Black-legged kittiwake (Rissa tridactyla)

Figure 1. Trends in abundance index of black-legged kittiwake 1986–2019 for the UK (red), Scotland (blue), England (black) and Wales (orange). 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 regional variation in black-legged kittiwake abundance trends since monitoring began in 1986, although all show varying levels of decline (Figure 1). The number of sites monitored annually in the UK (n=~250 colonies) and Scotland (n=~146 colonies) are similar, hence their trend lines match closely. The indices for the UK and Scotland declined rapidly from 1995, reaching their lowest values in 2013. Since then they have increased overall, although are still at least 50% below the 1986 baseline. The England and Wales trends have declined less severely over the same period. Abundance data submitted to the SMP of black-legged kittiwake in Northern Ireland, Republic of Ireland, the Channel Islands, and the Isle of Man are sparse; thus, no meaningful trend can be provided.

Productivity

Figure 2. Trend in breeding productivity (no. of chicks fledged per pair) of black-legged kittiwake 1986–2019 for the UK (red), Scotland (blue), England (black), Wales (orange) and all-Ireland (green). Further details can be found in the sections below. Based on SMP data; view the methods of analysis.

There has been pronounced regional variation in black-legged kittiwake productivity trends since monitoring began in 1986, although all trends show varying degrees of decline until 2008. Since then, the number of chicks fledged per pair in each region has increased slightly (Figure 2).

Figure 3. Trend in UK abundance index (solid line) of black-legged kittiwake 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

Combining census results with the SMP sample indicates that, while black-legged kittiwake numbers increased by approximately 24% between the late-1960s and mid-1980s (see UK population estimate table above), there are probably now 50% fewer than in the late 1960s (Figure 3). The UK kittiwake abundance index has declined rapidly since the early 1990s and by 2013 had decreased to 70% below the 1986 baseline, although appears has been slowly increasing since then. In 2019, the index was 52% below the baseline.

Table 1 shows how numbers have changed at some of the most important colonies in the Special Protection Area (SPA) network in the period since they were surveyed for Seabird 2000. Some of the largest declines recorded have been at colonies in the north and north-west. Colonies in Shetland have been declining at an average rate of 13.8% per annum; those in Orkney at 10.0%; and those in the North West at 11.9%. Along the east coast of the UK, and in the Minch and Irish Sea, the average rate of decline has been slower, at an estimated 2.1%, 3.4% and 3.9% per annum, respectively.

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of black-legged kittiwake 1986–2019. Based on SMP data; view the methods of analysis.

SMP sample data show that average productivity between 2014 and 2019 (0.62 chicks fledged per pair) was higher than it had been for a number of years. This increase in productivity could contribute to a larger number of birds being recruited to the breeding population in future years.

Black-legged kittiwake productivity in the UK declined markedly between 1986 and 2008 but has increased since then (Figure 4). This appears to have been related to declines in abundance of their sandeel prey which in certain regions is negatively correlated with sea surface temperatures¹ (SST) that have risen due to climate change. Productivity (and adult survival) was also negatively affected by the presence of a sandeel fishery that operated off SE Scotland from 1990 to 2000^2,3,4.

Lesser sandeels Ammodytes marinus constitute a significant proportion of kittiwake diet during the breeding season^{2,4,5,6,7,8,9,10}. Consequently, kittiwakes are highly sensitive to fluctuations in sandeel abundance. Warming of waters around the UK over the last three decades (between 0.1 and 0.5 ̊C per decade¹¹) has led to substantial changes in species composition and abundance at lower trophic levels^{12,13,14,15,16}, with detrimental effects on sandeels^14,16. Lesser sandeels are sensitive to changes in SST^1,14 and have restricted capacity to shift distribution^17,18. Species such as black-legged kittiwake are particularly vulnerable to food shortages as they can only take prey when it occurs at or near the surface of the sea, unlike diving species such as auks which have access to a greater variety of prey in the water column. This has exacerbated the effects of low prey abundance⁵.

In the North Sea, significant changes in the timing of key life history events of sandeels and their copepod prey¹⁹ have resulted in changes in their length and, therefore, energy value^16,20,21,22. Kittiwakes and other seabirds have not kept pace with these changes and, as a result, are now relying on prey of lower calorific value during the chick rearing period when energy demands are highest^16,23. This trophic mismatch appears to be prevalent within the North Sea pelagic food web; however, important regional differences are apparent, with these effects appearing stronger in the North Sea than the Irish Sea, the Celtic Sea and the English Channel^24,25,26. In addition, over-winter survival of adult black-legged kittiwakes breeding in eastern Scotland was lower following winters with higher SST, and breeding success one year later was reduced^2,27,28,29. This relationship, though varying greatly between colonies, has also been demonstrated at other kittiwake colonies along the North Sea coast^30,31. There is evidence that breeding success is positively related to sandeel abundance^32,33. Effects of climate on these bottom-up processes is likely to intensify if – as predicted – warming continues.

In addition to kittiwake productivity being correlated with sandeel availability^33,34, diet composition can also impact on chick growth and survival. A study in Norway showed that whilst adult black-legged kittiwakes are foraging on energy rich prey items in both the oceanic habitat and the fjords, a diet dominated by sandeel Ammodytes spp. resulted in higher growth and survival of chicks compared to a diet consisting of mesopelagic fish³⁵.

Figure 5. Trend in abundance index (solid line) of black-legged kittiwake in Scotland, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

Census results indicate black-legged kittiwake numbers in Scotland changed little between Operation Seafarer and the Seabird Colony Register (SCR) but fell thereafter with Seabird 2000 recording 21% fewer AON than the SCR. Data collected by the SMP indicate a steady decline since the late 1980s, with the index in 2019 60% below the 1986 baseline (an increase since its lowest point in 2013 at 76% below).

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of black-legged kittiwake in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.

Black-legged kittiwakes breeding in Scotland underwent a sustained decline in productivity from 1986, culminating in a very poor breeding season in 2008 when, on average, only one chick fledged per five pairs. Productivity between 2009 and 2019 was higher than it had been for several years, possibly due to increased availability of prey. In 2019, average productivity in Scotland was 0.80 chicks fledged per pair and the Isle of May recorded an above average season with 0.89 chicks fledged per pair¹⁰.

Between 1980 and 2010, SST in the Forth and Tay region increased significantly; however, in the last decade. cooler conditions have led to a decrease in SST⁵. This decrease in SST is thought to have had a direct positive effect on the number of black-legged kittiwake chicks that were fledged on the east coast of Scotland between 2009 and 2019 (average 0.83) which is apparent in Figure 6.

Figure 7. Trend in abundance index (solid line) of black-legged kittiwake in England, 1986–2019 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.

In England, the abundance index peaked in 1992 (cf. 1989 in Scotland). A decline occurred between 1995 and 1998 but, since then, the index has been relatively stable (Figure 7). The abundance index for England did not decline to the extent seen in Scotland and, in 2019 was 20% below the 1986 baseline. Another difference between the two countries is that between Operation Seafarer and the Seabird Colony Register, the English population more than doubled, while in Scotland it showed little change. However, the proportional population decline in England between the Seabird Colony Register and Seabird 2000 censuses was nearly twice as much as Scotland.

In contrast, Wales (n=20 colonies) and all-Ireland (n= 39) showed an early decline during the late 1980s and early 1990s, briefly recovered between 1993 and 1995, and declined thereafter but to a lesser degree compared to Scotland and England.

Recent studies^24,25,26 have indicated that regional variation in the effects of climate change on seabirds is apparent. In particular, there is growing evidence that climate impacts recorded in the North Sea are not replicated elsewhere. The negative effect of winter SST on black-legged kittiwake productivity outlined in the Scotland section above is not apparent throughout the UK³⁰ and analyses of the Irish Sea, the Celtic Sea and the English Channel have found only weak climate effects on seabird demography^25,26.

Productivity

Figure 8. Trend in productivity (no. of chicks fledged per pair) of black-legged kittiwake in England, 1986–2019. Based on SMP data; view the methods of analysis.

Productivity at English colonies, usually more successful than those in Scotland, decreased from 1986 to its lowest point in 2004 but has increased since then. Monitored colonies were least productive in 1998, 2004 and 2008. However, 2009 to 2012 inclusive were four of the more successful breeding seasons in the last decade. Productivity in 2019 was 0.58 chicks fledged per pair.

The relatively high productivity in England compared to Scotland may explain why the species' abundance has not fallen to the same extent. The reasons for higher mean productivity in English colonies could be related to recent findings about weaker climate effects on seabird demography which is not apparent throughout the UK^25,26,28.

Figure 9. Trend in abundance index (solid line) of black-legged kittiwake in Wales, 1986–2019 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.

National censuses found that black-legged kittiwake numbers in Wales had changed little between Operation Seafarer and Seabird 2000, but more recent measures of abundance using the SMP dataset suggest the population is now lower than in 1969-70. Factors affecting colonies in Scotland, appear to have had less impact in Wales but the reasons for this are not well understood. In common with the Scottish and English trends, Wales reached its lowest point in 2013, at 58% below the baseline (Figure 9). Currently, the index is 39% below the 1986 baseline; 9 colonies surveyed in 2019 held 2,548 AON, 27% fewer than were recorded during Seabird 2000 (3,492 AON).

Productivity

Figure 10. Trend in productivity (no. of chicks fledged per pair) of black-legged kittiwake in Wales, 1986–2019. Based on SMP data; view the methods of analysis.

Productivity of black-legged kittiwakes in Wales has fluctuated widely over the recording period with no clear trend, although it appears to have been depressed over much of the last decade. In general, Welsh colonies are less productive than those in England and there are more years when productivity falls below 0.50 chicks per pair compared with both England and Scotland. Productivity in 2017 was the lowest on record, although the details as to why are limited. On Skomer, one of four colonies regularly monitored, low breeding success of 0.33 chicks fledged per pair in 2017 was partly attributed to above average rainfall (twice the normal amount for Pembrokeshire in June)³⁶. In 2019, black-legged kittiwake productivity in Wales had increased to 0.51³⁷.

Figure 11. Trend in productivity (no. of chicks fledged per pair) of black-legged kittiwake throughout Ireland, 1986–2019. Based on SMP data; view the methods of analysis.

In Ireland, there are few years when black-legged kittiwake productivity is higher than 0.80 chicks fledged per pair. A particularly poor breeding season was recorded in 2008, probably due to the very wet summer that year impacting on young birds in nests. Since then, productivity has increased (Figure 11). The practice of pair-trawling of spawning inshore sprat has increased in recent years and a herring fishery is operating in the Irish and Celtic Seas. The existence of these fisheries, operating within the foraging areas of black-legged kittiwakes, may have an impact on the productivity of this species, particularly if the fisheries target young sprat⁴¹.

Figure 12. Abundance of black-legged kittiwake on the Calf of Man, 1986–2017.

During the Operation Seafarer census, 908 black-legged kittiwake AON were recorded on the Isle of Man. The Seabird Colony Register census recorded 1,376 AON, an increase of over 50%. The population had declined 25% (1,045 AON) by Seabird 2000, and a further 36% (672 AON) by the most recent census in 2017. During Seabird 2000, 182 black-legged kittiwake AON were recorded on the Calf of Man but, by 2017, the colony was extirpated (Figure 12). The stronghold of breeding kittiwakes on the Isle of Man is located at the Sugar Loaf, which held 580 AON during Seabird 2000 and had increased 5% by 2017 to 553 AON⁴². No abundance data have been submitted to the SMP since 2017.

Productivity

No black-legged kittiwake productivity data from the Isle of Man have been submitted to the SMP recently but, between 1986 and 2005, an average of 0.27 chicks were fledged per nest each year.

Figure 13. Percentage of sandeels (by weight) in the diet of young black-legged kittiwakes at the Isle of May, 1987–2019.

Breeding success of black-legged kittiwakes on the Isle of May, which had declined whilst the fishery was in operation, increased during the period 2000–2003, suggesting that the industrial sandeel fishery on the Wee Bankie had adversely affected this species³². However, from 2004, breeding success and adult return rate declined, despite the fishing ban still being in operation. Over the same period, there were changes in kittiwake diet, with the sudden appearance of snake pipefish Entelurus aequoreus the most dramatic. Although numerous, this prey is difficult to digest and of poor nutritional value⁴⁴. Pipefish have now disappeared from the diet of seabirds on the Isle of May.

Sandeels (Ammodytes sp.) are an energy-rich food compared with some alternative prey and the proportion of sandeels by weight can be used as a measure of 'diet quality'. Figure 13 shows that the percentage of sandeels in the diet of black-legged kittiwakes on the Isle of May in south-east Scotland has been high for the majority of years but between 2012 and 2016 declined considerably (from 87% to 54%). A recent study on Isle of May seabirds found that the proportion of sandeels in their diet has declined in both the summer and winter over the last three decades, linked to increasing SST⁵. The amount of sandeels fed to chicks, (particularly the size 1+ group fish) has decreased, and a shift to 0 group fish has occured⁵. Black-legged kittiwake chicks were increasingly fed clupeids, predominantly sprat Sprattus sprattus, indicating a potential reduction in their reliance on sandeels and a diet composition shift⁵. This is reflected in the low breeding success values recorded on the Isle of May between 1990 and 2013. Only recently have kittiwakes produced more than one chick fledged per pair¹⁰. Therefore, sandeel abundance might be still important to black-legged kittiwakes but less so when there are alternative prey in poor sandeel years. Sandeels regained the status of the main food of black-legged kittiwakes in 2019. The proportion of sandeel in the diet (93% by biomass) was well above average, whereas the proportion of clupeid (3% by biomass) was below average¹⁰.

Figure 14. Annual return rate of black-legged kittiwakes breeding on the Isle of May, 1987–2019.

The return rate of black-legged kittiwakes on the Isle of May (Figure 14) declined between 1986 and 1998 and then typically fluctuated at a low level until 2009, after which it increased. Particularly low rates occurred from 2007 to 2009 but, in 2010, the return rate (89.0%) was the highest since 1989 and well above the average for the period prior to this (77.7, 95% CI = 74.5–81.0). In 2019, the return rate of black-legged kittiwakes (80.5%) was average (1986-2018 average = 80.0%, 95% CI =76.9–82.2).

Figure 15. Estimated adult survival rate of black-legged kittiwakes on Skomer, 1986–2018.

Survival rate of kittiwakes on Skomer (Figure 15) declined between 1991 and 1997 and then increased up to 2002, after which another decline is evident. The survival rate in 2007 was particularly low (70.4%). Over the period of the study (1978–2018), survival of breeding adults averages 0.86%. There continues to be a cyclic fluctuation in adult breeding survival over the course of the study, and maybe a long-term decline in survival rate, but this requires further analysis as part of a demographic study that draws together the population parameters measured on Skomer³⁷. The survival rate of breeding adult black-legged kittiwakes in 2017-18 was 0.88, a decrease from 0.96 in 2018.

The waters around the UK have been warming since the 1980s and recent studies have demonstrated links between kittiwake adult survival and climate^{38,45,46,47,48}. Adult survival is largely affected by winter mortality, and additionally by the carry-over effects of the previous breeding season. These effects may become increasingly important for all seabirds. Most climate models predict that future warming will be associated with increasing climate variability and hence frequency of extreme weather events^49,50. A recent study using geolocation data loggers to examine the non-breeding season distribution of kittiwakes found tagged birds from Skomer and Rathlin remained relatively near the colony but those from the Isle of May migrated more widely with the main areas of use split between the North Sea and the Central and West Atlantic⁵¹. Based on the longer distance from the breeding grounds, black-legged kittiwakes breeding on the Isle of May may, therefore, be at higher risk of winter mortality and lower adult survival based on environmental conditions in the West Atlantic. Any major changes in food availability in this part of the west Atlantic (e.g. projected changes in distribution of C. finmarchicus) could affect the entire Atlantic population⁵².