Guillemot (Uria aalge)

Figure 1. Trends in abundance index of guillemot 1986–2019 for the UK (red) and Scotland (blue). 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 guillemot abundance trends during the monitoring period (Figure 1). The UK and Scotland trends were closely matched until 2001 but, since then, have drifted apart, with Scotland remaining below the baseline between 2004 and 2016 and the UK trend above. The number of guillemot colonies monitored frequently is only sufficient to allow the production of meaningful abundance trends for the UK and Scotland. In 2019, a large number of small colonies and a few large colonies (>10,000 individuals) were counted and included in the abundance index analysis. Guillemot numbers at the majority of these colonies had increased and this has led to a pronounced increase in the UK 2019 trend index value. This should be treated with caution as the large number of smaller colonies included in the sample are likely to have had a disproportionate influence on the index.

Productivity

Figure 2. Trends in guillemot productivity (no. of chicks fledged per pair) 1986–2019 for 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.

Most SMP data on guillemot productivity in the UK have been collected at Scottish colonies, so these trends are closely matched (Figure 2). Productivity in Scotland was relatively stable between 1986 and 2002, after which it declined rapidly until 2007 and has since then recovered. The England trend fluctuated between 1986 and 2007 but has been more stable since then. The number of colonies monitored frequently is only sufficient to allow meaningful productivity trends to be produced for the UK, Scotland and England.

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

National census results show that the UK guillemot population increased by over 130% between the Operation Seafarer (1969-70) and Seabird 2000 (1998–2002) censuses. The UK abundance index for guillemot (Figure 3) declined between 1986 and 1990 to its lowest point during the monitoring period, at 28% below the baseline. It then increased gradually and, by 2001, was 41% above the baseline. The trend then remained relatively stable until 2015 when it began to increase again, reaching its highest value to date in 2019, at 98% above the 1986 baseline. In 2019, a large number of small colonies and a few large colonies (>10,000 individuals) were counted and included in the abundance index analysis. Guillemot numbers at the majority of these colonies had increased and this has led to a pronounced increase in the UK 2019 trend index value. This should be treated with caution as the large number of smaller colonies included in the sample are likely to have had a disproportionate influence on the index.

Table 1 (below) shows how guillemot numbers have changed at important colonies within the Special Protect Area (SPA) network since the Seabird 2000 census. The largest declines recorded have been at SPAs in Scotland, while those in England, Wales and Northern Ireland have all increased. In Scotland, Fowlsheugh is the only SPA to have increased since Seabird 2000.

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

After remaining relatively stable from 1986 to 2002, UK guillemot productivity (Figure 4) declined steeply until 2007, by which time a mean of just 0.23 chicks per pair were fledged. Productivity has increased since then, although values recorded between 2009 and 2019 are still lower than those recorded prior to 2002. In 2019, productivity was 0.62 chicks fledged per pair.

The decline in productivity coincided with food shortages and primarily affected colonies in the north and east of the UK where sandeels (rather than sprats and their relatives) are the main prey. Detailed studies on the Isle of May found reduced energy content in fish brought to chicks at this time¹ and that the proportion of sandeels in the diet had decreased at east coast colonies over the last 15–30 years, with sprat mainly fed to chicks since 2000². This would appear to be related to declines in abundance of their sandeel prey which in certain regions is negatively correlated with sea surface temperatures that have risen due to climate change^2,3,4. However, in recent years, sandeels have been more abundant at some colonies and this has coincided with increased productivity, although productivity at colonies where the predominant prey species is sprat has also been high.

Figure 5. Trend in abundance index (solid line) of guillemot in Scotland, 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 census data show an increase of 82% in the Scottish guillemot breeding population between 1969-70 and 1985-88, with a further increase of 24% up to the time of Seabird 2000. The population trend for guillemot in Scotland (Figure 5) was stable during the early 1990s, after which it climbed slightly over a few years before levelling off. After Seabird 2000 (1998–2002), the index declined and was lower than the 1986 baseline from 2004 to 2016. Since then, it has risen and, in 2019, was 18% above the baseline.

However, such a broad scale picture masks regional differences. For instance, numbers of guillemots in study plots on mainland Shetland have been falling at a considerable rate since peak figures (as measured post-1986) were recorded in 2000; plots held 48% fewer guillemots in 2019, although numbers have been slowly increasing since 2009⁵. Data from study plots between 2000 and 2018 indicate declines of 41% on Handa and 24% at St. Abb's Head but an increase of 22% at Fowlsheugh. Recent increases in productivity across Scotland, coupled with increased return rates (assuming measurements of this at the Isle of May (Figure 6, below) are typical of what is happening elsewhere), may lead to future increases throughout Scotland.

Productivity

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

Productivity was relatively stable until 1996, falling slightly thereafter with a steep decline from 2003 to 2007 when it reached its lowest value of 0.19 chicks fledged per pair. Declines in productivity primarily affected colonies in the north and east (e.g. Shetland and Isle of May), coinciding with shortages in sandeels, the main prey for guillemots breeding in those areas. Overall, productivity has increased in the last few years across Scotland, when sandeels were apparently more abundant near some colonies. However, some regions have still recorded very low levels of productivity during the last few years. For example, from 2011 to 2013, mean productivity in Shetland was 0.14 and in Orkney 0.19, compared to 0.62 in north-west Scotland and 0.72 in south-east Scotland. In 2018 and 2019, guillemots had a relatively successful season, although values recorded were still lower than the years prior to 2000. In 2019, guillemots fledged an average of 0.33 chicks per pair in Shetland; 0.30 in Orkney; 0.58 in north-west Scotland; and 0.70 in south-east Scotland.

At Sumburgh Head, the median laying date in 2019 was four days earlier than in 2018 (Figure 11). Productivity in 2019 was 0.76 chicks fledged per apparently incubating pair which was above the long-term mean of 0.54 (1989–2019)⁵. It was also the fourth highest recorded since 1989 and 41% higher than in 2018. At Sumburgh Head, large gull predation was likely to have been a contributing factor to chick losses in 2017⁷ and 2018⁶ but not 2019⁵.

Figure 7. Trend in productivity (no. of chicks fledged per pair) of guillemot in England, 1986–2019. Based on SMP data; view the methods of analysis.

Productivity at the few English colonies studied (Bempton Cliffs, Berry Head, Lundy Island, Farne Islands and St. Aldhelm’s Head) has been relatively high during the monitoring period compared to Scotland. However, lower levels of productivity occurred in 1986, 1997, 2006 and 2007. The reasons for this are not always reported but, in 1997, severe gales at the end of June caused major losses of guillemot eggs and chicks at colonies on the east coast of England.

In 2019, an average of 0.66 and 0.68 chicks per pair were fledged at Flamborough Head and Bempton Cliffs, and Lundy Island NNR respectively.

Figure 8. Abundance of guillemot on the Calf of Man, 1986–2019.

The number of guillemots breeding on the Isle of Man increased four-fold between Operation Seafarer and Seabird 2000. Few colonies have been regularly surveyed since then, other than on the Calf of Man, which held c. 15% of the total Isle of Man guillemot population during the SCR (350 individuals) and c. 10% during Seabird 2000 (416 individuals) although both figures could be underestimates as counts were done from land only. A decline occurred at the colony after Seabird 2000, with recent surveys recording between 25% (e.g. 2009, 2012, 2014, 2015 and 2017) and 50% (e.g. in 2010 and 2013) of the census total. In 2019, 191 individual guillemots were recorded at the Calf of Man. In 2017, a seabird census of the Isle of Man recorded a total of 5,217 individual guillemots, an increase of 14% since Seabird 2000. The most significant increase was found to have occurred at the Anvil-Sugarloaf Chasms, which had increased from 707 individuals in 1985-86 to 3,848 individuals (mean count) by 2017¹⁴.

Productivity

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

Figure 9. Median laying date of guillemots on Skomer (blue dots) and Sumburgh Head (red dots), 1991–2019. Skomer data from 1991–2013 reproduced with kind permission of Professors Tim Birkhead (University of Sheffield/National Resources Wales¹⁶). Sumburgh data reproduced with kind permission of Will Miles (SOTEAG⁵).

Figure 10. Percentage of sandeels (by weight) in the diet of young guillemots at the Isle of May, 1987–2019.

Feeding watches at Sumburgh Head (Shetland) throughout the main chick-rearing period (mid-June to mid-July) in 2019, identified the main prey delivered to chicks as gadoids (92.6%) and sandeels (6.4%), which combined account for 99% of identified food items each year⁵. This contrasts with 2017, when gadoids comprised 50.7% of chick diet and sandeels 46.4%7. Years when few sandeels are delivered to chicks saw an increase in gadids and vice versa. At what point this shift from a diet dominated by sandeels took place is unknown, as data at Sumburgh Head have only been collected since 2007; the last year when sandeels made up more than 50% of the chick’s diet¹⁹. A similar shift in diet on Fair Isle (40 km south-west of Sumburgh Head) seems to have occurred at some time between 2000 and 2002¹². Since 2003, weights of chicks near fledging at Compass Head (1.3 km north of Sumburgh Head) have been lower than during the 1990s and average productivity at Sumburgh Head has also reduced⁶.

Figure 11. Annual return rate of guillemot breeding on the Isle of May, 1987–2019.

The winter of 2013/14 saw a succession of severe storms from late January to the beginning of March result in a large 'wreck' of seabirds along Atlantic coasts from England and Ireland to Spain. A minimum of 54,000 seabirds, mostly auks, were washed ashore dead or dying. Examination of many corpses revealed birds were emaciated with empty stomachs indicating starvation as the main cause of death, although a small proportion showed signs of oil contamination²¹. Overall, about 30% of the casualties were guillemots^21,22.

Biometric data from 30 of these corpses indicated birds were typical of the subspecies albionis which breeds from south-west Scotland down to Iberia. Rings recovered from guillemot corpses (from beaches in France) also indicated birds originated from colonies around the UK and Ireland. The majority of birds examined were found to be adults. The total mortality will be much higher than reported because not all beaches were checked, birds were washed ashore over a number of weeks and many birds will be lost unrecorded at sea¹⁷.

On Skomer, following the ‘wreck’ of winter 2013/14, the data from 2014 strongly suggested a very high level of mortality among adult birds between 2013 and 2014 and this was consistent with the very high number of guillemot ringing recoveries in the weeks following the ‘wreck’. The data from 2015 reaffirmed that mortality following the ‘wreck’ had been high²³. The survival of adult guillemots, based on re-sightings only (not on statistical analyses) suggested that survival was slightly higher in 2016 than in 2015, but lower than in many previous years²⁴. In 2019, survival was similar overall to that of the last five years²⁵.

Figure 12. Annual return rate of guillemot breeding on Skomer, 1986–2013. Reproduced with kind permission from Professors Tim Birkhead and Ben Hatchwell (University of Sheffield)/Countryside Council for Wales¹³.