Sandwich tern (Sterna sandvicensis)

Figure 1. Trends in abundance index of Sandwich tern 1986–2019 for the UK (red) 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.

A large proportion of the colonies monitored for Sandwich tern abundance are in England and, as such, the UK and English trends are closely matched (Figure 1). Both trends decreased between 1986 and 2008, although since then have gradually increased. The number of colonies monitored frequently are only sufficient to allow meaningful abundance trends to be produced for the UK and England.

Productivity

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

Most data on Sandwich tern productivity in the UK have been collected at English colonies, so the two trends in Figure 2 are closely matched. Both trends appear to have gradually decreased until 2014, after which an increase was observed until 2017. The number of colonies monitored are only sufficient to allow the production of meaningful productivity trends for the UK and England.

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

Between censuses in 1969-70 and 1985-88, the United Kingdom Sandwich tern population increased from 10,500 pairs to 14,800, probably as a result of increased legal protection helping to reduce disturbance from recreation. The Sandwich tern breeding abundance index declined from 20% above the 1986 baseline in 1988 to 20% below in 1995 (Figure 3). It then increased to 8% above the baseline in 2002 and then fluctuated considerably until 2015 when it was at the same level as 1986. The spike in the index in 2009 is due to an influx of Sandwich terns, apparently from continental Europe, nesting at Minsmere (Suffolk); 550 AON were recorded there in 2009, compared to only one pair the year before and none for several years after that. Sandwich terns are increasing again at Minsmere with 1 AON counted in 2014 and 50 in 2019. In 2019, the index is at pare with the baseline.

Productivity

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

A marked decline in productivity is obvious since 2000 when Sandwich terns fledged a record number of chicks. In the 14 years prior to 2000 productivity showed no clear trend, although in 1991 and between 1997 and 1999 it was low. Few chicks fledged in these years due to bad weather, predation, and disturbance by a variety of mammals and gulls; food shortage was also implicated at only one colony. Predation on eggs and chicks by foxes Vulpes vulpes is probably the most prevalent factor determining productivity, and abandonment of a colony is often the result¹. Fox populations are thought to have increased during the past few decades due to less intensive management by gamekeepers. Nature reserve managers use electric fences to exclude foxes but these are not always successful. As Sandwich terns nest on low-lying ground close to the water’s edge, their nests are also vulnerable to tidal inundation; predictions of increased storminess and sea-level change under climate change scenarios may lead to increased prevalence of such events. In 2019, average UK productivity was 0.41 chicks fledged per pair.

Figure 5. Number of Sandwich tern AON at Sands of Forvie National Nature Reserve between 1986 and 2019.

The Scottish population of Sandwich terns changed little between the national censuses in 1969-70 and 1985-88 but a large decline then occurred as numbers fell 53% by Seabird 2000. The majority of the Scottish population now nest at Sands of Forvie, a well monitored colony which held almost half of Scotland's Sandwich terns during Seabird 2000. The low numbers nesting at this site between 1992 and 1998 were largely due to fox Vulpes vulpes predation². Since then, numbers have increased largely due to an electric fence being erected to minimise predation by foxes and badgers Meles meles. In addition, disturbance from human activities has been minimised by restricting access in line with the Scottish Outdoor Access Code, enforced by regular patrols by staff³. In 2019, 1,010 AON were recorded at Sand of Forvie.

Many other large colonies in Scotland had become extinct by the mid to late 1990s (e.g. Loch of Strathbeg, Inchmickery and McDermott's), while at others (e.g. Long Craig and Isle of May) nesting became sporadic. Gulls extirpated the colony on Inchmickery but causes of decline or extinction have not been determined for other colonies.

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern in Scotland, 1986–2019. Based on SMP data.

Productivity of Sandwich terns at colonies monitored in Scotland has fluctuated considerably since recording began. 2000 and 2001 were the only years on record with relatively high levels of productivity, with an average of 0.57 chicks being fledged per pair between 1986 and 2019. In 2019, Sandwich terns at Sands of Forvie again occupied a breeding colony among black-headed gulls. In 2018, productivity at this colony rose slightly to 0.73 chick fledged per pair (624 chicks from 852 pairs), the highest values recorded since 2013 when 0.80 chicks fledged per pair (Figure 6). In 2019, productivity decreased to 0.53³.

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

In contrast to Scotland, Sandwich tern numbers in England have generally fared better, with an increase between Operation Seafarer and the Seabird Colony Register. Numbers have since been fairly stable, although between 2002 and 2008 there was evidence of a slight decline. The annual abundance index spiked in 2009 when over 500 pairs nested at Minsmere, possibly as a result of birds abandoning a colony on the continent (Figure 7). The index declined again in 2010 but, by 2018, had risen to 5% above the 1986 baseline, although by 2019 had declined to 6% below. The Minsmere colony was abandoned in 1977 and again from 2010 to 2013, with one pair attempting to breed there in 2014 and none in 2015. Since 2016 Sandwich terns are breeding there again but in very low numbers (11–50 AON). Several other sites now hold no breeding Sandwich terns (e.g. Dungeness, Foulness, Foulney, Havergate, Chichestera and North Solent). The largest colonies in England are on the Farne Islands, Coquet Island, Blakeney Point and Scolt Head Island where over 6,662 Sandwich terns nested in 2019.

Productivity

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

Sandwich tern productivity at English colonies is usually higher than those in Scotland but is variable due to the influences of predation and tidal inundation. After peak productivity was recorded in 2000 there was a downward trend culminating in very low levels of chick production in 2014 (Figure 8). Productivity at colonies across England then increased until 2017, although declined in 2018 to 0.57 chicks fledged per pair and then increased slightly to 0.60 in 2019. Scolt Head Island NNR had four very successful breeding seasons, with an average of 0.84 chicks fledged per pair between 2015 and 2019. In contrast, neighbouring Blakeney Point only fledged an average of 0.13 chicks per pair across the same four years. A similar scenario happened in 2000, when all nests at Blakeney Point failed and Scolt Head recorded one of the highest productivity values since monitoring began in 1986.

Figure 9. Number of Sandwich tern AON at Cemlyn Lagoon, 1986–2019.

Sandwich terns are confined to just one location in Wales, Cemlyn Lagoon on Anglesey. No birds were recorded there during Operation Seafarer, although there was an influx of 50 AON mid-season in 1970, which were possibly birds displaced from elsewhere. By the time of the Seabird Colony Register (1985-88), 450 AON were nesting at the lagoon and, aside from a peak of short duration from 1987–1989, numbers remained relatively stable until Seabird 2000. Numbers then increased substantially from 2001 onward, with the colony regularly holding over 2,000 pairs (although only 409 pairs nested in 2008) up to a peak in 2015 (2,650 AON). The 2017 and 2018 breeding seasons had lower numbers, with 1,980 and 519 AON being recorded respectively. Numbers increased in 2019 to 1,200 AON.

Productivity

Figure 10. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern at Cemlyn Lagoon, Wales, 1986–2019.

Sandwich terns nesting at Cemlyn have been relatively successful compared with those at other UK colonies, with an average of over one chick per pair being fledged in 1997, 2003 and 2006. This may explain the increased numbers nesting at the site in recent years, as young from successful breeding seasons themselves return to nest at the colony. In 2007 and 2008, there was almost complete failure with very few young fledged due to predation and desertion of nests. In 2017, no chicks fledged from 1,980 AON due to disturbance and predation by otters Lutra lutra (Chris Wynne pers. comm.). In 2019, productivity was 0.68 chicks fledged per pair.

Figure 11. Abundance of Sandwich tern at five colonies in Northern Ireland, 1986–2019.

National census data show that numbers of Sandwich terns nesting in Northern Ireland increased between Operation Seafarer and the Seabird Colony Register by 80% but then fell 11% by Seabird 2000. The decline between 1987 and 2000 is reflected by data from five well monitored colonies (Cockle Island, Carlingford Lough, Strangford Lough, Larne Lough and Lower Lough Erne) presented above (Figure 11). Previous reports state that Strangford Lough was abandoned in 1986 and 1988 but this was an error; in actual fact, 1,418 AON were recorded in 1986 and 2,228 in 1988. Comprehensive monitoring of these five colonies between censuses suggest numbers in Northern Ireland declined after the SCR until the early 1990s, before increasing steadily until 2005 when the total for the five colonies reached a peak of 3,319 AON. Numbers at these sites then declined rapidly until 2013, when just 1,299 AON were recorded, but have recovered slightly since with 1,698 AON being recorded in 2019⁴.

Productivity

Few systematic data on the productivity of Sandwich terns in Northern Ireland have been collected as part of the SMP. On average, 0.31 chicks have been fledged per pair per year between 1990 and 2019.

Figure 12. Abundance of Sandwich tern at Lady's Island Lake, 1986–2019.

Operation Seafarer recorded 1,005 AON of Sandwich tern in the Republic of Ireland. This number increased during each subsequent census with 1,762 AON being recorded during Seabird 2000. A recent seabird census in the Republic of Ireland recorded 2,519 AON, an increase of 43% since Seabird 2000. Sandwich terns breed in small numbers in colonies along the Atlantic coastline, and at the larger colonies of Lough Swilly and Lady’s Island Lake in the southeast which hold approximately 84% of the national population. Numbers at Lady's Island Lake (Figure 12) have increased substantially during the last decade, with a peak of 1,958 AON in 2009, although there was a slight decline after that to 1,617 AON in 2014. However, numbers have increased again with 1,739 AON being recorded in 2019⁵.

Productivity

Few systematic data on the productivity of Sandwich terns in the Republic of Ireland have been collected as part of the SMP. Particularly low productivity was recorded in 1993, 1999 and 2008 but in most years since 2000 Sandwich terns have been relatively productive. No productivity data have been submitted to the SMP since 2008.

Figure 13. Number of Sandwich tern AON at six colonies throughout Ireland, 1986–2019.

National census data show a 56% increase in Sandwich tern numbers between Operation Seafarer and the Seabird Colony Register (SCR). The Seabird 2000 census recorded similar numbers to the SCR but, in the decade following that census, the five main colonies in Northern Ireland and the colony at Lady's Island Lake in Republic of Ireland all increased, with a peak combined count in 2009 of 4,918 AON (Figure 13). The total number for all six colonies then declined to 2,968 AON in 2013 but rose again with 3,643 AON being recorded in 2018. In 2019, 3,437 AON were recorded. A recent seabird census in the Republic of Ireland recorded 2,519 Sandwich tern AON Between 2015 and 2018.

Productivity

Figure 14. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern throughout Ireland, 1986–2019. Based on SMP data; view the methods of analysis.

Few systematic data on Sandwich tern productivity in the whole of Ireland have been collected as part of the SMP. Particularly low levels of productivity were recorded in 1993, 1999 and 2008 but, in most years since 2000, Sandwich terns have been relatively productive. On average 0.54 chicks were fledged between 1990 and 2018. No productivity data was submitted to the SMP in 2019.