Seabirds

Across Scotland some seabird colonies have seen large declines, particularly in the Northern Isles where black-legged kittiwake and Arctic skua populations have been greatly reduced.

However, a few Scottish colonies have recorded increases, northern gannet is strongly increasing at most of its Scottish colonies. Between 2000 and 2014 northern gannet numbers at St Kilda, Sule Stack and Scar Rocks were stable, but all other colonies had increased, some spectacularly. Numbers of razorbill had been steadily increasing since the SCR survey, although there was decline in the early 2000s. Since 2009 the overall increase in numbers has resumed.

Some species trends appear to be stabilising at a new level which differs from the 1986 baseline used in this indicator. Foster et al. (2017) showed a link between declining fishery landings and gull numbers which led to steep declines, followed by a stabilisation of numbers on the Isle of Canna. However, when compared with information from the 1930s, the lower numbers of gulls found recently appear to be more typical than the peaks observed in the mid-1980s.

Since 2011 both number of seabirds and breeding success of seabirds have shown no strong trend overall, despite annual fluctuations. Since 2011 the breeding numbers index has fluctuated between 59 (in 2016) and 49 (in 2011), remaining considerably below the baseline value of 100 over the whole period.

Arctic skua

Out of the 11 species assessed in 2018 for breeding numbers, Arctic skua had experienced the largest declines reaching an 80% decline in 2018 compared to 1986 level. Since 2013 there has been little overall change but between 2011 and 2018 Arctic skua numbers have declined by 5% (Figure b).

Figure b: Trend in Arctic skua in Scotland from 1986 to 2018. The trendline is the solid line, with upper and lower 95% confidence intervals shown by dashed lines. Figure from JNCC (2020).

The decline is driven by consistently low productivity of Arctic skua (Figure c), affected by top down and bottom-up pressures of predation and low prey availability (Meek et al., 2011; Perkins et al., 2018).

Figure c: Arctic skua productivity in Scotland from 1986 to 2018.

The Northern Isles are the breeding stronghold for Arctic skua in Scotland and changes to the availability of sandeels, which they obtain from other seabirds by chasing to make them release their food, has led to declines. Similar patterns of declines have also occurred for the species they parasitise, particularly black-legged kittwakes (Figure d) and terns (Figures e and f).

Figure d: Productivity of Arctic skua (red line) and black-legged kittiwake (purple line) in Shetland 2002 to 2016.

Common tern

The number of breeding common terns increased in 2016 (Figure e) and has been maintained since. As with many colonial nesting tern species they can respond to favourable breeding conditions rapidly, resulting in higher numbers at breeding colonies in some years.

Figure e: Common tern numbers in Scotland. The trendline is the solid line, with upper and lower 95% confidence intervals shown by dashed lines. Figure from JNCC (2020).

Arctic tern

The numbers of breeding Arctic tern were at their lowest point in 2011 (Figure f). A gradual increase in numbers is shown through to 2014 with some fluctuation in numbers from 2014 to 2018.

Figure f: Trend in Arctic tern numbers in Scotland from 1986 to 2018. The trendline is the solid line, with upper and lower 95% confidence intervals shown by dashed lines. Figure from JNCC (2020).

Currently there is limited information for detailing changes at the scale of Scotland’s seas. The results of the 4^th national census, when available in 2022, will enable changes in population size between the 3^rd census ‘Seabird 2000’ and the current period to be assessed. However, regular monitoring at individual sites can provide valuable information between census periods, with the understanding that it is a sample of the overall population only. Furthermore results from tracking work such as the Future of the Atlantic Marine Environment fameproject.eu (FAME) and Seabird Tracking and Research (STAR) seabird tracking projects, and work planned in conjunction with offshore renewable development is helping to provide details of the distribution of breeding seabirds when at sea around Scotland.

Regionally there are differences in the population and productivity trends shown by some species, although the general pattern of long-term declines since the mid-1980s, with some recovery or slowing of decline since 2011 is true for most. See more information by Scottish Marine Region in the sections below.

Forth and Tay Scottish Marine Region

The Isle of May is a SMP key site and CEH produces annual breeding season summaries. Figures of breeding success below have been taken from Newell et al. (2016), and figures of breeding numbers from Outram et al. (2018).

The 2016 breeding season on the Isle of May was mixed following the generally successful seasons of 2014 and 2015. Breeding in 2016 commenced early for European shags but late for black-legged kittiwakes and Atlantic puffins.

Of the six study species, European shags had their most successful season on record, while black-legged kittiwakes and northern fulmars were above average. Atlantic puffins had an average season while common guillemots had a below average breeding season. Razorbills had their worst season on record. Return rates were above the long-term average in all study species except Atlantic puffins, which was very low. Sandeels (Ammodytes spp.) remained the main food of young razorbill, Atlantic puffins, European shags and black-legged kittiwakes. The diet of common guillemots was dominated by clupeids. The main results were:

Northern fulmar

Breeding succes in 2016 (0.43 chicks per incubating pair) was above average (Figure g). In 2015 the success was 0.52 chicks per pair. In 2017 it was 0.40 chicks per pair – close to the long-term average.

Figure g: Northern fulmar breeding success at Isle of May 1986 to 2019. Average value shown by solid blue line.

Northern fulmar numbers have been relatively stable for a long period, although since 2011 numbers have been below the peaks of 381 recorded in 2010 and 382 in 1997 (Figure h). The average number from 2011 to 2017 was 300 apparently occupied sites (AOS). Breeding success from 2011 to 2017 has been generally above the long-term average.

Figure h: Number of breeding northern fulmar on Isle of May 1990 to 2018.

European shag

These had the most successful season on record in 2016 (2.1 chicks per pair). Breeding success has been high for the last decade (Figure i). The number of breeding birds remains relatively stable (474 apparently occupied nests (AON) in 2017) and well below peaks recorded in 2003 (968 AON) and 1992 (1,634 AON) (Figure j). Return rate was above average at 88.9%. Diet was dominated by sandeels which occurred in 73.5% of samples.

Figure i: European shag breeding success at Isle of May, 1986 to 2019.

Figure j: Number of pairs of breeding European shag on Isle of May, 1990 to 2018.

Black-legged kittiwake

Black-legged kittiwakes had their most productive season since 1987 in 2014 with productivity of 1.17 chicks per completed nest. This was well above average for the whole period 1986 to 2019 which is 0.61 chicks per nest. Since 2011 average productivity has been 0.86 chicks per nest. The breeding success has been variable but generally high from 2011 to 2019 (Figure k).

Figure k: Breeding success (young reared per pair) of black-legged kittiwake on the Isle of May, 1986 to 2019.

The number of pairs of black-legged kittiwake nesting has been overall stable or shown a slight increase from 2011 to 2017 (Figure l), however, the population size is less than 50% of that recorded in the early 1990s.

Figure l: Number of breeding black-legged kittiwake (AON) on the Isle of May 1990 to 2018.

Common guillemot

Common guillemots had a relatively poor breeding season in 2016 (0.66 chicks leaving per pair laying) although breeding success from 2011 to 2019 has been just above average compared to the 1986 to 2010 mean (0.72 cf. 0.71) (Figure m).

Figure m: Breeding success (young raised per pair) of common guillemot at Isle of May, 1986 to 2019.

Apart from a distinct dip in breeding success between 2004 and 2008 common guillemot breeding success has shown a very shallow decline over the period since 1986. Both post and prior to 2011 average breeding success was 0.72 chicks per pair, but from 1986 to 2004 average breeding success was 0.75 chicks per pair, although declining over the period. The slightly higher average breeding success from 1986 to 2004 is reflected in the increase in estimated population size between 1990 and 2004 (Figure n), Since 2004 there has been some reduction in the population size, but the number of common guillemot in 2018 is still slightly higher than it was in 1990.

Figure n: Number of estimated pairs (adjusted by annual k factor) of common guillemot at Isle of May, 1990 to 2018.

Razorbill

Razorbill breeding success in 2016 (0.45 chicks leaving per pair) was the lowest on record but adult return rate (87.5%) was above average (Figure o). Chick diet contained more sandeels (58% of loads) than clupeids (42%).

Figure o: Counts of estimated pairs of razorbill (adjusted by annual k factor) on the Isle of May, 1990 to 2018.

Atlantic puffin

Atlantic puffins had an average season in 2016 with 0.76 chicks fledging per pair laying (Figure p). There appears to be a long-term, slight decline in breeding success since the late 1980s, but little sign of change since 2011. Average breeding success prior to 2011 and since 2010 is hardly different (0.72 young per pair compared to 0.715 young per pair). Diet studies in 2016 showed that chicks were fed mainly sandeels (88.7% by number) with Clupeidae (mainly sprats) and rockling contributing 3.5% and 1.9% respectively.

Figure p: Productivity (breeding success) of Atlantic puffin (young reared per pair) at Isle of May, 1986 to 2019.

Survival, except for years 2007 and 2008 is generally good (Figure q). It is high and steady from 2011 to 2015.

Figure q: Survival (return rates) of adult Atlantic puffin on Isle of May 1987 to 2015.

Since 1984 the population of Atlantic puffin on the Isle of May has increased from 12,000 pairs to 39,200 in 2017 (Figure r). Although during 1998 to 2013 the population was higher: the peak count being 69,300 pairs in 2003 and 46,200 pairs in 2013. During the period 2011 to 2017 therefore, the population declined but is still well above that in 1984.

Figure r: Number of pairs of Atlantic puffin on Isle of May, 1989 to 2017.

Herring gull and lesser black-backed gull

Figure s: Herring gull and lesser black-backed gull breeding numbers on Isle of May 1982 to 2018. Total nests (yellow line), herring gull nests (grey), lesser black-backed gull nests (red).

Moray Firth and North Coast Scottish Marine Region

For northern mainland Scotland there is no SMP key site, but as part of the National Seabird Census recent counts have been commissioned for the North Caithness Cliffs SPA and East Caithness Cliffs SPA which allow site specific trends to be determined. North Caithness Cliffs SPA falls within the North Coast and Moray Firth SMRs, while East Caithness Cliffs SPA is within the Moray Firth, but it is not sensible to draw indices for the complete SMR from either site as other major colonies also exist within each region.

The latest counts for East Caithness Cliffs in 2015 and North Caithness Cliffs in 2016 revealed:

Northern fulmar

There were 13,864 AOS at East Caithness Cliffs in 2015, 3.6% lower than 1999 and part of a slow decline since 1986. The results were similar for North Caithness Cliffs with 13,405 AOS in 2015/16, 7% lower than 1999 and showing a continued slow decline since 1986.

Great cormorant

This species is only recorded breeding at East Caithness Cliffs where there were 53 AON in 2015, 41.1% lower than 1999 and a continuation of a long term 81% decline since 1977.

European shag

At East Caithness Cliffs there were 1,098 AON, 3.6% higher than 1999, but overall 53% down on the peak recorded in 1986. The total of 170 AON at North Caithness Cliffs was a much larger increase at 66% higher than 1999.

Black-legged kittiwake

At East Caithness there were 24,460 AON, a decline of 39.5% since 1999. While at North Caithness there were 5,568 AON, a decline of 55% since 1999 and 64% since 1986. Both sites show a similar trend of similar magnitude.

Herring gull

The population of 3,267 apparently occupied territories (AOT) at East Caithness was 4.2% lower than in 1999 and a continuation of a 79% decline since 1977. Conversely at North Caithness Cliffs, herring gull numbers in 2016 were 33% higher than in 1999 at 232 AOT.

Great black-backed gull

Increased strongly at both SPAs. There were 266 AOT at East Caithness Cliffs, an increase of 47.8% since 1999, though still 74% lower than the peak recorded in 1977. While at North Caithness Cliffs there were 68 AOT in 2016, an increase of 81% since 1999.

Common guillemot

There were 149,228 birds counted at East Caithness Cliffs in 2015, a decline of 6.2% since 1999, but still 40% higher than the numbers present in 1977 and 1986. A much stronger decline was noted at North Caithness Cliffs with 38,863 birds present, a decline of 53% since 1999, but still 93% higher than the numbers present in 1986.

Razorbill

The East Caithness Cliffs population was measured at 30,042 birds, an increase of 69.5% since 1999 and a continuation of a long term 147% increase since 1977. Numbers at North Caithness Cliffs also showed a similar strong increase with 3,503 birds counted. This is an increase of 39% since 1999 and a continuation of a long term 112% increase since 1986.

Atlantic puffin

Only recorded in substantial numbers in North Caithness Cliffs 3,053 birds counted, 55% lower than the 1999 count, but just 13% lower than the numbers present in 1986.

The long-term trend for the four key SPA species shows that generally they have fared better at the North Caithness Cliffs SPA when compared with the overall Scottish trend. Although the long-term decline in black-legged kittiwake is similar to the Scottish trend, the decline in northern fulmar is far less and for common guillemot and razorbill the overall increase is greater.

Shetland Scottish Marine Region

Fair Isle is the SMP key site and the Shetland Bird report gives details of seabird trends as monitored by Shetland Oil Terminal Environmental Advisory Group (SOTEAG).

An index of seabird breeding success for the Fair Isle (Figure t) can be inferred from the data provided by monitoring on Fair Isle and compared with another high intensity monitored site, Noss NNR and sites monitored under the SOTEAG programme. Breeding success was at its lowest point in 2011. The index shows a level of stability during 2012-2018 (with 2013 and 2017 being poor years).

In the Shetland SMR most seabirds have declined, in particular there have been large declines for black-legged kittiwake and Arctic skua (and this is the same picture seen for the Northern Isles as a whole).

Summary species information for Fair Isle and Shetland is published in the Fair Isle Bird Observatory annual report and the Shetland Bird Report respectively.

Figure t: Index of seabird breeding success, Fair Isle.

Northern fulmar

Plot counts on Fair Isle show a decrease from 1986, but since 2011 the population has increased. In Shetland the number of AOS at NatureScot and SOTEAG monitoring plots was similar in 2017 and 2018, having increased slightly since 2011 (Figure u).

Figure u: Annual counts of northern fulmar AOS at four monitoring colonies in Shetland and mean count (green line) of all colonies 1990 to 2019.

The 2018 breeding success in Shetland was similar to the 1985 to 2017 average of 0.42 young / AOS at 0.4 chicks per AOS. Between 2011 and 2017 the productivity was above the long-term average in 3 years, but below in 5 years (Figure v).

Figure v: Average annual productivity of northern fulmar at four monitoring sites in Shetland 1984 to 2018.

European shag

Since 1986 breeding numbers have declined from around 1,099 apparently occupied nests (AONs) to 204 AONs in 2013 when the last all island count of Fair Isle was completed. Two monitoring plots have been the subject of long-term counting, one at Sumburgh Head is counted by SOTEAG; while Noss NNR is also counted annually by NatureScot. Between 2011 and 2017 the number of nests at Sumburgh Head and nearby No Ness has declined by 10% (from 100 to 90) and 21% (from 134 to 107 nests) respectively. Over the same period the Noss NNR population increased from 96 to 104 AON (+8%).

Northern gannet

The Fair Isle breeding population has increased since the early 2000s, reaching 3,409 AON in 2016, 3,882 AON in 2017 and 4,291 AON in 2018. Northern gannet first bred on Fair Isle in 1975.

On Noss the population increased from 9,767 AOS in 2008 to 11,786 AOS in 2014 and 13,765 AOS in 2019. Therefore, during the period 2014 to 2019 the population has increased by 17%.

At three monitored colonies in Shetland (Noss, Hermaness and Fair Isle) breeding success has ranged between 0.66 and 0.75 from 2011 to 2017. The mean breeding success was 0.71 chicks raised per nest.

Common guillemot

Since 1986 the number of common guillemot on Fair Isle has declined, although that rate of decline has slowed and even levelled off in recent years. Long-term monitoring shows that numbers are still around double those in the 1960s.

The population on Noss is counted approximately every six years with 22,065 individuals recorded in 2009 and 24,456 in 2015. It is likely that the population was stable or slightly increasing between 2011 to 2015.

Razorbill

On Fair Isle the population was around 5,000 birds in 1986 but has declined to 1,930 birds in 2015. On Noss, however, the numbers counted in 2009 and 2015 were similar (558 and 533 birds respectively). This is considerably smaller than counts in 2001 (1,984) or 1991 (1,180) indicating, like common guillemot, a historical decline that has now levelled out.

Black guillemot

On Fair Isle from 1986 there was a decline until around 1996, since then numbers have remained around 150 breeding adults. On Noss the monitoring plot shows that numbers have declined slightly from 2011 to 2017 from 55 birds in 2011 to 47 birds in 2017.

Great black-backed gull

Numbers on Noss declined between 2011 and 2017 from 66 AOT to 26 AOT, having varied between 47 and 80 AOT for the previous 20 years.

Herring gull

Although the number of breeding pairs on Noss was higher in the 1990s and early 2000s at around 60 AOT, between 2011 and 2017 the population was fairly stable at a lower level between 33 and 24 AOT.

Black-legged kittiwake

The population on Noss declined from 507 pairs in 2010 to 44 pairs in 2017 (-91%). This is a continuation of a steep decline from more than 10,000 pairs in 1970. At other colonies counted in Shetland the number of pairs declined, between 2010 and 2017, from 582 to 256 on Foula (-56%), from 161 to 46 at Compass Head (-71%) and from 386 to 254 at Sumburgh Head (-34%). All sites held much larger numbers of birds in previous decades, with peak numbers recorded around 1980 at most sites.

Arctic tern

Large declines in Arctic tern have been recorded in Shetland since the 1980s, and colonies are now relatively small and scattered. Regular monitoring takes place on Fair Isle and on Noss NNR.

On Fair Isle there were 322 AON in 2017, with only 0.06 chicks produced per AON, only 9 AON were recorded in 2011, but there were 377 AON in 2010. Numbers have been erratic, but generally low with low breeding success during the 2011-2017 period.

On Noss there were 51 AON in 2017, with similar productivity to Fair Isle (0.06 chicks per AON). In 2011 no Arctic tern nested on Noss, but between 2011 and 2017 the population has fluctuated between 0 and 72 pairs. Previously there were 140 pairs in 2004, 120 pairs in 1998, although the colony here has always been relatively small and fluctuated in size.

Great skua

On Noss the great skua population is stable at 465 AOT in 2013 and 478 AOT in 2018 having increased from around 400 pairs at the turn of the century. Productivity has been generally poor in recent years but fluctuates (Figure w). On Fetlar there were 887 AOT in 2017 compared to 593 recorded in Seabird 2000. On Yell numbers were little changed from seabird 2000. A whole island survey found 392 AOT in 2017, compared to 384 previously.

Figure w: Great skua breeding success in Shetland.

Arctic skua

Very few Arctic skua breed on Noss, with only 1 or 2 pairs between 2011 and 2017. This is down from 5 pairs in 2007, 10 pairs in 2000 and 15 pairs in 1990. As the great majority of Scotland’s Arctic skua breed in Shetland the regional trend is very similar to the national trend reported above.

Productivity at monitoring plots in Shetland was generally low between 2011 and 2017 (Figure x).

Figure x: Arctic skua breeding success in Shetland 2008 to 2017.

West Highland Scottish Marine Region

Canna is the west coast SMP key site. It is monitored mainly by the Highland Ringing Group.

An index of seabird breeding success for Canna and Sanday (Figure y) can be inferred from data provided by monitoring on Canna and Sanday. The index of breeding success (purple line) is consistently above the baseline (index value 100) during this assessment period.

Figure y: Seabird breeding success for all monitored seabirds on Canna and Sanday (purple line) and an index using only data from black-legged kittiwake, European shag and northern fulmar (red line).

Recent counts of seabirds on Canna show that European shag and black-legged kittiwake have shown increases in numbers. Black-legged kittiwake numbers have been around 1,000 pairs (between 820 and 1,200) in the period 2011 to 2018 (Figure z).

Figure z: Black-legged kittiwake population counts on Canna 2011 to 2019.

Pressures from activities

The main drivers of seabird population changes are thought to be climate change, introduction of invasive species and changes in prey abundance and quality. Other pressures include fisheries bycatch, collisions and displacement with anthropogenic structures and pollution. These may cause local or even national population-level effects. For all of these pressures there is evidence of impacts on Scottish seabirds. However, quantitative data to determine the full extent to which these pressures act on seabird populations, individually and in combination with each other, is lacking.

An assessment of all pressures acting on the seas around Scotland and the main activities causing these is provided for each of the Scottish Marine Regions under Pressures from Activities.

Fisheries: Fisheries impacts may operate directly through bycatch or indirectly through discarding ‘waste’ fish and through competing with seabirds for forage fish. Numbers of birds accounted for by this pressure are largely unknown (although see Bradbury et al., 2017 and Northridge et al., 2020). Studies of discard impacts and fisheries impacts on forage fish (e.g. Gremilet et al., 2018; Sydeman et al., 2017) show that pressures on seabirds are increasing, globally. More widely an analysis of discard fish available in the North Sea (Sherley et al., 2019) reports that the amount of discards in the area declined by 48% between 1990 and 2010, with a consequent decline in the number of scavenging seabirds that could be supported.

Inshore and non-commercial fish stocks are not well monitored, and this is a significant obstacle to understanding seabird declines. While sea (and air) temperatures are routinely monitored the impact on forage fish for seabirds is not well established as most forage fish are not commercially important (although some sandeel fisheries are an exception). Fisheries monitoring is also not targeted at understanding the links between forage fish and predators, and implementation of discard monitoring is in its early stages. Other fisheries may impact seabirds indirectly through habitat alteration leading to changes in prey densities or distribution, although information relating to this is currently limited.

Marine energy:  Collison and displacement by marine energy developments has not yet been shown to operate at a population scale. As increasing development takes place (see Scottish Government Sectoral Plan report) and currently consented projects are built, monitoring of these projects during operation will provide empirical evidence of the scale of any impact which has largely been assessed through theoretical modelling until now.

Wider pressures:  Pressures that are expected mainly to act through changing prey availability include: loss of sea area (to land or fresh water); barrier impacts; and removal and modification of seabed habitats (e.g. through dredging activities or harbour development) are associated with many activities. These pressures are not measured systematically, or on a wide scale with respect to their impact on birds.