Arctic tern (Sterna paradisaea)

Figure 1. Trends in abundance index of Arctic tern 1986–2019 for the UK (red), Scotland (blue), and England (black). Further details, including 95% confidence limits, can be found in the sections below. Based on SMP data; view the methods of analysis.

There has been regional variation in Arctic tern abundance trends since monitoring began in 1986 (Figure 1). The UK trend has fluctuated along the baseline for much of the monitoring period. In 1998 the Scottish abundance index was 230% above the baseline but has decreased considerably since then and, in 2019, was 57% below. In England, the trend increased gradually from 1999 to 2007 and, since then, has fluctuated along the baseline. The number of Arctic tern colonies monitored frequently are only sufficient to allow meaningful abundance trends to be produced for the UK, Scotland and England.

Productivity

Figure 2. Trends in productivity of Arctic tern 1986–2019 for the UK (red), Scotland (blue), and England (black). Further details can be found in the sections below. Based on SMP data; view the methods of analysis.

Artic tern productivity trends for the UK, Scotland and England all fluctuate considerably throughout the monitoring period. Average productivity in Scotland was 0.31 chicks fledged per pair between 1986 and 2019, the lowest of any seabird species breeding in the UK, whereas in England the average was 0.67. The number of Arctic tern colonies monitored frequently is only sufficient to allow meaningful productivity trends to be produced for the UK, Scotland and England.

Figure 3. Trend in UK abundance index (solid line) of Arctic tern 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

There is uncertainty, due to questions of compatibility of methods between censuses, of the magnitude of changes in UK Arctic tern population size between 1969-70 and 1985-88, although numbers probably increased over this period.

The UK Arctic tern abundance index showed a rapid increase, followed by a decrease, from 1986 to 1990 (influenced by changes at a few large colonies in the Northern Isles of Scotland during those years). Between 1990 and 2009, the index fluctuated between 100 and 130, excluding 2002–2004 when a dip was noticeable. Abundance fell steeply between 2009 and 2011 but rose again to its third highest value since monitoring began in 2014, at 47% above the baseline. This was probably due to an increase in numbers at colonies in England and Wales. Since then, it has fluctuated considerably and in 2019 was 1% above the 1986 baseline.

Declining abundance between 1988 and 2004 was in part caused by poor breeding seasons in the Northern Isles, which were attributed to sandeel Ammodytes spp. shortages linked to oceanographic changes (see below). Declines in the Arctic tern population have also been caused (in western Scotland) by American mink Neovison vison predation, a non-native invasive species. A successful mink eradication programme on the Western Isles and other control measures¹ are contributing towards conservation initiatives aimed at increasing the population.

Productivity

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of Arctic tern 1986–2019. Based on SMP data; view the methods of analysis.

Productivity of Arctic terns is consistently the lowest of any of the seabirds breeding in the UK. Since 1986, annual average productivity has only risen above 0.40 chicks per pair once, in 2000, and in most years lies below 0.30. Especially unproductive years occurred from 1988 to 1990. In 2004 and during four of the last five breeding seasons productivity was less than 0.12 chicks fledged per pair.

Sandeels are particularly important for terns in the Northern Isles, where most UK Arctic terns breed, and due to low availability of other small energy-rich fish species in the area² are the main prey fed to their chicks^3,4,5. Inter-annual variation in Arctic tern productivity in Shetland can be largely explained by variation in sandeel biomass⁶, and a low availability of 0-group sandeels during chick-rearing can result in total breeding failure⁷. However, sandeels are also crucial for adult Arctic terns in Shetland: a decline in the sandeel portion of adult diet from 100% to 20% between the early and late 1980s coincided with the number of breeding pairs declining by 70%⁸. Sandeel shortages in Shetland have probably been caused by oceanographic changes that affected larval sandeel transport and recruitment from spawning stock in Orkney^9,10. However, from 2015 to 2017, sandeel prey appeared to be plentiful and unseasonable weather conditions, strong tides and predation were the more likely causes of fewer chicks fledging than in 2014^11,12,13.

Figure 5. Trend in abundance index (solid line) of Arctic tern in Scotland, 1986–2019 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). Based on SMP data; view the methods of analysis.

The rapid increase and subsequent decline in Scotland’s Arctic tern population between 1986 and 1990 was in part caused by a period of poor breeding seasons in the Northern Isles, which were driven by sandeel shortages linked to oceanographic changes¹⁴. Since then, the index has declined considerably and, in 2019 was 57% below the 1986 baseline.

Declines are likely to have been caused by a combination of factors. In the Northern Isles, very low productivity (below 0.30) is evident in 30 years between 1986 and 2019, and linked to sandeel shortages driven by oceanographic changes^3,14. In western Scotland, declines have been caused by American mink taking adults, eggs and chicks from near-shore nesting islands¹, although successful mink eradication programmes there have led to increases at many sites^1,15. Scotland's Arctic tern population appears to not have changed significantly since Operation Seafarer, although there is some uncertainty over this due to questions concerning the compatibility of methods between the national censuses.

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of Arctic tern in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.

Arctic tern productivity in Scotland is consistently lower than common tern, and indeed the lowest of any seabird species breeding in Scotland. Annual average productivity has risen above 0.40 chicks per pair only twice in 30 years. Very low productivity (at or below 0.20) is evident in 19 out of 34 years.

Low productivity of Arctic terns in Scotland is associated with food shortages (especially sandeels around the Northern Isles, where most Arctic terns breed), exacerbated by poor weather and increased predation by gulls seeking alternative food sources. Sandeel shortages in Shetland have probably been caused by oceanographic changes that affected larval sandeel transport and recruitment from spawning stock in Orkney^9,10,14. At colonies in Lochaber, and in Argyll and Bute, detailed studies into the effects of predation by American mink revealed that colonies where mink were controlled fledged an average of 0.65 chicks per nesting pair compared to 0.50 at colonies with no, or unsuccessful, mink control¹⁵.

Figure 7. Trend in abundance index (solid line) of Arctic tern in England, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

Arctic tern numbers in England appear to have been stable between the first two censuses (1969-70 and 1985-88), however, this should be viewed with caution as there is uncertainty regarding how compatible the different methods used in each census were. Between the SCR census and Seabird 2000, numbers declined by 21%.

However, since 2000 (when all Arctic tern colonies were surveyed for Seabird 2000), numbers appear to have increased. With the addition of missing data from Long Nanny, Figure 7 now shows the abundance trend since 1986 (previous reports only showed numbers at five main colonies). Abundance declined from 1986 to 1991 before recovering in 1992 then falling again until 1999. Since then, numbers have increased and the baseline has been crossed in recent years, reaching a high point in 2014. Since then the index has fluctuated considerably and, in 2019, was 8% below the 1986 baseline. Three colonies monitored in 2019 (Farne Islands, Coquet, and Long Nanny) held 4,038 AON, which was 12% more than recorded for all colonies surveyed in England (c. 16 are known) during Seabird 2000.

Productivity

Figure 8. Trend in productivity (no. of chicks fledged per pair) of Arctic tern in England, 1986–2019. Based on SMP data; view the methods of analysis.

Productivity of Arctic terns in England has fluctuated considerably over the 33 year monitoring period. However, it has not been as consistently low as that recorded at Scottish colonies. Until recently, the lowest recorded productivity was in 2004 (0.18 chicks fledged per pair), however, in 2016 it was just 0.02, the lowest year since records began. While productivity has remained low at most reported colonies, Coquet Island achieved reasonably high productivity in 2018 and 2019, with 1.13 and 1.09 chicks fledged per pair respectively.

Figure 9. Abundance of Arctic tern on The Skerries, Isle of Anglesey, 1986–2019.

There are only a few Arctic terns colonies in Wales, all situated on the island of Anglesey. National census data show numbers increased from 436 AON in 1969-70 to 1,705 AON in 1998–2002. This increase has continued; for example, in 2008, over 2,800 AON were found at four colonies and in 2019 there were 3,206 in four colonies, so the Welsh population has almost doubled over the last 19 years. By far the largest colony, holding at least 95% of the Welsh population, is on The Skerries which held over 3,500 AON each year between 2013 and 2015. Numbers dropped to 2,770 AON in 2017 and 2,814 AON were recorded in 2019 (Figure 9).

Productivity

Figure 10. Abundance of Arctic tern at Strangford and Carlingford Loughs, and Cockle Island, 1986–2019.

In Northern Ireland, Arctic terns breed in just a few colonies, including the Copeland Islands, Strangford Lough, Belfast Harbour, Bird Island, Green Island, and Cockle Island. During Operation Seafarer, 121 Arctic tern AON were recorded in Northern Ireland. Numbers then increased in each subsequent census, with Seabird 2000 recording 767 AON.

Counts from three regularly monitored colonies, Strangford and Carlingford Loughs, and Cockle Island, show how numbers there have changed over time, reaching a peak in 2006 (926 AON) but declining steeply afterward with just 178 AON present at the three sites in 2012 (Figure 10). Since then, numbers have fluctuated but are currently increasing again, with 604 AON recorded in 2018. The Copeland Islands, the largest colony of Arctic terns in Northern Ireland, held between 800 and 1,000 AON each year from 2003 to 2012, with a new peak of 1,250 AON being recorded in 2013. No full survey has taken place on all three islands of the Copeland Islands in the past five years, however, Arctic terns were counted on Lighthouse Island in 2019, when 150 AON were recorded. In 2019, numbers still compared favourably against the total figure from the last census, with 751 AON recorded at seven of eight colonies, especially as the largest, Big Copeland Island, is not included in this total.

Productivity

Arctic terns in Northern Ireland fledged an average of 0.30 chicks per pair per year between 1991 and 2019. Productivity has been extremely low in some years, especially between 2007 and 2011 when the main colonies on the Copeland Islands failed due to a combination of adverse weather and predation by gulls and otters Lutra lutra. Complete breeding failure was recorded in 2019, due to high levels of predation from Jackdaws Corvus monedula¹⁶.

Figure 11. Abundance of Arctic tern at Rockabill and Lady's Island Lake, 1986–2019.

In the Republic of Ireland, Arctic tern numbers increased between every national census, with Seabird 2000 recording a population three times higher than Operation Seafarer. The Rockabill and Lady's Island Lake (Figure 11) colonies increased to a combined peak of 1,102 AON in 2010 but, by 2014, numbers had fallen to 808 AON. The majority of these (787 AON) were at Lady's Island Lake with the colony on Rockabill decreasing considerably since 2009. In 2019, just 19 AON were recorded on Rockabill and 518 AON at Lady’s Island Lake^17,18. A recent seabird census of the Republic of Ireland recorded 2,778 Arctic tern AON, a slight increase since Seabird 2000¹⁹.

Productivity

Arctic terns in the Republic of Ireland on average fledged approximately 0.28 chicks per pair per year between 1991 and 2013; there was no statistically significant variation over time. No productivity data have been submitted to the SMP since 2014.