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Common tern (Sterna hirundo)

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:

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International importance

UK Population

Biogeographic Population

% World Population

11,800 AON*

4.2 (ssp. hirundo)

2.2

*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)

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.

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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 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.

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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

common-tern-uk-ab.jpg

Figure 1. Trend in UK abundance index (solid line) of common tern 1986–2018 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 abundance trend 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 was 6% above the baseline in 2018, the highest value recorded to date. 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

common-tern-uk-prod.jpg

Figure 2. Trend in UK productivity (no. of chicks fledged per pair) of common tern 1986–2018. Based on SMP data; view the methods of analysis.

 

Like most of the other tern species, the productivity of common terns has fluctuated over the recording period and, although the species has never been as successful as roseate tern, seldom has it been as unproductive as Arctic tern. 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 tide 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

common-tern-scotland-ab.jpg

Figure 3. Trend in abundance index (solid line) of common tern in Scotland, 1986–2018 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 prolonged decline evident thereafter up to Seabird 2000 and beyond (with some fluctuation). In 2013, the index increases to the baseline recorded in 1986 before decreasing again to 26% below the baseline in 2015. It then rose again in 2016 and has remained stable between 18% and 22% above the baseline since then. The largest colonies of common terns in 2018 were at Leith Docks (514 AON), Invergordon Town and Docks (289 AON) and Sands of Forvie (278 AON). As for many tern species, maintaining population levels depends on the management at most breeding sites, with predator control, habitat creation and disturbance reduction being key to their success.

 

Productivity

common-tern-scotland-prod.jpg

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

 

Although productivity of common terns in Scotland has fluctuated somewhat over the recording period, there are very few years when the species has nested successfully; in most years, productivity fell below 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 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 2017, average productivity of common terns decreased to 0.4 chicks fledged per pair, although in 2018 had increased again to 0.57.

 

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

common-tern-england-ab.jpg

Figure 5. Trend in abundance index (solid line) of common tern in England, 1986–2018 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

 

Common tern numbers in England decreased by 24% between Operation Seafarer and Seabird Colony Register (SCR) censuses, in contrast to populations in Scotland and Wales, with Seabird 2000 recording approximately the same number as the SCR. The abundance trend in Figure 5 shows considerable fluctuation since Seabird 2000, although has usually been close to the 1986 baseline. In 2018, the index rose to its highest level to date, at 33% above the 1986 baseline, suggesting that the English common tern breeding population may now be larger than it was at the time of Seabird 2000. The largest colonies in England, between 2017 and 2018, were Coquet Island (1667 AON in 2018), Saltholme (291 AON in 2018) and Scroby Sands (250 AON in 2017). 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

common-tern-england-prod.jpg

Figure 6. Trend in productivity (no. of chicks fledged per pair) of common terns in England, 1986–2018. 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, with 2018 averaging at 0.73 chicks fledged per pair. Peak productivity was recorded in 1995 (1.02) however, it has seldom been high and, although poor in some years, common terns in England usually fare better than those breeding at Scottish colonies.

 

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

common-tern-wales-ab.jpg

Figure 7. Trend in abundance index (solid line) of common tern in Wales, 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 abundance of common terns in Wales has decreased steeply since a peak value in 2017 of 115% above the 1986 baseline was recorded. It dipped below the 1986 baseline between 2009 and 2013 and then increased until 2017 where it was 72% above the 1986 baseline, the third highest value recorded to date. In 2018, the index had decreased again but was still 52% above the baseline. The decline in 2017 was largely due to the abandonment of the largest colony at Shotton Steelworks where common terns last successfully bred in 2008 (624 AON). Data received from the same three colonies (Shotton Steelworks, The Skerries and Cemlyn Lagoon) in 2016, 2017 and 2018, recorded a total of 698, 820 and 662 breeding pairs respectively, figures which are likely to be fairly representative of the current Welsh population.

 

Productivity

common-tern-wales-prod.jpg

Figure 8. Trend in productivity (no. of chicks fledged per pair) of common terns in Wales, 1986–2018. Based on SMP data; view the methods of analysis.

 

Common terns at Welsh colonies are among the most productive in the UK, regularly fledging more than one chick per pair. Near complete failure was recorded in 1990 with the next poorest breeding season occurring in 2012. No real trend is apparent over the recording period as productivity has fluctuated widely. However, 2014 was the most productive breeding season since 1996 with 1.53 chicks per pair, with 2015 slightly lower at 1.32. For the first time since 2008, common terns bred at Shotton Steelworks in 2014 and fledged 2.1 chicks per pair. The following year was also a successful year, with an increase in breeding pairs and high productivity at 1.6 chicks fledged per pair. This was due to a combination of new fox-deterrent security measures around the Shotton Steelworks colony and an abundant food supply in the Dee estuary. Volunteers have also been tirelessly working to improve suitable nesting space for the terns at this site. The Shotton Steelworks colony has faced a number of threats over recent years, the greatest being the rapid expansion of black-headed gulls using the site and extensive weed growth which discourages the terns from nesting (Peter Coffey pers. comm.). On average this colony produced 1.33 chicks fledged per pair between 2014–2018.

 

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

common-tern-ni-ab.jpg

Figure 9. Abundance of common terns at six colonies in Northern Ireland, 1986–2018.

 

The number of common terns breeding in Northern Ireland increased by 55% between the Seabird Colony Register and Seabird 2000 censuses, from 1,096 to 1,704 AON. In 2006, the six largest colonies in the east of Northern Ireland (Cockle Island, Carlingford Lough, Strangford Lough and Copeland Island, Larne Lough and Belfast Lough ), held 2,369 AON (cf. 1,570 in 2000), representing an increase in the national population, although numbers at these colonies declined to 1,119 AON in 2018 (Figure 9). 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 extant 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 likely that the population of breeding common terns in Northern Ireland is now slightly larger than recorded during the last census.

 

Productivity

Productivity data for common terns in Northern Ireland showed no statistically significant variation over time. An average of 0.68 chicks were fledged per pair per year between 1999 (the first year the SMP has data for) and 2018. Common tern productivity has been recorded at Portmore Lough since 2014 and has averaged 1.05 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

common-tern-rockabill-ab.jpg

Figure 10. Abundance of common terns on Rockabill, 1986–2018.

 

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, Rockabill, numbers increased almost exponentially from 1986 onwards (Figure 10), possibly due to immigration from other colonies around the Irish Sea, although have now become more stable with 2,039 AON recorded there in 2018 (cf. peak of 2,191 in 2011). A recent seabird census of the Republic of Ireland recorded 5,058 AON, an increase of 104% since Seabird 20007. This increase in the national population may have begun in the late 1990s. There is little doubt that the strong national increase in the common terns population is driven by the long-standing and on-going direct conservation actions at Lady’s Island5 Lake and Rockabill6. There were insufficient data submitted to the SMP to allow a national trend to be generated.

 

Productivity

common-tern-ireland-prod-1986-2014.jpg

Figure 11. Trend in productivity (no. of chicks fledged per pair) of common terns in the Republic of Ireland, 1986-2014. Based on SMP data; view the methods of analysis.

 

Productivity in the Republic of Ireland was very high in the early 1990s but, since 1998, has been more typical of levels recorded in Britain. Productivity has been very low, seldom above 0.50 chicks fledged per pair, in several years since 2007. No data on common tern productivity has been provided to the SMP since 2014.

 

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

common-tern-ireland-ab.jpg

Figure 12. Trend in abundance index (solid line) of common tern throughout Ireland, 1986–2015 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 total Irish population of common terns during Seabird 2000 was similar to that recorded by Operation Seafarer 30 years previously, although numbers decreased between these two censuses. The common tern abundance trend for the whole of Ireland shown in Figure 12 was upward between 1986 and 2006, then increased from Seabird 2000 until 2014 but has declined since then. Rockabill, the largest colony in the Republic of Ireland, has increased substantially during the last 28 years from 70 AON in 1986 to over 2,000 AON in 2018. However, in Northern Ireland, numbers had similarly increased until a peak in 2006 but then declined steeply until 2015. A recent seabird census of the Republic of Ireland recorded 5,058 AON, an increase of 185% since Seabird 2000. Therefore, it is very likely that the all-Ireland population of common terns is increasing.

 

Productivity

The trend in productivity of common terns throughout Ireland is similar to that shown for the Republic of Ireland, where 54% of the data have been collected however, only 10 productivity records have been supplied to the SMP between 2009 and. Productivity was very high in 1990 but very low in seven years in the last decade. Between 1998 and 2006 productivity appears to have been relatively stable, albeit with some fluctuation, but apparently has declined since then. No data from the Republic of Ireland on common tern productivity was provided to the SMP since 2014.

 

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

% change since previous census

n/a

+600

0

*AON = Apparently Occupied Nests

 

Breeding abundance

Only a few pairs of common terns have bred on the Isle of Man. One pair was found during Operation Seafarer which had risen to seven pairs during the Seabird Colony Register. No pairs were recorded during Seabird 2000 and none have been reported nesting since8.

 

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. Prior to this, numbers had doubled from 107 AON in 1969-70. On Jersey common terns breed on the islands of Les Ecrehous and Les Minquiers. On Guernsey, a colony on Alderney has been regularly monitored since Seabird 2000 with numbers increasing from 20 to 32 AON. During a recent Channel Island Seabird Census 250 AON were counted, an increase of 44% compared to Seabird 20009.

 

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.

 

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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 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 Wright, D and Clarke, A. 2014. Cemlyn wardens report. Unpublished NWWT report, North Wales Wildlife Trust.

4 Wright, D and Clarke, A. 2015. Cemlyn wardens report. Unpublished NWWT report, North Wales Wildlife Trust.

5 Daly, D., Murphy, B., O’Connor, B. and Murray, T. 2018. Lady’s Island Lake Tern Report 2018. Unpublished report.

6 Acampora, H., Ní Dhonnabháin, L., Miley, D. and Newton, S. 2018. Rockabill Tern Report 2018. BirdWatch Ireland Seabird Conservation Report.

7 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.

8 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.

9 Veron, M. and Veron, C. 2016. Seabird Count 2015; monitoring the status of Guernsey’s Seabirds. La Société Transactions, Channel Islands.

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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 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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SMP Report 1986–2018

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