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Sandwich tern (Sterna sandvicensis)

The following has been adapted from original text by Norman Ratcliffe in Seabird Populations of Britain and Ireland (with permission from A&C Black, London).

 

Sandwich terns exhibit the most erratic population trends and distribution of any seabird breeding in the UK. The population fluctuates dramatically among years due to large variations in the proportion of mature birds attempting to breed and distribution varies owing to mass movements between colonies. The species is distributed widely but patchily around the coasts of the British Isles, broadly reflecting the availability of favoured nesting habitat: low-lying offshore islands, islets in bays or brackish lagoons, spits or remote mainland dunes. Despite frequent changes in the sites used, the broad distribution in the UK has changed little over the last 30 years. Sandwich terns are among the most gregarious of all seabirds, with the population confined to a small number of relatively large colonies in which birds nest at very high densities.

Tern populations in NW Europe were brought to the brink of extirpation at the end of the 19th century by egg collection for food and hunting of adults for the millinery trade, but recovered in response to protective legislation in the early 20th century. Sandwich terns in the UK increased from the 1920s to the mid 1980s, with protection from increasing recreational disturbance on beaches as well as from persecution probably facilitating this recovery. Annual counts of the main colonies demonstrated that there was a sustained increase between the first two national surveys, but that the population fluctuated erratically around this trend.

Conservation status

Sandwich tern is currently identified as a conservation priority in the following:

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International importance

UK Population

Biogeographic Population

% World Population

12,500 AON*

16.9 (ssp. sandvicensis)

9.6

*AON = Apparently Occupied Nests

The UK population figure (rounded to the nearest hundred) was derived from data in Mitchell, P.I., Newton, S.F., Ratcliffe, N. and Dunn, T.E. (eds.) 2004. Seabird Populations of Britain and Ireland. Poyser, London. This was also the source of figures for the Biogeographic and World populations.

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UK population estimates and change 1969–2002 (census data)

Coverage of Sandwich tern colonies was comprehensive during all three of the national surveys, and so long-term trends will be real rather than artifacts of survey coverage. However, the size of the breeding population fluctuates erratically from year to year so trends based on comparison of two widely spaced surveys must therefore be viewed with caution, since one of them may have coincided with a year of temporarily depressed population size. Because whole colonies may move site within a year or two in response to changing conditions, such movements have the potential to produce severe bias in national population estimates that rely on summing counts from colonies surveyed in different years. To minimise this, all Sandwich tern colonies in the UK (except for one) were surveyed in 2000. During the SCR census, counts of colonies within regions were often taken from different years so, if colonies moved, some pairs may have been double counted and others omitted.

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(2000–2001)

UK Population estimate (AON*)

11,068

14,766

12,490

% change since previous census   

n/a

+33

-15

*AON = Apparently Occupied Nests

For census results for individual countries and Ireland, the Channel Islands and the Isle of Man, see under relevant sections below.

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Distribution/abundance

The Seabird 2000 census provides the most comprehensive recent assessment of the distribution and abundance of breeding seabirds. Numbers of Sandwich tern found in different regions, and a map showing the locations and size of colonies, is provided in the Seabird 2000 Sandwich tern results page.

The locations sampled during the annual Seabird Monitoring Programme provide some information on distribution and are accessible via the Seabird Monitoring Programme online database.

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Annual abundance and productivity by geographical area

With reference to the regional accounts below please note the following:

Breeding abundance: graphs of abundance index with 95% confidence limits are only shown for a region where the trend produced has been deemed accurate (see methods of analysis). Where a trend was thought to be inaccurate, graphs of abundance at major colonies in a region may be shown instead, particularly if such colonies hold greater than 10% of the regional population, are monitored frequently and may thus help illustrate regional population fluctuations outwith national censuses. Occasionally, too few data have been collected regionally to produce either of these.

Productivity: graphs of productivity are only shown if analysis of breeding success data produced a significant result for regional and/or year effects (again see methods of analysis). If results were not significant, then a regional mean productivity value is given. However, on some occasions, too few data are available from which to provide a meaningful average. Furthermore, for 11 species where the quality of monitoring data available was considered high, population viability analysis was undertaken at the UK level and the results of this are also reported.

 

United Kingdom

Breeding abundance

sandwich-tern-uk-ab.jpg

Figure 1. Trend in UK abundance index (solid line) of Sandwich tern 1986–2018 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 population increased from 10,500 pairs to 14,800, probably as a result of increased legal protection, helping to reduce disturbance from recreation. The breeding abundance of Sandwich terns in the UK declined from 20% above the 1986 baseline in 1988 to 20% below in 1995. It then increases to 8% above the baseline in 2002 and then fluctuated considerably until 2015 when it was at the same level as 1986. In 2016, the index decreased to 23% below the baseline but increased again to 4% above in 2018. 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 several years before and after 2009. Sandwich terns are increasing again at Minsmere with 1 AON counted in 2014 and 32 AON in 2018.

 

Productivity

sandwich-tern-uk-prod.jpg

Figure 2. Trend in UK productivity (no. of chicks fledged per pair) of Sandwich tern 1986–2018. 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 it could be argued that 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 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 result of predation1. 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 tide 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.

Analysis of the SMP dataset found productivity of Sandwich terns averaged 0.66 chicks per nest per year between 1986 and 2008 and remained relatively stable2. The quality of the dataset meant any change of 10% or more could be detected with confidence. Using existing life history information (population size, clutch size, age at first breeding and survival rates of different age classes) to parameterise population viability analysis, it was predicted that, if this level of breeding performance were to remain unchanged, the population of Sandwich terns would decline by 62% over 25 years. Such a decline could be averted, and the population could be stabilised, if productivity rose to 1.10. In 2018, average UK productivity was 0.54 chicks fledged per pair.

 

Scotland

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

2,465

2,286

1,068

% change since previous census   

n/a

-7

-53

*AON = Apparently Occupied Nests

 

Breeding abundance

sandwich-tern-scotland-ab.jpg

Figure 3. Trend in abundance index (solid line) of Sandwich tern in Scotland, 1986–2018 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 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 by 53% by the Seabird 2000 census. The majority of the Scottish population nest at the Sands of Forvie, a well monitored colony, which held almost half of Scotland's Sandwich terns during Seabird 2000. The low trend between 1987 and 1998 is largely governed by the decline observed at Sands of Forvie, caused by several years of fox predation3. Since 1999, numbers have increased after an electric fence was erected at the start of the season, in order to minimise predation of breeding birds by foxes Vulpes vulpes and Badgers Meles meles. In addition, disturbance from human activities was minimised4. Recently, AON are fluctuating between 1,315 and 565 AON. However, other large colonies became extinct during this period, most 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 ascertained for other colonies. In 2018, the index was 47% below the 1986 baseline.

 

Productivity

sandwich-tern-scotland-prod.jpg

Figure 4. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern in Scotland, 1986–2018. Based on SMP data; view the methods of analysis.

 

The 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 the average being 0.67 chicks fledged per pair. In 2018, 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 per pair (624 chicks from 852 pairs), the highest since 2013 when 0.80 chicks fledged per pair.

 

England

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

7,392

9,844

9,018

% change since previous census   

n/a

+33

-8

*AON = Apparently Occupied Nests

 

Breeding abundance

 

sandwich-tern-england-ab.jpg

Figure 5. Trend in abundance index (solid line) of Sandwich tern in England, 1986–2018 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 index spikes in 2009 when over 500 pairs nested at Minsmere, possibly as a result of birds abandoning a colony on the continent. The index declined again in 2010 but by 2018 had risen to 5% above the 1986 baseline. 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 terns are breeding there again but in very low numbers (11–32 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 where over 8,500 Sandwich terns nested in 2018. These colonies probably now hold over 70% of the English population.

 

Productivity

sandwich-tern-england-prod.jpg

Figure 6. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern in England, 1986–2018. Based on SMP data; view the methods of analysis.

 

Annual average productivity at English colonies is usually higher than that recorded in Scotland but is very variable due to the influences of predation and tidal inundation. However, after peak productivity was recorded in 2000 there was a downward trend culminating in very low levels of chick production in 2014. Productivity in colonies across England then increased until 2017, although declined slightly in 2018 to 0.57 chicks fledged per pair. Scolt Head Island NNR had four very successful breeding seasons, with an average of 0.87 chicks fledged per pair between 20015 and 2018. 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 showed one of the highest productivity since monitoring began in 1986.

 

Wales

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

0

450

450

% change since previous census   

n/a

n/a

0

*AON = Apparently Occupied Nests

 

Breeding abundance

sandwich-tern-wales-ab.jpg

Figure 7. Abundance of Sandwich tern at Cemlyn Lagoon, 1986–2018.

 

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, 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 have both seen a reduction in numbers, with 1980 and 519 AON being recorded respectively.

 

Productivity

sandwich-tern-wales-prod.jpg

Figure 8. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern at Cemlyn, Wales, 1986–2017. Based on SMP data; view the methods of analysis.

 

Sandwich terns nesting at Cemlyn have been relatively successful compared with other British 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 since 2002, 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. Between 2009 and 2017, average productivity was 0.73 chicks fledged per pair however, between 2014 and 2017 it was between 0.45 and 0.55 chicks fledged per pair. Productivity increased in 2016 to 0.73 chicks fledged per pair but declined again in 2017 with no chicks fledged from 1,980 AON, due to some predation but mostly repeated disturbance by otters Lutra lutra (Chris Wynne pers. comm.). In 2018, productivity was not recorded.

 

Northern Ireland

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

1,211

2,186

1,954

% change since previous census   

n/a

+80

-11

*AON = Apparently Occupied Nests

 

Breeding abundance

sandwich-tern-ni-ab.jpg

Figure 9. Abundance of Sandwich tern at five colonies in Northern Ireland, 1986–2018.

 

Sandwich tern has the most complete monitoring record over the longest period of any seabird species in Northern Ireland. 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 by 11% to 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 9). Previous reports stated that Strangford Lough was abandoned in 1986 and 1988 but this appears to be in error; in actual fact, 1,418 nested in 1986 and 2,228 in 1988. Comprehensive monitoring of these five colonies between censuses suggest total numbers in Northern Ireland declined after the SCR until the early 1990s, before increasing steadily until 2005, reaching a peak of 3,319 AON. Numbers then declined rapidly until 2013 when just 1299 AON were recorded but have recovered slightly, with 1,863 AON being recorded in 2018.

 

Productivity

Few systematic data on the productivity of Sandwich terns in Northern Ireland have been collected as part of the SMP. On average, 0.30 chicks have been fledged per pair per year between 1990 and 2018.

 

Republic of Ireland

Population estimates and change 1969–2018 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Republic of Ireland Census

(2015-18)

Population estimate (AON*)

1,005

1,281

1,762

2,519

% change since previous census   

n/a

+27

+38

+97

*AON = Apparently Occupied Nests

 

Breeding abundance

sandwich-tern-ladys-island-lake-ab.jpg

Figure 10. Abundance of Sandwich tern at Lady's Island Lake, 1986–2018.

 

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 37% since Seabird 2000. Sandwich terns breed in small numbers in colonies along the Atlantic coastline, and at the larger colonies at Lough Swilly and Lady’s Island Lake in the southeast which hold approximately 84% of the national population. Numbers at the largest colony, Lady's Island Lake (Figure 10), have increased substantially during the last decade with a peak of 1,958 AON in 2009. There was a slight decline thereafter with 1,617 AON recorded in 2014, however, numbers have increased again with 1,780 AON recorded in 2018.

 

Productivity

sandwich-tern-ireland-prod-1990-2008.jpg

Figure 11. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern in the Republic of Ireland, 1990–2008. Based on SMP data; view the methods of analysis.

 

Few systematic data on the productivity of Sandwich terns in the Republic of Ireland have been collected as part of the SMP. Breeding Sandwich terns in the Republic of Ireland show the typical fluctuating fortunes found at colonies in the United Kingdom. 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.

 

All Ireland

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

2,216

3,467

3,716

% change since previous census   

n/a

+56

+7

*AON = Apparently Occupied Nests

 

Breeding abundance

sandwich-tern-ireland-ab.jpg

Figure 12. Abundance of Sandwich tern at six colonies throughout Ireland, 1986–2018.

 

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 at Lady's Island Lake in Republic of Ireland all increased, with a peak combined count in 2009 of 4,918 AON. Numbers then declined to 2,968 AON in 2013 but have risen again with 3,643 AON being recorded in 2018. A recent seabird census in the Republic of Ireland recorded 2,519 Sandwich tern AON. Taking smaller colonies in Northern Ireland that have not been counted recently (of which there are very few) into account, the total Irish population at present probably lies between 3,643 and 3,700 AON.


Productivity

sandwich-tern-all-ireland-prod-1986-2018.jpg

Figure 13. Trend in productivity (no. of chicks fledged per pair) of Sandwich tern throughout Ireland, 1986–2018. Based on SMP data; view the methods of analysis.

 

The trend in breeding performance shown for the whole of Ireland closely matches that shown for the Republic of Ireland where most data have been collected, but with slightly lower average values. Low levels of productivity were recorded in 1993, 1999 and 2008 but, in most years since 2000, Sandwich terns have been relatively productive. No productivity data have been submitted since 2008.

 

Isle of Man

Sandwich tern does not breed on the Isle of Man.

 

Channel Islands

Sandwich tern does not breed on the Channel Islands.

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UK phenology, diet, survival rates

No data have been collected as part of the Seabird Monitoring Programme.

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References

1 Ratcliffe, N., Pickerell, G. and Brindley, E. 2000. Population trends of Little and Sandwich Terns Sterna albifrons and S. sandvicensis in Britain and Ireland from 1969 to 1998. Atlantic Seabirds, 2, 211-26.

2 Cook, A.S.C.P. and Robinson, R.A. 2010. How representative is the current monitoring of breeding success in the UK? BTO Research Report, No. 573, British Trust for Ornithology, Thetford.

3 Short, D. 2014. Breeding of four species of tern and Black-headed Gull at Forvie National Nature Reserve, 2013. Unpublished SNH report, Scottish Natural Heritage, Edinburgh.

4 Short, D. 2019. Breeding of four species of tern and Black-headed Gull at Forvie National Nature Reserve, 2018. Unpublished SNH report, Scottish Natural Heritage, Edinburgh.

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Partners

Data have been provided to the SMP by the generous contributions of its partners, other organisations and volunteers throughout Britain and Ireland. Partners to the SMP are: BirdWatch Ireland; The British Trust for Ornithology; UK Centre for Ecology and Hydrology; Natural Resources Wales; Department of Environment, Food and Agriculture (Isle of Man); Department of Environment, Heritage and Local Government (Republic of Ireland); States of Guernsey Government; JNCC; Manx Birdlife; Manx National Heritage; The National Trust; National Trust for Scotland; Natural England; Northern Ireland Department of Agriculture, Environment and Rural Affairs; The Royal Society for the Protection of Birds; Scottish Natural Heritage; Seabird Group; Shetland Oil Terminal Environmental Advisory Group; Scottish Wildlife Trust. More about the SMP partners >>

 

Image of Sandwich tern appears courtesy of Ian Rendall ©, is subject to international copyright law and may not be reproduced in any form whatsoever.

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SMP Report 1986–2018

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