Arctic Tern Sterna paradisaea

Breeding abundance

Figure 1: Trend in UK abundance index (solid line) of Arctic tern 1986-2010 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

There is uncertainty, due to questions of compatibility of methods between censuses, of the magnitude of changes in Arctic tern population size between 1969/70 and 1985-88, though it probably increased over this period. The Arctic tern abundance index based on the SMP sample showed an apparent rapid increase, followed by decrease, during 1986 to1990 (influenced by changes at a few large colonies in the Northern Isles during those years). Thereafter, the index declined  to a low in 2004, after which increases occurred. Declining abundance up to and including 2004 was in part caused by period poor breeding seasons in the Northern Isles, in turn attributable to sandeel shortages linked to oceanographic changes (see below). Declines in the Arctic tern population has also been caused (in western Scotland) by predation by American mink, a non-native invasive species. A successful mink eradication programme on the Western Isles and other control¹ is contributing towards population increases seen in  recent years.

Productivity

Figure 2: Trend in UK productivity (no. of chicks fledged per pair) of Arctic tern 1986-2010. Based on SMP data; view the methods of analysis (PDF 158 kb).

The productivity of Arctic terns is consistently amongst the lowest of any of the seabirds breeding in the UK. Since 1986, the annual average productivity has only risen above 0.40 chicks per pair once, in 2000, and in most years it fell below 0.30. Very unproductive years were recorded from 1988 to 1990 and in four of the last seven seasons, associated with marked shortages in prey (especially sandeels around the Northern Isles, where most Arctic terns breed) the effects of which may be exacerbated by poor weather hampering foraging and chilling eggs and chicks, together with 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 ^2,3.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

Figure 1: Trend in abundance index (solid line) of Arctic tern in Scotland, 1986-2010 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

The abundance of Arctic terns in Scotland appears to show a rapid increase between 1986 and 1988. Thereafter, the index declined to a low in 2004, after which increases occurred. This decline was in part caused by a period of poor breeding seasons in the Northern Isles, which were driven by sandeel shortages linked to oceanographic changes. In western Scotland, declines have been caused by American mink taking adults, eggs and chicks from near-shore nesting islands. Successful mink eradication programmes on the west coast are contributing towards population increases seen recently. Over the longer term, Scotland's Arctic tern population may not have changed much since Operation Seafarer although there is some uncertainty over this due to questions of compatibility of census methods between the national surveys.

Productivity

Figure 2: Trend in productivity (no. of chicks fledged per pair) of Arctic tern in Scotland, 1986-2010. Based on SMP data; view the methods of analysis (PDF 158 kb).

Compared with their conspecific, the common tern, the productivity of Arctic terns is consistently lower and indeed amongst the lowest of any seabird breeding in Scotland. Annual average productivity has risen above 0.40 chicks per pair only twice in 25 years, and in most years it is lower than 0.30. Very low productivity was recorded between 1988 and 1990, 1998, 2001, 2004 and 2005, associated with marked 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 ^2,3.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

Figure 1: Trend in abundance index (solid line) of Arctic tern in England, 1986-2010 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

There is uncertainty, due to questions of compatibility of methods between censuses, of the magnitude of changes in Arctic tern population size between 1969/70 and 1985-88. In England, numbers appear to have been stable but had declined by one-fifth Seabird 2000. The abundance index based on the SMP sample broadly agrees with this but suggests an increase since then, to 2006 at least after which numbers have fallen slightly again although confidence intervals are very wide.

Productivity

Figure 2: Trend in productivity (no. of chicks fledged per pair) of Arctic tern in England, 1986-2010. Based on SMP data; view the methods of analysis (PDF 158 kb).

The productivity of Arctic terns in England has fluctuated widely over the 25 years it has been recorded by the SMP. However, it has not been as consistently low as that recorded at Scottish colonies and, despite several poor years, 2004 is the only year in common between the two countries when productivity was very low.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

There are only a few breeding sites for Arctic terns in Wales, all situated on the island of Angelsey. Numbers have increased during each national census, from 436 pairs in 1969/70 to 1,705 pairs in 1998-2002. This increase has continued and in 2010 over 3,500 pairs were found in two colonies so the national population will have more than doubled over the last decade. By far the largest colony is on The Skerries, which held 2,912 pairs in 2010.

Productivity

Figure 2: Trend in productivity (no. of chicks fledged per pair) of Arctic tern in Wales, 1986-2010. Based on SMP data; view the methods of analysis (PDF 158 kb).

In Wales, the trend in productivity of Arctic terns was upward during the period from the early to late 1990s after which there was a few years of relatively consistent productivity. However, since 2003 productivity has fluctuated widely possibly due to food shortage in some years.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

Figure 1: Abundance of Arctic tern at Strangford and Carlingford Loughs, 1986-2010.

During Operation Seafarer, only a few hundred Arctic terns nested in Northern Ireland. Numbers increased in each subsequent national census with Seabird 2000 recording 767 pairs. The national population has increased since then as counts from two of the more regularly monitored colonies, Strangford and Carlingford Loughs, show (Figure 1). In addition the Copeland Islands, the main concentration of nesting Arctic terns, have held between 800-1,000 pairs in each year since 2003. So in 2006 numbers approached 2,000 pairs nationally (there are few other colonies) and even in 2010, when counts were lower at Strangford and Carlingford, more than 1,400 pairs nested at six colonies.

Productivity

Few systematic data on the productivity of Arctic terns at colonies in Northern Ireland has been collected as part of the SMP.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

In the Republic of Ireland, Arctic tern numbers also increased in each subsequent national census. Counts during Seabird 2000 were three times higher than that recorded by Operation Seafarer. Few Arctic tern colonies are frequently monitored, so the current status of the population is unknown. Counts at Rockabill and Lady's Island Lake have been increasing over the last decade but these colonies may not be representative of the national trend.

Productivity

Relatively few data are available on productivity but Arctic terns in the Republic of Ireland on average fledged approximately 0.41 chicks per pair per year; there was no statistically significant variation over time.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

Seabird 2000 found that Arctic terns numbers throughout Ireland had increased by 261% since Operation Seafarer. In the decade since Seabird 2000, numbers in Northern Ireland have at least doubled although it is unknown whether a similar increase has also occurred in the Republic of Ireland due to a paucity of data.

Productivity

Figure 2: Trend in productivity (no. of chicks fledged per pair) of Arctic tern throughout Ireland, 1986-2010. Based on SMP data; view the methods of analysis (PDF 158 kb).

Productivity data on Arctic terns collected from colonies throughout Ireland certainly appears to have been declining over the last decade with very few chicks fledged since 2007. Prior to this data is lacking for several years so no clear trend is evident.

Population estimates and change 1969-2002 (census data)

* AON = Apparently Occupied Nests

Breeding abundance

Figure 1: Abundance of Arctic tern at The Ayres, 1986-2010.

The small population of Arctic tern nesting on the Isle of Man are mostly found near The Ayres or Rue Point. Twenty-nine pairs were recorded during Operation Seafarer and 22 pairs by the Seabird Colony Register. This had declined somewhat by Seabird 2000 when only five pairs were recorded although numbers had been lower during the early 1990s. Since Seabird 2000, greater numbers have nested but the colony still holds fewer than half the number it once did.

Productivity

Very few systematic data on the productivity of the Arctic tern have been collected on the Isle of Man as part of the SMP.

This species does not breed on the Channel Islands.