Arctic Terns Sterna paradisaea are an understudied species in Greenland. Outside of the largest colonies, little is known about their current population status and no long-term annual monitoring program has been established. Previous surveys outside of key breeding colonies have been limited to a small number of colonies for single-season studies. This limits our understanding of population dynamics as terns have been shown to have a high degree of variation in annual reproduction and readily move between nearby colonies. Here we present results of the first multi-year systematic survey for breeding Arctic Terns in the High Arctic of northwest Greenland. Surveys took place from 2009 to 2017 and identified eight islands where terns currently nest. A total of 426 nests were counted and annual counts ranged from 25 to 92 nests. Complete reproductive failure in at least one year was noted for seven of the eight colonies, and three colonies comprised nearly 90% of the total population. Mean clutch size ranged from 1.4 to 1.7 eggs. When compared with historical data from the late 1960s through late 1990s, the population has declined by over 50%, with the greatest decline occurring over the past two decades. Four previously identified colonies have been extirpated while three colonies have undergone significant reductions in size.

The Arctic Tern Sterna paradisaea breeds throughout coastal Greenland and nests mostly on islands (Salomonsen 1950). The current population in Greenland is estimated at approximately 65,000 pairs, with the largest colonies located in the Upernavik and Disko Bay regions, the latter having an estimated 20,000 pairs (Kitsissunnguit) (Boertmann et al. 1996; Egevang & Frederiksen 2011). Arctic Tern distribution elsewhere is sparse, with hundreds of kilometers often separating breeding areas, and colony sizes that generally range from single to hundreds of pairs (Salomonsen 1950; Boertmann 1994; Boertmann et al. 1996). Arctic Terns are currently listed as near threatened in Greenland (Boertmann 2007). While there is no annual monitoring program, limited data strongly suggest that the overall population has declined over the past 50–75 years (Salomonsen 1950; Egevang et al. 2004; Burnham et al. 2005; Boertmann 2007). However, concern exists that current survey methods are insufficient to provide an accurate appraisal of changes in population size. To our knowledge, only one multiyear census has been conducted for Arctic Terns in Greenland (Egevang & Frederiksen 2011).

Results from previous Arctic Tern nesting studies have highlighted the difficulties of making accurate assessments of colony size and population trends (Kress 1983; Hatch 2002; Ratcliffe 2004; Egevang & Frederiksen 2011). Significant annual variation in colony size and reproduction has been shown, including total abandonment of the colony and/or complete reproductive failure. Causes of large scale variation include: predation of eggs and/or chicks by mammals (Craik 1995; Levermann & Tøttrup 2007; Egevang & Frederiksen 2011), lack of prey (Monaghan et al. 1989; Suddaby & Ratcliffe 1997; Furness 2007), and predation by other bird species (Hatch 1970; Whittam & Leonard 2000). Furthermore, although nest site fidelity in Arctic Terns can be high at a regional scale, movement between neighbouring colonies can be common and often negatively correlated with geographic distance (Drury 1973; Brindley et al. 1999; Møller et al. 2006a; Devlin et al. 2008). To better overcome the above concerns, Egevang & Frederiksen (2011) recommend that Arctic Tern monitoring programs in Greenland should survey colonies in the same season within a large overall geographic area.

The Avanersuaq District in northwest Greenland (Figure 1) is home to the largest remaining seabird and waterfowl colonies in Greenland (Boertmann et al. 1996; Burnham et al. 2012) and a diverse assemblage of both Nearctic and Western Palearctic bird species (Salomonsen 1950; Boertmann 1994). The Avanersuaq District is also the northern edge of many of those species’ breeding range, and in some instances only a few pairs nest in the area each year (Boertmann et al. 1996; Burnham et al. 2012). Although Arctic Terns do breed in the area, information on colony location and density is limited and no systematic multi-year survey has been conducted there prior to this study.

The aim of our research was to: 1) conduct a multi-year survey identifying the location and size of Arctic Tern colonies in the southern portion of the Avanersuaq District, and 2) determine if Arctic Tern abundance has changed over the past 50 years.

Thank you to Calen Offield, Jack Stephens, Claire Behnke, Sara Baugh, Jeremy Hughes, Ryan Biester feld, Fallon Meyer, and Aaron Trost who helped conduct surveys. We thank the Greenland Home Rule Government for permits to work in Greenland and the Danish Liaison Office and the United States Air Force for access to Thule Air Base. Thanks to Polar Field Services, the 109th Air National Guard, the National Science Foundation, the Bureau of Land Management, and Greenland Contractors for their assistance with logistics. Financial support for this project was provided by the Offield Family Foundation, Wolf Creek Charitable Trust, Patagonia, University of North Texas, Augustana College, The Seabird Group, the Nisbet Award from the Waterbird Society, Kim Pelle, the Trout/Jarvis and Cafferty Families, and many others who have donated to the High Arctic Institute. Additional thanks to the residents of Thule Air Base for their longstanding support of all of our research projects in northwest Greenland.

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