NB6a/NB17a Abundance and productivity of breeding seabirds

Scotland is internationally important for its twenty four species of breeding seabirds, hosting most of the UK population (SNH, 2018). Climate change is considered to be the principal driver of declines in seabird populations. Warmer winter sea temperatures in the North Sea have affected the food chain: changes in the species composition and abundance of plankton have had a detrimental effect on sandeel (the major food source for most seabirds) numbers and quality (Daunt et al, 2013). Breeding phenology of seabirds is changing; some species are breeding later and some earlier. This is causing a temporal mismatch between prey availability and the breeding season. This de-synchronisation of interactions between species can disrupt the functioning of ecosystems (Daunt et al, 2013).

Scotland has a network of fifty Special Protection Areas currently designated for their seabird populations. Regionally, Orkney and Shetland are particularly important, hosting the largest seabird colonies. This current network may have to change in future, for instance if specific sites lose the seabird populations they are designated for they may become redundant. New species using Scotland for part of their migration pattern may create opportunities for the establishment of new designated sites. Management of the designated site network will therefore have to include monitoring of changes in seabird population numbers and distribution to ensure the network remains effective. Changes in the numbers or distribution of seabird populations may impact on land management in some areas (Brown et al, 2012).

This indicator includes data for sixteen species of seabird, out of the twenty-four that breed in Scotland regularly. It measures both the numbers of breeding seabirds and breeding success (each of these metrics is based on 12 species, for which sufficient data is available), to help us understand whether climate change is impacting upon the numbers of existing migratory seabird populations in Scotland. Breeding success measures the number of chicks surviving to fledging. It tends to vary far more year to year than seabird numbers, and is a good indicator of within season effects, whereas population numbers are likely to change slowly over time. It is a Scottish Biodiversity Indicator and the information provided is drawn from Scottish Natural Heritage (SNH) analyses of these indicators and underpinning data (SNH, 2018).

Seabirds are a good indicator of the overall status of marine environments and, because they breed on land, can be monitored relatively easily. All of the species included in this indicator are Red or Amber listed as Birds of Conservation Concern; European shag, Arctic skua, Atlantic puffin, black-legged kittiwake and herring gull are Red listed (Eaton et al, 2015; cited in SNH (2018).

Related Indicators:

NB6b/NB17b: Numbers of wintering water birds

Numbers of breeding birds

Between 1986 and 2016, after peaking in 1991, mean numbers of breeding seabirds declined steadily until 2011, when numbers were 50% below the 1986 level. Subsequently there has been a slight recovery. In 2016, breeding numbers were 62% of the 1986 level.

Breeding success

The mean breeding success of 12 breeding seabird species has declined (SNH, 2018).

The indices of numbers of breeding birds and breeding success are shown in Figure 1.

Figure 1: Numbers of breeding seabirds and breeding success of seabirds in Scotland, 1986-2016 (Source: SNH, 2018 (based on data provided by JNCC))

Note: Axes are not scaled to zero.

Table 1 shows the trends in numbers and breeding success for the individual species for which data is available.

Table 1: Changes in numbers and breeding success of Scotland’s seabirds (1986-2015)

¹ Coastal colonies

Source: JNCC, 2016

Scotland hosts the majority of the UKs breeding seabirds. Over much of the 20^th century seabird populations in the UK overall increased in abundance, with a 55% increase recorded between the late 1960s and the late 1990s, although for some species numbers had peaked before the end of the century. Except for the black guillemot, all species have declined more rapidly in Scotland than in the UK overall (Daunt et al, 2013).

As the climate changes, the species composition of Scotland’s seabird population is likely to change. Many seabird species in Scotland are at the southern edge of their range and northwards range shifts are expected. Some species that currently breed in Scotland may move to new locations and new species move in as the climate becomes more suitable.

The Marine Climate Change Impacts Partnership (MCCIP) paper ‘Impacts of climate change on seabirds’ (Daunt et al, 2013) provides more detail on projected climate change impacts. Potential impacts include:

• Climate projections indicate that by 2100 the UK climate will no longer suit Arctic skua and great skua, while breeding colonies of black guillemot, common gull and Arctic terns will be restricted to Shetland, Orkney and the northernmost tip of mainland Scotland.
• More frequent extreme weather events may impact breeding habitat and foraging conditions, causing increased mortality of both adult birds and chicks. Further climate-driven changes in prey availability or quality may significantly affect seabirds.
• Jellyfish have recently increased in abundance in UK waters. This has been linked to both climate change and overfishing. Jellyfish compete directly with seabird prey such as sandeels for planktonic food and could therefore have a detrimental impact on availability of prey for seabirds (Purcell et al, 2007, Brotz et al, 2012, cited in Daunt et al, 2013).
• Ocean acidification may adversely affect the food chain (Heath et al, 2012, cited in Daunt et al, 2013).
• Climate change may increase the risk of disease (Lafferty, 2009, cited in Daunt et al, 2013).
• Effects of pollutants may be increased by climate change.
• Sea level rise (especially in the Southern North Sea) will directly affect ground-nesting birds; it may be possible to mitigate this by creating new nesting habitat upshore (Solomon, 2007, cited in Daunt et al, 2013).
• Marine renewables may impact seabirds through collision and displacement. Range shifts may put future breeding sites close to renewable energy developments (Masden et al, 2010; Langton et al, 2011, cited in Daunt et al, 2013).

Numbers of breeding birds

There is considerable variation in the trends among the 12 species assessed for breeding numbers. Arctic skua has experienced the largest decline, 77% since 1986. Common tern is the only species to have increased numbers in 2016, at 159% of the 1986 level, after a long trend of declining numbers.

Breeding success

Across the time series since 1986, breeding success has varied among the 12 species assessed. In 2016 it was above the long-term average for six species (Arctic tern; black-legged kittiwake; common tern; little tern; northern gannet and Sandwich tern), while two species (Great skua and herring gull) had lower breeding success. The other four species (Arctic skua; Atlantic puffin; Guillemot and Northern fulmar) were around the long-term average.

There is significant regional variation in seabird numbers and breeding success across Scotland, with the Northern Isles seeing the most serious declines.

Average winter sea surface temperatures (SST) in the North Sea have increased by around 1oC since the early 1980s, altering the food chain to the detriment of seabirds (van Deurs et al, 2009, cited in Daunt et al, 2013). However currently climate impacts are much weaker in other UK seas such as the Irish Sea (Lauria et al, 2012, 2013; cited in Daunt et al, 2013)

The reasons behind declines in seabird numbers and breeding success are complex, involving a combination of changing food availability, climate change and the impact of non-native species (Foster & Marrs, 2012).

SNH (2018) link the large declines in numbers of Arctic skua to the declining availability of sandeels around their Northern Isles breeding stronghold, together with increased predation from great skua (Meek et al, 2011; cited in SNH, 2018).

Some species that have seen a declining trend in numbers may now be stabilising at a lower level (compared to the 1986 baseline). Research by Foster et al (2017) indicated a link between fishery landings and gull numbers; whereby a steep decline was followed by stabilising of numbers on the Isle of Canna. Given the increase in seabird populations over much of the 20th century (see ‘What has happened in the past?’ above), these lower numbers may be more typical of population sizes prior to the increases from the 1960s (SNH, 2018).

Some scavenging species have benefitted from discards from fishing vessels; it is thought this may have led to populations of great skua and northern fulmar increasing beyond a level that can be sustained by natural food sources (Tasker & Furness, 1996, cited in Daunt et al, 2013). Discards have been reduced as a result of measures to conserve fish stocks and scavengers such as great skua have had to find other sources of food, including predation of other seabirds (Votier et al, 2004, cited in Daunt et al, 2013). The 2013 reform of the EU Common Fisheries Policy, which banned the discarding of fish at sea, is likely to increase the pressure on scavenging species (Bicknell et al, 2013, cited in Daunt et al, 2013).

MCCIP (Daunt et al, 2013) provide more detailed information on these and other factors, including:

• Disturbance by coastal leisure and recreation activities and the offshore renewables industry
• Seabird by-catch by long-line fisheries
• Contamination by oil spills
• Marine litter ingested by surface feeders

Information is also available on the major threats to specific migratory seabird species:

Fulmar: predation by brown rats and American mink; changes in food supply; long-line fishing (Tasker, 2007 in Foster & Marrs, 2012).

Arctic skua: depredation by great skuas; sandeel scarcity; breeding habitat loss; human persecution (Furness, 2007 in Foster & Marrs, 2012).

Kittiwakes: food shortage; climate change; localised predation (Heubeck, 2007 in Foster & Marrs, 2012).

Arctic tern: food availability (mainly small shoaling fish and sandeels); presence of non-native predatory mammals (Craik, 2007 in Foster & Marrs, 2012).

Common guillemot:  oiling; overfishing of sandeels; climate change; disturbance to colonies (Harris and Wanless, 2007 in Foster & Marrs, 2012).  On the Isle of May it has been noted that the failure of guillemots to breed was due to the lower quality (energetic value) of sandeels rather than fewer fish being brought into the colony (Wanless, 2005 in Foster & Marrs, 2012).

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This is a Scottish Biodiversity Indicator and the information provided is drawn from SNH analysis of the data (SNH, 2018). Simon Foster (SNH) provided the data and interpretation of trends. The underpinning data is from the Seabird Monitoring Programme. The species trend data provided in Table 1 is from the Joint Nature Conservation Committee’s ‘Seabird Population Trends and Causes of Change: 1986-2015 Report’ (JNCC, 2016).

Suzanne Martin (RBGE) contributed content to the 2016 version of this indicator.