. Fish and Wildlife Service (2006), and the total population was therefore placed in the range 11,000-13,000 mature individuals. A subsequent range-wide assessment of population and trends estimated a minimum worldwide breeding population of 31,131 birds across 202 colonies, with 18% (5,529 birds in 110 colonies) in Alaska and 82% (25,602 birds in 92 colonies) in Russia (Renner et al. 2015). It does not account for colonies that have not been surveyed in recent years or for the fact that the surveys conducted were neither systematic nor inclusive of all potential habitats (Renner et al. 2015) and thus the total population may be higher. Based on the minimum total recorded by Renner et al. (2015), the global population estimate has therefore been revised to 31,000 mature individuals.

Trend justification

Numbers at known colonies in Alaska have declined 8.1% annually since 1960 or 92.9% over three generations (33 years; 95% CI = 83.3%–97%), with large colonies experiencing greater declines than small colonies . Trends at known colonies within discrete geographic regions of Alaska (Aleutian Islands, Bering Sea, Chukchi Sea, Gulf of Alaska and Kodiak Island) were consistently negativeIt is possible that observed trends in Alaska could be explained by the establishment of large, undiscovered colonies in new locations within Alaska, or major shifts between Alaska and Russia, but at present this seems unlikely. The geographic distance would be far greater than any typical breeding dispersal distance of other tern species, and there is no precedence for terns to move their established breeding sites in a biased direction from such a wide geographic scale. All the recently discovered colonies in Alaska are of small size, and unless there are some major colonies remaining to be discovered, it is unlikely to account for the observed declines (M. Rauzon / Pacific Seabird Group in litt. 2017). 

Quantitative trend information from the colonies in the Russian Far East remains lacking, but declines have been reported in several colonies in the Anadyr and Chukotka region (C. Zockler in litt. 2014). However, numbers in some regions in Russia appear to have increased substantially in recent decades, especially on Sakhalin Island and the southern coast of the Koryak Highland (Renner et al. 2015). 

It is difficult to determine the overall trend, but it is unlikely that the extremely rapid declines observed in Alaska have been outweighed by the trend in Russia and a rapid overall decline is therefore suspected to be taking place.

Distribution and population

This species breeds in the north Pacific Ocean on the coasts of Sakhalin and Kamchatka, Russia, on the Bering and Pacific coasts of Alaska (U.S.A.) and on the Aleutian Islands (U.S.A.). It is strongly migratory, and although the wintering range is poorly known, it is believed to lie off Indonesia and Malaysia.

Ecology

This species is found over the waters of the Arctic and subarctic coastal plains. It feeds mainly on small fish which it catches by surface-dipping. Laying mainly occurs in June, usually in small monospecific colonies on a variety of habitats up to 20 km inland (del Hoyo et al. 1996).

Threats

No single main driver of declines has been identified but several factors including habitat degradation, predation, egg harvesting and disturbance by humans likely play a substantial role in population change at local scales, and may have a cumulative impact at the population level. Eggs and chicks are reportedly depredated by introduced species such as Arctic Fox Vulpes lagopus and Red Fox V. vulpes, Brown Rats Rattus norvegicus, and domestic dogs. Natural predators include American Mink Mustela vison, bears Ursus spp. and a variety of bird species. Some chicks may also be killed by Arctic Terns Sterna paradisaea (U.S. Fish and Wildlife Service 2006), however breeding success in the species is lower in the absence of Arctic Terns as they benefit from their aggressive colony defence. Aleutian Terns are very sensitive to disturbance at colonies and may seasonally or even permanently abandon colonies in response to human disturbance (Gochfield et al. 2018). Complete colony abandonment has been observed following a single visit by humans (Haney et al. 1991).

Widespread declines across a coastline as large as Alaska’s suggest that declines are not strictly caused by local factors. Shifts in prey distribution due to climate change impacts may be having a particular impact on the species at a large scale. Within a mixed colony, provisioning rates of Arctic Terns were found to be higher than for Aleutian Terns (Gochfield et al. 2018), implying a divergence in preferred prey availability. The cause of this prey limitation may be highly relevant to the declines in the species: it may be that ongoing shifts in the marine habitat have particularly negative consequences for this species.

It is also possible that threats that are yet to be determined are acting on the species along its migratory route and in the wintering areas, which are particularly poorly known (M. Rauzon / Pacific Seabird Group in litt. 2017).

Conservation actions

Conservation and research actions underway
It has been designated as a species of concern by several agencies and NGOs, including ADF&G, Audubon Alaska, U.S. Fish and Wildlife Service (USFWS), and The North American Waterbird Conservation Plan, as well as a Forest Service Sensitive Species. An Aleutian Tern Working Group was set up in Alaska in 2007; it identified the need to develop an accurate population estimation method as the highest priority for managing the species, with the second highest priority to identify its migration pathway and timing. In 2010 work began to deploy geolocators on terns from colonies in Alaska (Oehlers et al. 2010).

Conservation and research actions needed
Conservation actions are limited in part because we lack data on basic breeding biology, ecological requirements, and causes of possible declines. Continue regular monitoring of all known colonies in Alaska and expand monitoring work in Russia if possible. Continue work to establish the location of the main wintering grounds. Carry out urgent research to clarify threats and main drivers of recent declines. Establish protection at key colonies to reduce disturbance and egg-harvesting.

Acknowledgements

Text account compilers
Calvert, R., Ekstrom, J., Ashpole, J, Butchart, S., Martin, R., Stuart, A., Symes, A.

Contributors
Zockler, C., Yu, Y., Renner, H., Tirtaningtyas , F., Rauzon, M., Andres, B.A.


Recommended citation
BirdLife International (2019) Species factsheet: Onychoprion aleuticus. Downloaded from http://www.birdlife.org on 21/07/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 21/07/2019.