This discussion was first published as part of the 2014 Red List update, but remains open for comment to enable reassessment in 2015.
Current species factsheet for Aleutian Tern: http://www.birdlife.org/datazone/speciesfactsheet.php?id=3285
Aleutian Tern Onychoprion aleuticus is currently listed as Least Concern on the basis that it was not thought to approach the thresholds for Vulnerable under any of the IUCN Red List criteria.
It breeds entirely in the north Pacific Ocean on the coasts of Sakhalin and Kamchatka, Russia, on the Bering and Pacific coasts of Alaska (USA) and on the Aleutian Islands (USA). It is strongly migratory, and although the wintering range is poorly known it is believed to lie off Indonesia and Malaysia.
Published population estimates for Alaska range from 9,000 to 12,000 birds, and estimates for Russia range from 7,200 to 13,000, although they are based on data that are more than 20 years old (H. Renner in litt. 2013). Wetlands International estimate 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).
A 2013 status assessment by the U.S. Fish and Wildlife Service compiled new data on Alaskan colonies (including surveying more than half of the known active colonies in 2013) and suggested that the population at surveyed colonies had declined by 79% since 1995, with perhaps fewer than 5,000 individuals in Alaska as a whole. This number is not 79% lower than the published population estimate owing to the discovery of previously undocumented colonies. However, colonies with data points both before and after 1995 have declined 79%. In addition, few new colonies (3) have been discovered since 1995, while 56% of colonies active prior to 1995 (24) have been reduced to zero (H. Renner in litt. 2013).
Surveys on the Kodiak Archipelago in 2008-2010 visited 86% of known colonies (324 out of 377) and recorded only 326 Aleutian Terns at 11 colonies (Corcoran 2012). Four of the five largest colonies that supported from 240-3,000 individuals in the 1970s now are used intermittently and at reduced numbers with counts ranging from 22-120 terns. Although the species is difficult to monitor since colonies often shift from year to year, given the few sightings in the region in the past decade it is reasonable to conclude that this species has declined rather than relocated within the Kodiak Archipelago (Corcoran 2012).
Data available for Aleutian Tern colony sizes are not optimal for a variety of reasons, including variable survey methodology as well as extreme variation in attendance, common breeding failure, and occasional colony movement. Nonetheless, there appears to be sufficient evidence of a dramatic decline in Alaska.
Data on the Russian breeding population are currently missing from this picture. Personal communications with Russian ornithologists indicate the species is stable in Russia (H. Renner in litt. 2013), but no published data are available.
No single main driver of declines has been identified but several factors including habitat modification, 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 preyed on by introduced species such as arctic (Alopex lagopus) and red (Vulpes vulpes) foxes, Norway rats (Rattus norvegicus), and domestic dogs. Natural predators include mink (Mustela vison), bears (Ursus spp.), and a wide variety of other bird species. Some chicks may also be killed by Arctic Terns (U.S. Fish and Wildlife Service 2006). There is limited information regarding response to predation, but Aleutian Terns are not as aggressive as Arctic Terns and are very sensitive to disturbance at colonies. Individuals frequently hover high over the colony if disturbed by humans. They will dive at avian predators, but often rely on the more aggressive Arctic Terns to chase intruders away.
If evidence points towards a decline approaching 30% (typically 25-29%) over three generations, estimated by BirdLife to be c.33 years, either in the past, in the future, or over a period including both the past and the future, then the species may be eligible for uplisting to Near Threatened under the A criterion. A decline of 30-49% over 33 years would qualify the species for uplisting to Vulnerable, and a decline of 50-79% over the past 33 years, predicted to occur in the next 33 years, or over a specified period of time including both the past and the future, would warrant uplisting to Endangered.
Data from the Russian population and comments on the likely global population trend are requested.
Corcoran, R.M. 2012. Aleutian tern counts from seabird colony and nearshore marine bird surveys in the Kodiak Archipelago, Alaska 1975-2012 Unpubl. Refuge Report 01-12. U.S. Fish and Wildlife Service, Kodiak National Wildlife Refuge, Kodiak, Alaska
U.S. Fish and Wildlife Service 2006. Alaska Seabird Information Series: Aleutian Tern Onychoprion aleutica
Wetlands International (2014). “Waterbird Population Estimates” . Retrieved from wpe.wetlands.org on Friday 14 Mar 2014