Common Tern Sterna hirundo

Status;  International importance;  Population estimates;  Distribution;  Annual abundance/ productivity; Phenology/diet/survival

 

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Description

Common Tern vignette

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)

EC Birds Directive - listed in Annex 1 and as a migratory species

(further information on Conservation Designations for UK Taxa)

Amber listed in Birds of Conservation Concern in Ireland 2014-2019 (2014 update)

 


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.

 


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

 

All terns breeding in the British Isles 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 (PDF, 1.1 mb).

 

An interactive map is available on the NBN Gateway, where you can filter to display only the Seabird 2000 data.  For more recent, but less comprehensive, coverage view the distribution on the NBN with all available contributing datasets.

 

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% CLs 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.   

 



 

Breeding abundance

 

UK common tern abundance trend

Figure 1: Trend in UK abundance index (solid line) of common tern 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Between the first census in 1969-70 and second census in 1985-88, common terns increased by 9%, although a fall in numbers was recorded by Seabird 2000. Since then, the trend in abundance index appeared to be upward, albeit with some fluctuation, until 2006, but in recent years a decline is obvious. In 2015, the index has produced an upward trend based at 19% below the 1986 baseline. Trends at finer spatial scales have varied considerably and these are likely to reflect varying pressures facing common terns in different habitats across their wide geographic range. Increased predation by non-native species such as American mink Neovison vison1 and the native fox Vulpes vulpes have caused declines in some areas, but conservation management to ameliorate these problems is being undertaken. The species has also benefited from habitat creation by man in the form of gravel pits, tern rafts in reservoirs, islets in industrial lagoons and structures in ports, and from maintenance of habitat on reserves by control of vegetation succession and gull competition. Maintaining the population is likely to depend on the continuation of such management in perpetuity.

 

Productivity

 

UK Common tern productivity trend

Figure 2: Trend in UK productivity (no. of chicks fledged per pair) of common tern 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

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, though 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.

 

 

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

 

Scotland common tern abundance trend

Figure 1: Trend in abundance index (solid line) of common tern in Scotland, 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Numbers of common terns in Scotland during the Seabird Colony Register were 58% higher than during Operation Seafarer but by Seabird 2000 had fallen by 29%. The index above perhaps 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). The abundance index reaches a low point in 2011 and in 2015, based at 61% and 56% below the 1986 baseline, respectively. As for many tern species, maintaining population levels depends on management of breeding sites. Predator control, habitat creation, competition for nesting sites and reducing disturbance are key management factors at most breeding localities.

 

Productivity

 

Scotland Common tern productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of common terns in Scotland, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Although the 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 falls 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 at colonies 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.

 

 

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

 

England common tern abundance trend

Figure 1: Trend in abundance index (solid line) of common tern in England, 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Common tern numbers in England decreased by 24% between Operation Seafarer and the Seabird Colony Register, in contrast to populations in Scotland and Wales. Seabird 2000 recorded approximately the same number as during the 1985-88 survey. The abundance trend above shows much fluctuation since Seabird 2000 and suggests numbers in 2014 and 2015 may be similar to that recorded during Seabird 2000. The species has benefited from habitat creation by man in the form of gravel pits, tern rafts in reservoirs, islets in industrial lagoons and structures in ports, and from management to maintain habitat on reserves by control of vegetation succession and to reduce competition with, and predation by, gulls.

 

Productivity

 

England Common tern productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of common terns in England, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

The productivity of common terns in England has fluctuated since 1986 but appears to have been in decline since the mid-1990s, with 2015 averaging at 0.40 chicks fledged per pair. Peak productivity was recorded in 1995 (1.08) 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.

 

 

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

 

Wales common tern abundance trend

Figure 1: Trend in abundance index (solid line) of common tern in Wales, 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

The abundance of common terns in Wales has decreased steeply since 2007, falling by two-thirds and actually dipping below the 1986 baseline from 2009 to 2013. This steep decline was largely due to the abandonment of the largest colony at Shotton Steelworks (Clwyd) where common terns last successfully bred in 2008 (624 AON); between 2009 and 2013, common terns arrived at the site in varying numbers but either left before breeding had commenced, or the few pairs attempting to nest failed early. However, in 2014 and 2015, common terns nested successfully with a total of 225 and 344 nests. The population, as shown by national census data, has never been large and there are few colonies, but numbers increased during each subsequent census after Operation Seafarer. Data received from the same three colonies (Shotton Steelworks (Clwyd), The Skerries and Cemlyn Lagoon (both Gwynedd)) in 2014 and 2015, held a total of 561 and 684 breeding pairs respectively, figures which are likely to be representative of the entire Welsh population (some small colonies holding a few pairs may not have been surveyed). 

 

Productivity

 

Wales Common tern productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of common terns in Wales, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

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 1.56 chicks per pair, with 2015 slightly lower at 1.34. In the colony at Cemlyn (Gwynedd), the first incubating adult was seen on 25th May, only two days later than 20143. However, despite reasonable weather conditions at this colony, only 40 chicks were thought to have fledged from 67 nesting pairs. For the first time since 2008, common terns bred at Shotton Steelworks in 2014 and fledged 2.1 chicks per pair. 2015 also proved to be a successful year with increased numbers of pairs, high productivity at 1.6 chicks fledged per pair and low mortality. This was due to a combination of new fox-deterrent security measures around the colony and an abundant food supply in the Dee estuary.

 

 

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 abundance graph 1986-2015

Figure 1: Abundance of common terns at four colonies in Northern Ireland, 1986-2015.

 

The number of common terns breeding in Northern Ireland increased by 55% between the Seabird Colony Register and Seabird 2000 to 1,704 pairs. In 2006, the six largest colonies (Cockle Islandm Carlingford Lough, Strangford Lough and Copeland Island (all Co. Down), Larne Lough and Belfast Lough (both Co. Antrim)), in the region held 2,369 pairs (cf. 1,570 in 2000) - overall there had been an increase in the national population - but numbers at these colonies have now declined with only 1,319 AON recorded in 2015 (Figure 1). A further two colonies, where monitoring is less frequent, held 114 nests; so eight colonies surveyed totalled 1,436 breeding pairs which is an increase on the 2014 total of 989. Few other extant colonies were found during the comprehensive coverage of Seabird 2000 and these are unlikely to hold more than 100-200 pairs in total. Hence, it seems the national population is now much lower (maybe 15% lower) than that recorded during the last census.

 

Productivity

 

Productivity data for common terns in Northern Ireland showed no statistically significant variation over time. An average of 0.57 chicks was fledged per pair per year between 1999 (the first year the SMP has data for) and 2015. Breeding success was high again at Portmore Lough (Co. Antrim) in 2015 where 84 nests fledged 101 young (1.20 chicks per nest).

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 2,804 1,574 2,485
% change since previous census    N/a -44 +58

 

* AON = Apparently Occupied Nests

 

Breeding abundance

 

Common tern Rep Ireland abundance 2015

Figure 1: Abundance of common terns on Rockabill (Co. Dublin), 1986-2015.

 

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 pairs than during the first census. However, increases have been recorded at the main colonies over the last decade. At the largest, Rockabill (Co. Dublin), numbers increased almost exponentially from 1986 onwards (Figure 1), possibly due to immigration from other colonies around the Irish Sea, with 2,153 pairs recorded there in 2014 (cf. peak of 2,191 in 2011). Rockabill is just one of at least 30 colonies in the country; it and two other colonies monitored in 2014 held a total of 3,451 pairs - more than was recorded during any of the national censuses. No data was submitted to the SMP for the 2015 breeding season.

 

Productivity

 

Eire Common tern productivity trend

Figure 2: 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 (PDF 158 kb).

 

Productivity in the Republic of Ireland was very high in the early 1990s, but since 1998 has been more typical of levels recorded in the countries of the British Isles. Productivity has been very low, seldom above 0.50 chicks fledged per pair, in several years since 2007. No data on common tern was provided to the SMP for 2015.

 

 

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

 

All-Ireland common tern abundance trend

Figure 1: 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 (PDF 158 kb).

 

The total Irish population of common terns during Seabird 2000 was similar to that recorded by Operation Seafarer 30 years previously, although numbers decreased in between these two censuses. Colonies sampled for the SMP show that the trend in abundance has generally been upward since 1986, although continuing post Seabird 2000, it appears to have levelled off in recent years. Rockabill, the main colony in the Republic of Ireland, has increased substantially during the last 28 years from 70 pairs in 1986 to over 2,000 pairs. However, in Northern Ireland, numbers had similarly been increasing but reached a peak in 2006 and have been declining steeply since then until 2015. Nevertheless, colonies sampled in 2014 (about one-third of those known during Seabird 2000) held almost 4,361 pairs - a similar amount to that recorded for the whole of Ireland during Operation Seafarer and Seabird 2000. No data on common tern was submitted by the Republic of Ireland to the SMP for the 2015 breeding season.

 

Productivity

 

All-Ireland Common tern productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of common terns in throughout Ireland, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

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 breeding success records have been supplied to the SMP since 2009. 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.

 

 

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 7 N/a
% change since previous census    N/a +600 N/a

 

* 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 in the decade since.

 

Productivity

 

No systematic data on the productivity of common terns on the Isle of Man have been submitted to the SMP.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 107 227 174
% change since previous census    N/a +112 -23

 

* AON = Apparently Occupied Nests

 

Breeding abundance

 

Common terns are the only species of tern to breed in the Channel Islands. During Seabird 2000, 174 pairs were recorded which represented a decline of 23% since the Seabird Colony Register. Prior to this, numbers had doubled from 107 pairs in 1969-70. The small colony on Alderney (20 AON in 2000) appears to be the only colony visited since Seabird 2000; 15 AON were recorded there in 2005, 11 in 2007, five in 2012, 14 in 2013 and 25 in 2014 (a peak count since 1986). No data were submitted to the SMP in 2015. With so little data collected during the last decade, the current status of common terns on the Channel Islands is unknown.

 

Productivity

 

Data submitted to the SMP on the productivity of common terns in the Channel Islands are sparse; thus, no meaningful average productivity value can be given.

 

 


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

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

 

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; 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 Environment Agency; 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.