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).
Common terns are not the most abundant UK tern species, but are probably the most familiar because their breeding range extends around much of the British Isles coastline plus inland on lakes, reservoirs and gravel pits along the large river valleys of SE and Central England, notably the Thames, Ouse, Humber and Trent, and along rivers in SE Scotland. They are absent from most of Wales and SW England, and are largely replaced in the Northern and Western Isles by Arctic terns.
All tern populations in NW Europe were brought to the brink of extirpation at the end of the 19th century by hunting of adults for the millinery trade, but recovered in response to protective legislation in the early 20th century. However, over the last three decades, the UK common tern population has remained broadly stable.
Conservation status
Common tern is currently identified as a conservation priority in the following:
- Amber listed in Birds of Conservation Concern 4 (2015 update)
- Further information on Conservation Designations for UK Taxa
- Amber listed in Birds of Conservation Concern in Ireland 2020–2026
- EC Birds Directive – listed in Annex 1 and as a migratory species
International importance
UK Population |
% Biogeographic Population |
% World Population |
11,800 AON* |
3.0 (ssp. hirundo) |
2.2 |
*AON = Apparently Occupied Nests
The UK population figure (rounded to the nearest hundred) and the World populations were 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. The source for the Biogeographic population is AEWA CSR7.
UK population estimates and change 1969–2002 (census data)
All terns breeding in Britain and Ireland show a low degree of site faithfulness from one year to the next; in response to predation or habitat change, and especially in areas where islands and other suitable habitat are plentiful, adults may move en masse between different sites. Hence, in order to gain an accurate national estimate of tern numbers, a simultaneous census was planned to cover all colonies in Britain within a single year. Thus, the majority of British tern colonies were surveyed in 2000, including extensive surveys of Orkney and Shetland. The main exception was in the Western Isles; most Lewis and Harris tern colonies were surveyed in 1999, with those from the Sound of Harris to Barra Head surveyed in 2002. In Northern Ireland, results from the All-Ireland tern survey conducted in 1995 were utilised. During the SCR, counts were made in different years within regions, and inter-colony movements may have caused greater inaccuracies. Also, survey coverage of the Northern Isles was poor. This was overcome by inclusion of data from the 1980 survey of terns in Orkney and Shetland. Coverage of inland sites was probably more extensive during Seabird 2000, so the assessment of changes in range and status inland should be made with caution.
Breeding populations can also fluctuate among years owing to variations in the proportion of mature birds attempting to nest. However, comparison with annual counts from sites throughout the UK indicated that counts during the SCR and Seabird 2000 were not atypically low. As such, trends estimated between the two surveys should be reasonably robust.
|
Operation Seafarer (1969-70) |
Seabird Colony Register (1985-88) |
Seabird 2000 (1998–2002) |
UK Population estimate (AON*) |
11,978 |
13,053 |
11,838 |
% change since previous census |
n/a |
+9 |
-9 |
*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.
Distribution/abundance
The Seabird 2000 census provides the most comprehensive recent assessment of the distribution and abundance of breeding seabirds. Numbers of common tern found in different regions, and a map showing the location and size of colonies, is provided in the Seabird 2000 common 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.
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 estimated reproductive success (hereinafter referred to as productivity) are only shown if data analysis 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.
Note: for clarity, the Figure 1 ‘Overview of all regions’ abundance graph does not include confidence limits. These can be found in the respective regional accounts below.
Overview of all regions
Breeding abundance
Figure 1. Trends in abundance index of common 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 slight regional variation in common tern abundance trends since monitoring began in 1986 (Figure 1). The trends for the UK and England are similar and fluctuate around the baseline, although the England trend shows slightly higher index values for much of the monitoring period. The Scottish trend is more variable and has periods with comparatively high (early 1990s and 2016 to 2018) and low index values (e.g. 1988 and 2019) to the UK and England trends.
The number of colonies monitored frequently are only sufficient to allow the production of meaningful abundance trends for the UK, Scotland and England.
Productivity
Figure 2. Trends in productivity of common 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.
Common tern productivity trends in the UK, Scotland and England (Figure 2) all show a gradual decline over time. Productivity in England has, however, been higher than in the UK and Scotland throughout much of the monitoring period.
The number of colonies monitored frequently are only sufficient to allow the production of meaningful abundance trends for the UK, Scotland and England.
United Kingdom
Breeding abundance
Figure 3. Trend in UK abundance index (solid line) of common tern 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.
Between the Operation Seafarer (1969-70) and Seabird Colony Register (1985-88) censuses, common terns in the UK increased by 9%, although had returned to Operation Seafarer levels by the time of the Seabird 2000 census. Since then, the annual abundance index has shown a similar pattern, where numbers increase and then decrease again over respective five- to six-year periods. The lowest ever recorded index value occurred in 2012 when it was at 25% below the 1986 baseline. Since then, the index has risen and in 2019 was 10% below the baseline (Figure 3).
Trends at finer spatial scales have varied considerably and are likely to reflect varying pressures facing common terns in different habitats across their wide geographic range. Increased predation by species such as American mink Neovison vison1 and red fox Vulpes vulpes have caused declines in some areas, although conservation management to ameliorate these problems is being undertaken. Common terns have also benefited from habitat creation in the form of gravel pits; tern rafts in reservoirs; islets in industrial lagoons; structures in ports; and from habitat improvement on reserves through control of vegetation succession and gull competition. Maintaining the population is likely to depend on the continuation of such management in perpetuity.
Productivity
Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of common tern 1986–2019. Based on SMP data; view the methods of analysis.
Like most other UK tern species, productivity of common terns has fluctuated considerably over the monitoring period. In 2019, an average of 0.45 chicks were fledged per pair. There rarely is a single reason for years of poor productivity, which are usually due to several factors such as predation2, bad weather and poor feeding conditions, although common terns have a broader diet than many tern species and are less affected by changes in prey availability. As common terns often nest on low-lying ground close to the water’s edge, their nests are vulnerable to erosion and tidal inundation; predictions of increased storminess and sea-level change under climate change scenarios may lead to increased prevalence of such events, though managed realignment of coastal defences may create new opportunities for nesting.
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*) |
4,285 |
6,784 |
4,784 |
% change since previous census |
n/a |
+58 |
-29 |
*AON = Apparently Occupied Nests
Breeding abundance
Figure 5. Trend in abundance index (solid line) of common tern in Scotland, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.
Numbers of common terns in Scotland during the Seabird Colony Register (SCR) were 58% higher than during Operation Seafarer but by Seabird 2000 had fallen by 29%. The index above suggests common tern abundance actually continued to rise after the SCR and peaked in the early 1990s with a gradual decline evident thereafter up to Seabird 2000 and beyond. From 2008, the index began to increase again, peaking at 28% above the baseline in 2016, although by 2019 had decreased to its lowest value since monitoring began at almost 50% below. The largest Scottish common tern colonies in 2019 were at Leith Docks (246 AON), Sands of Forvie (242 AON) and Methil Docks (190 AON).
As for many tern species, maintaining common tern population levels depends on the management at most of their breeding sites, with predator control, habitat creation and disturbance reduction being key to success.
Productivity
Figure 6. Trend in productivity (no. of chicks fledged per pair) of common terns in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.
Productivity of common terns in Scotland has fluctuated considerably over the monitoring period and there are very few years when it has been above 0.60 chicks fledged per pair. Productivity was particularly low in 1996, 2002 and 2008 due to the additive effects of predation, bad weather and poor feeding conditions. In 2014, detailed studies in Lochaber, and Argyll and Bute, found that colonies that had suffered predation (by American mink Neovison vison and large gulls) fledged 0.52 chicks per nest (n=135 nests) compared to 0.93 chicks fledged per nest (n=393 nests) from colonies with no, or little, predation. In 2018, average common tern productivity decreased to 0.18 chicks fledged per pair, although by 2019 had increased again to 0.40.
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*) |
6,099 |
4,659 |
4,676 |
% change since previous census |
n/a |
-24 |
<+1 |
*AON = Apparently Occupied Nests
Breeding abundance
Figure 7. Trend in abundance index (solid line) of common tern in England, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.
Common tern numbers in England decreased by 24% between the Operation Seafarer and Seabird Colony Register (SCR) censuses and Seabird 2000 recorded approximately the same number as the SCR. Since Seabird 2000, the common trend index has fluctuated close to the 1986 baseline (Figure 7). In 2019, the index was 18% above the baseline, suggesting that the English common tern breeding population may now be larger than it was at the time of Seabird 2000.
In 2019, the largest colonies in England were Coquet Island (1,652 AON) and Saltholme (210 AON). The species has benefited from habitat creation in the form of gravel pits; tern rafts in reservoirs; islets in industrial lagoons; structures in ports; and from management to maintain habitat on reserves through control of vegetation succession and to reduce competition with, and predation by, gulls.
Productivity
Figure 8. Trend in productivity (no. of chicks fledged per pair) of common terns in England, 1986–2019. Based on SMP data; view the methods of analysis.
The productivity of common terns in England has fluctuated considerably since 1986 but appears to have been in decline since the mid-1990s. Peak productivity was recorded in 1995 (1.02 chicks fledged per pair), although it has seldom been high during the monitoring period. In 2012 the productivity index reached its lowest value at 0.24 chicks fledged per pair. Since then, an increase has become evident with an average of 0.74 chicks fledged per pair in 2019.
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*) |
292 |
514 |
674 |
% change since previous census |
n/a |
+76 |
+31 |
*AON = Apparently Occupied Nests
Breeding abundance
Figure 9. Number of common tern AON at three colonies in Wales, 1986–2019. Based on SMP data.
The Welsh common tern population increased between Operation Seafarer and the Seabird Colony Register by 76% from 292 to 514 AON. By the time of Seabird 2000, the population had increased by a further 31%.
Data submitted to the SMP on the abundance of common terns in Wales are sparse; thus, no meaningful abundance trend can be provided. However, data from three of the largest, and most frequently monitored, colonies indicate contrasting fortunes since Seabird 2000. Common terns at Ynys Feurig have been slowly increasing since 1998, while at Cemlyn Lagoon they have declined during the last decade. Numbers at Shotton Steels increased steadily until 2008 but the colony was abandoned in 2009 due to fox depredation. Numbers there have been increasing again, reaching 412 AON in 2019. This has been attributed to the installation of fox-deterrent measures, an abundant food supply and good weather.
Productivity
Figure 10. Trend in productivity (no. of chicks fledged per pair) of common tern at Shotton Steels in Wales, 1986–2019.
Common terns at Welsh colonies are among the most productive in the UK, regularly fledging more than one chick per pair. Complete failure at Shotton Steels was recorded in 1990 and between 2008 and 2013 and was largely due to fox depredation and adverse weather conditions (Peter Coffey pers. comm). Volunteers have been tirelessly working at this colony to improve suitable nesting space for common terns and to install fox-deterrent security measures. These appear to have been working, with an average of 1.27 chicks being fledged per pair between 2014–2019. However, rapid expansion of black-headed gulls using the site and extensive weed growth, which discourages the terns from nesting, are also a problem (Peter Coffey pers. comm.).
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*) |
1,302 |
1,096 |
1,704 |
% change since previous census |
n/a |
-16 |
+55 |
*AON = Apparently Occupied Nests
Breeding abundance
Figure 11. Number of common terns AON at six colonies in Northern Ireland, 1986–2019.
The number of common terns breeding in Northern Ireland increased by 55% between the Seabird Colony Register and Seabird 2000 censuses (1,096 to 1,704 AON respectively). In 2006, the six largest colonies in the east of Northern Ireland (Cockle Island, Carlingford Lough, Strangford Lough, Copeland Island, Larne Lough, and Belfast Lough), held 2,369 AON (cf. 1,570 in 2000), although numbers at these colonies had declined to 1,119 AON in 2018 (Figure 11). A further six colonies, where monitoring is less frequent, held 433 AON in 2018; therefore, all 11 colonies totalled 1,552 AON. The few other colonies that were found during the comprehensive coverage of Seabird 2000 are unlikely to hold more than 400-500 AON in total. Hence, it is possible that the population of breeding common terns in Northern Ireland in 2018 was slightly larger than that recorded during the last census. In 2019 less colonies were surveyed and a similar comparison could not be undertaken.
Productivity
An average of 0.65 chicks were fledged per pair per year between 1999 (the first year the SMP has data for) and 2019. Common tern productivity has been recorded at Portmore Lough since 2014 and has averaged 1.01 chicks fledged per pair.
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*) |
2,804 |
1,574 |
2,485 |
5,058 |
% change since previous census |
n/a |
-44 |
+58 |
+104 |
*AON = Apparently Occupied Nests
Breeding abundance
Figure 12. Number of common terns AON on Rockabill, Republic of Ireland 1986–2019.
Between Operation Seafarer and the Seabird Colony Register common terns in the Republic of Ireland declined by 44% and, although the population had increased again by 1998-2002, Seabird 2000 still recorded fewer AON than during this first census. At the largest colony, Rockabill, numbers increased almost exponentially from 1986 to 2011 (Figure 12), possibly due to immigration from other colonies around the Irish Sea. Rockabill has since then become more stable with 1,833 AON recorded there in 2019 (cf. peak of 2,191 in 2011). A recent seabird census of the Republic of Ireland (2015–2018) recorded 5,058 common tern AON, an increase of 104% since Seabird 20003. There is little doubt that the strong common tern population increase in the Republic of Ireland is being driven by the on-going conservation actions at Lady’s Island4 Lake and Rockabill5. There are insufficient data submitted to the SMP to allow an annual trend to be generated.
Productivity
The number of colonies monitored in the Republic of Ireland is insufficient to allow a meaningful productivity trend to be produced.
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*) |
4,106 |
2,670 |
4,189 |
% change since previous census |
n/a |
-35 |
+57 |
*AON = Apparently Occupied Nests
Breeding abundance
The Northern Ireland common tern population has increased since the Seabird 2000 census and is now thought to be approximately 2,000 AON. A recent seabird census (2015–2018) of the Republic of Ireland recorded 5,058 AON6. It is, therefore, likely that the all-Ireland common tern population has increased by over 60% since Seabird 2000.
The number of colonies monitored in both Northern Ireland and the Republic of Ireland are insufficient to allow a meaningful combined annual abundance trend to be produced.
Productivity
The number of colonies monitored in both Northern Ireland and the Republic of Ireland are insufficient to allow a meaningful combined productivity trend to be produced.
Isle of Man
Population estimates and change 1969–2018 (census data)
|
Operation Seafarer (1969-70) |
Seabird Colony Register (1985-88) |
Seabird 2000 (1998–2002) |
Isle of Man Census (2017-18) |
Population estimate (AON*) |
1 |
7 |
0 |
0 |
% change since previous census |
n/a |
+600 |
0 |
0 |
*AON = Apparently Occupied Nests
Breeding abundance
Only a few pairs of common terns have bred on the Isle of Man. One AON was recorded during Operation Seafarer and seven during the Seabird Colony Register. None were recorded during Seabird 2000 and none have been reported since6.
Productivity
No systematic data on the productivity of common terns on the Isle of Man have been submitted to the SMP.
Channel Islands
Population estimates and change 1969–2015 (census data)
|
Operation Seafarer (1969-70) |
Seabird Colony Register (1985-88) |
Seabird 2000 (1998–2002) |
Channel Islands Census (2015) |
Population estimate (AON*) |
107 |
227 |
174 |
250 |
% change since previous census |
n/a |
+112 |
-23 |
+44 |
*AON = Apparently Occupied Nests
Breeding abundance
Common terns are the only species of tern to breed in the Channel Islands. During Seabird 2000, 174 AON were recorded which represented a decline of 23% since the Seabird Colony Register. During the Channel Island Seabird Census in 2015, 250 AON were recorded, an increase of 44% since Seabird 20007.
Productivity
Data submitted to the SMP on the productivity of common terns in the Channel Islands are sparse; thus, no meaningful value can be provided.
UK phenology, diet, survival rates
No data have been collected as part of the Seabird Monitoring Programme.
References
1 Craik, J.C.A. 1997. Long-term effects of North American Mink Mustela vison on seabirds in western Scotland. Bird Study, 44, 303–309.
2 Craik, J.C.A. 1995. Effects of North American Mink Mustela vison on the breeding success of terns and smaller gulls in west Scotland. Seabird, 17, 3–11.
3 Cummins, S., Lauder, C., Lauder, A. and Tierney, T. D. 2019. The Status of Ireland’s Breeding Seabirds: Birds Directive Article 12 Reporting 2013 – 2018. Irish Wildlife Manuals, No. 114. National Parks and Wildlife Service, Department of Culture, Heritage and the Gaeltacht, Ireland.
4 Daly, D., Daly, L. and M. Murray. 2020. Lady’s Island Lake Tern Report 2019. Department of Culture, Heritage and the Gaeltacht, Dublin.
5 Gill, L., McManus, A., Tiernan, E. and Newton, S. 2019. Rockabill Tern Report 2019. BirdWatch Ireland Seabird Conservation Report, Kilcoole.
6 Hill, R.W., Morris, N. G., Bowman, K. A., and Wright, D. 2019. The Isle of Man Seabird Census: Report on the census of breeding seabirds in the Isle of Man 2017-18. Manx BirdLife. Laxey, Isle of Man.
7 Veron, M. and Veron, C. 2016. Seabird Count 2015; monitoring the status of Guernsey’s Seabirds. La Société Transactions, Channel Islands.
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; Department of Agriculture, Environment and Rural Affairs (Northern Ireland); Department of Environment, Food and Agriculture (Isle of Man); Department of the Environment, Climate and Communications (Republic of Ireland); JNCC; Manx Birdlife; Manx National Heritage ; The National Trust; National Trust for Scotland; Natural England; Natural Resources Wales; NatureScot; The Royal Society for the Protection of Birds; Scottish Wildlife Trust; Seabird Group; Shetland Oil Terminal Environmental Advisory Group; States of Guernsey Government; UK Centre for Ecology and Hydrology. More about the SMP partners >>
Image of Common 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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