Common tern (Sterna hirundo)

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.

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.

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.

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.

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.).

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.

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.