Seabirds: strategic ornithological compensatory measures: review

4 Species accounts

For focal species, i.e. those assessed most likely to require compensatory measures (Table 4), we provide some background on their current conservation status and ecology. Species of large gulls and auks were grouped into combined accounts.

4.1 Black-legged Kittiwake Rissa tridactyla

The Black-legged Kittiwake (Rissa tridactyla, hereafter ‘Kittiwake’) is a medium sized gull species with a circumpolar distribution (breeding in northerly latitudes of both the Pacific and Atlantic). They nest on coastal cliffs often in large colonies and are the most oceanic of gull species, foraging primarily on small pelagic shoaling fish species (‘forage fish’). Kittiwake are migratory with those breeding in northwest Europe dispersing widely across the North Atlantic during winter though with some birds breeding in the Britain and Ireland remaining relatively locally in winter (Frederiksen et al., 2012).

Over recent decades Kittiwake have been declining locally and within the UK, leading to corresponding negative conservation status (Table 13). The causes of declines are not fully understood but are thought to be largely driven by reduction in prey availability mediated primarily by climate impacts but also fisheries, this has led to reductions in both adult survival and productivity (Mitchell et al., 2020). Kittiwake are particularly sensitive to prey availability, being surface feeders and thus unable to dive for prey. Kittiwake were impacted by the Highly Pathogenic Avian Influenza (HPAI) outbreak in 2022 and 2023 though, on average, populations had increased between the Seabirds Count and HPAI censuses (Tremlett et al., 2024), but this masks some sites that suffered significant declines (e.g. -29% for Forth Islands).

Scotland holds a high proportion (56%) of Kittiwake breeding in the UK and are widely distributed around Scotland recognised as breeding features for 29 colony SPAs (Table 13), with the majority breeding on the east coast and in the Orkney and Shetland islands (Figure A 1). They are also a listed feature for one marine SPA, the Outer Firth of Forth and St Andrews Bay Complex SPA, both during the breeding and non-breeding seasons.

Kittiwake are thought to be vulnerable to offshore wind developments both through collision mortality and displacement. They were scored as having a vulnerability of 0.53 to collision and 0.6 to displacement placing them as the 4^th and 5^th most vulnerable, respectively, of the 11 species assessed by Searle et al. (2020) based on the methodology of Certain et al. (2015) using earlier assessments by Furness et al. (2013) and Wade et al. (2016). Based on in combination impacts from existing and planned offshore wind developments, the HRAs undertaken for the Sectoral Marine Plan for Offshore Wind Energy concluded that further offshore wind developments off eastern and northeastern Scotland in certain plan option (PO) sites (E3, NE2-NE4, and NE6) could lead to AEOSI. Kittiwake is therefore one of the species most likely to require compensatory measures were further offshore wind developments consented in these POs following HRA derogation (this has already occurred for developments in English Waters).

Table 13. Fact box for Black-legged Kittiwake

(sources included as table footnotes), values are means (± one standard deviation) unless otherwise stated.

This table can be viewed in Excel format.

4.2 Large gulls: Lesser Black-backed Gull Larus fuscus, Herring Gull Larus argentatus, and Great Black-backed Gull Larus marinus

There are three large gull (all within Larus genus) species that breed in Scotland; Lesser Black-backed Gull, Herring Gull, and Great Black-backed Gull. All three species are dietary generalists feeding on a wide variety of terrestrial and marine prey. The Great Black-backed Gull is an apex predator that while a dietary generalist will also predate other birds and small mammals. All three gulls are ground nesting with natural nesting sites associated with coastal cliffs and islands but also nesting on artificial or urban sites especially on rooftops of buildings. Great Black-backed Gull are the largest of all gulls globally and are distributed throughout northern and low Arctic parts of the North Atlantic mostly being coastal but also associated with inland water bodies and larger rivers. Herring Gull (full common name European Herring Gull, L. argentatus, to distinguish from Arctic Herring Gull/American Herring Gull, L. smithsonianus, which were previously grouped as one species, see Sangster et al. (2007)) are distributed around north and northwest Europe, they are most associated with coasts but increasingly also occur inland. Lesser black-backed gull breed in northwestern Europe and northern Russia. All three species are somewhat migratory, though with many British breeding birds being largely resident or performing only shorter-range migrations within Britain and Ireland or to southwestern Europe.

For all three species the global conservation status is Least Concern, however the species have different population trends (Table 14). Within the UK the population status assessments have, to date, been based on the component of the gull populations breeding on natural sites (e.g. coastal colonies), at these sites the species have been decreasing so all three species are listed as of medium (Amber) or high (Red) concern. The number of Herring Gull and Lesser Black-backed Gull breeding on artificial sites (largely on building rooftops) has been increasing over the past few decades, and for England it is was estimated that the majority of both species nest primarily on artificial sites (Burnell, 2021). This was subsequently confirmed with the latest seabird census, Seabirds Count, including ‘Urban’ and ‘Natural’ populations (Burnell et al., 2023). Estimates of the proportion of Great Black-backed Gull using artificial nesting sites is not available for England nor Scotland, however they are recorded to use artificial sites too; e.g. small numbers were recently recorded nesting in Cardiff though at much lower levels than either Herring Gull or Lesser Black-backed Gull (Rock, 2022).

A number of potential causes of population change for gulls have been described. However, the relative contribution of these is unclear, particularly in terms of understanding to what extent reductions in natural (usually coastal) colonies has resulted from redistribution to artificial breeding sites. Causes of declines (particularly for natural colonies) include: reduction in availability of fishery discards following changes in discards practice and regulations (Sherley et al., 2020); botulism from foraging and landfill sites and culling (Coulson, 2015). Gulls were impacted by the HPAI outbreak in 2022 and 2023; Herring Gull decreased slightly between the Seabirds Count and HPAI censuses while Great Black-backed Gull declined significantly (for sites counted), and Lesser Black-backed Gull declined by an intermediate amount (Tremlett et al., 2024), however, overall trends masks some sites that suffered more significant declines (e.g. -29% for Lesser Black-backed Gull and -34% for Herring Gull from Forth Islands, ).

Scotland hosts most of the UK’s breeding Great Black-backed Gull, over half of natural nesting Herring Gull though less than half of all nesting Herring Gull, and a smaller proportion of Lesser Black-backed Gull (Table 14). Within Scotland there are 8 colony SPAs including Herring Gull as a designated (breeding) feature with most of these in the east or northeast of Scotland (two colonies on the west coast) and one marine SPA which is designated for both breeding and non-breeding birds (Figure A 1). There are only two Scottish SPAs (both breeding colonies) for Lesser Black-backed Gull corresponding to the relatively low proportion of the British population breeding in Scotland. For Great Black-backed Gull there are five colony SPAs designated, all in the north with the majority in the Orkney Isles (Figure A 1), and it is not a non-breeding feature for any SPAs.

The three large gulls are considered to be vulnerable to offshore wind developments primarily through collision mortality but potentially also via displacement. Herring Gull and Lesser Black-backed Gull were both scored as having a vulnerability of 0.60 to collision and 0.52 to displacement placing them as the joint 2^nd and 6^th most vulnerable, respectively, of the 11 species assessed by Searle et al. (2020) based on the methodology of Certain et al. (2015) using earlier assessments by Furness et al. (2013) and Wade et al. (2016). Great Black-backed Gull were not included in that assessment but would be expected to have similar vulnerability.

Table 14. Fact box for large gulls

(sources included as table footnotes), values are means (± one standard deviation) unless otherwise stated.

This table can be viewed in Excel format.

4.3 Northern Gannet Morus bassanus

The Northern Gannet (Morus bassanus, here after referred to as ‘Gannet’, note other Gannet species exist in the Southern Hemisphere) is the largest species within the Sulidae family and is the largest breeding seabird in the Northern Atlantic (Nelson, 2002). Its distribution spans throughout both sides of the North Atlantic (BirdLife International, 2018). Currently, the UK holds 56% of the world’s breeding Gannet populations (JNCC, 2021). These birds primarily establish breeding colonies on offshore islands, although a few colonies can also be found on mainland coastal cliffs. They are monogamous birds, often forming lifelong partnerships, and both parents take turns incubating the single offspring. At the end of the breeding season, both adults and juveniles from UK colonies migrate south as far as West Africa (Fort et al., 2012; Lane et al., 2021). The IUCN classifies Gannet as a species of Least Concern (Table 15), considering their wide distribution and increasing population trends worldwide (BirdLife International, 2018). The latter, however, does not account for the devastating effects caused by the ongoing outbreak of Highly Pathogenic Avian Influenza (HPAI) on Gannet populations. Consequently, current population trends are unknown.

Gannets primarily feed on pelagic fish such as mackerel and herring but can also feed on Sandeels when these are abundant. Their foraging behaviour consists mostly of plunge-diving from significant heights. This strategy entails spotting prey from elevated positions and then swiftly diving towards it to surprise and capture it. Additionally, Gannet can use their wings underwater to pursue and catch prey (Ropert-Coudert et al., 2009). Gannet have also been recorded feeding on discards from fishing vessels, where their large size provides them with a competitive advantage over other scavenging species.

Gannet populations have been increasing worldwide, including in the UK. Despite this positive trend, Gannet are categorised as Amber in the Birds of Conservation Concern 5 status (Table 15). This classification is based on two Amber list qualifying criteria, their breeding localisation (i.e. relatively few important breeding locations) and of their breeding international importance (i.e. that a high proportion of Gannet globally breed in the UK). Gannet are vulnerable to fisheries bycatch and have been recorded as one of the more frequently bycaught seabird species in UK waters (Northridge et al., 2020). Similarly in an assessment of fisheries bycatch in Portuguese Atlantic waters, Gannet were the most frequently bycaught seabird species, particularly in demersal longline operations (Oliveira et al., 2015). An emerging threat is from the rising presence of marine litter and pollution, particularly plastic debris, which poses risks through entanglement and ingestion (Rodriguez et al., 2013; Merlino, 2019). In the past, Gannet populations and colony distributions were significantly depleted due to the practice of harvesting eggs and chicks. Nonetheless, there has been a significant decrease in such practices over time. In the UK, there is presently only one licensed harvest site for Gannet chicks (see Ecological feasibility: End of the Gannet harvest at Sula Sgeir). Gannet also potentially face threats in the non-breeding season, with potential for both bycatch and competition with fisheries for prey off West Africa (Gremillet et al., 2015). Recently, HPAI has been shown to have significant detrimental effects on this species (Lane et al., 2023; Jeglinski et al., preprint). Across the UK, Gannet decreased 25% from the pre-HPAI baseline to 2023, with significant declines for key Scottish colonies (-27% for Forth Islands, including Bass Rock, and -37% for Hermaness, Saxa Vord and Valla Field) (Tremlett et al., 2024).

Scotland holds a significant proportion (>80%) of breeding Gannet within the UK, with the world’s largest colony located on the Bass Rock in the Firth of Forth. Gannet are distributed around Scotland, mostly in the northern isles (Figure A 1), with breeding as a designated feature in 8 colony SPAs (Table 15). They are also a listed feature for two marine SPAs, the Outer Firth of Forth and St Andrews Bay Complex SPA and the Seas Off St Kilda SPA. No SPAs were designated for non-breeding.

Gannet are considered vulnerable to offshore wind developments both through collision mortality and displacement. They were scored as having a vulnerability of 0.67 to collision and 0.85 to displacement placing them as the most vulnerable and the 3^rd most vulnerable of the 11 species assessed by Searle et al. (2020) based on the methodology of Certain et al. (2015) using earlier assessments by Furness et al. (2013) and Wade et al. (2016).

Table 15. Fact box for Northern Gannet (sources included as table footnotes), values are means (± one standard deviation) unless otherwise stated.

This table can be viewed in Excel format.

4.4 Auks: Common Guillemot Uria aalge, Razorbill Alca torda, Atlantic Puffin Fratercula arctica

Auks, belonging to the Alcidae family, are small to medium-sized seabirds that are characterised by barrel-shaped bodies, short tails, small wings, and short legs set far back on their bodies. These morphological features are primarily adaptations for their (wing-propelled) diving (Pennycuick, 1987; Thaxter et al., 2010). Their walking ability is limited but they possess a distinctive ability to stand upright. Their flight is characterised by low and fast movements with whirring wings and limited manoeuvrability, but they are excellent swimmers and divers, using their wings to propel themselves underwater. Although there are five species of auks that breed or occur regularly in Scotland, this project will focus on three: Common Guillemot Uria aalge, Razorbill Alca torda, and Atlantic Puffin Fratercula arctica (Black Guillemot Cepphus grylle and Little Auk Alle alle are excluded). Guillemot exhibit a vast circumpolar distribution, inhabiting low-Arctic and boreal waters of the North Atlantic and North Pacific, it is one of the most abundant seabirds in the temperate and colder parts of the northern hemisphere. Similarly, Razorbills have a similar distribution but confined to the Atlantic Ocean and into the western Mediterranean. Puffins can be found across the North Atlantic, primarily in arctic and low-Arctic waters (IUCN, 2023; RSPB, 2023). When not breeding, auks are mostly found offshore within the North Atlantic.

The three auks here considered are pelagic species, spending most of their adult lives at sea and only going ashore to breed. During this time, they gather in large colonies on cliffs or rocky islands. Guillemots form dense nesting colonies along cliff edges, with colonies containing up to tens of thousands of individuals. Razorbills breed in colonies on cliff edges, in cracks in rocky cliffs or among boulders on rocky shores but tend to have smaller colonies and individuals are more widely spread compared to Guillemots. Both species do not build nests but lay a single egg on the bare rock, guano, or soil. Puffins, on the other hand, typically nest underground in burrows and more rarely in rocky crevices. Auks exhibit monogamous behaviour, and high fidelity to the nest site. Each pair lays one egg per breeding season, with the possibility of relaying if the first egg is lost early in the season. Both parents actively participate in parental care. However, unlike Guillemots and Razorbills, Puffins can leave their chicks unattended as their burrows provide protection. These species are piscivorous, mainly foraging on Sandeels, herring, sprat, juvenile gadoids, and capelin, but can also feed on crustaceans and worms (Harris and Wanless, 1986). All three auks swim and chase after fish underwater (‘pursuit diving’) but the duration spent underwater and the depth they reach depends on the species (Thaxter et al., 2010). Guillemots catch a single large fish during each feeding trip, while Razorbills and Puffins can collect several smaller fish during a single trip (Harris and Wanless, 1986).

The global conservation status for Guillemots and Razorbills is of Least Concern, with populations increasing worldwide. Atlantic Puffins, however, are categorised as Vulnerable and its populations are declining (Table 16). Despite these trends, auks face multiple threats and populations have varied accordingly. The major threat that auks are facing is starvation. The effects of overfishing and climate change have changed the proportion, distribution, nutrition quality, and size of fish they prey on which have collectively had a significant effect on the body condition, breeding success and survival of these birds (Sandvik et al., 2005; Wanless et al., 2005a; Häkkinen et al., 2022). Other threats involve high bycatch rates in some fisheries, due to their pursuit diving strategy (Costa et al., 2018; Northridge et al., 2020), oil spills (Votier et al., 2005), outwith the UK unregulated hunting at possibly unsustainable levels (Naves and Rothe, 2023), invasive mammalian predators, extreme weather conditions primarily winter storms (Häkkinen et al., 2022) and future sea level rises that may cause flooding in some parts of its geographical range (Sandvik et al., 2005). Across the UK, Common Guillemot decreased by 7% from the current baseline to 2023, following the HPAI outbreak in 2022, with significant declines for some key Scottish colonies (-42% for Forth Islands, and -34% for St Abb’s Head to Fast Castle) (Tremlett et al., 2024). Razorbill and Puffin were not included in the HPAI census, though mortality is thought to have been lower than for Guillemot.

Scotland hosts most of Britain’s breeding auks (>60% for all three species) (Table 16) with SPA colonies distributed around the whole of Scotland (Figure A 1). Within Scotland, Guillemots have 30 terrestrial colony SPAs as a designated (breeding) feature, and three marine SPAs, two of which are designated for both breeding and non-breeding birds (Table 16). Razorbills have smaller populations and have a more restricted distribution. Within Scotland, they have 16 colony SPAs as a designated (breeding) feature and one marine SPA designated for non-breeding birds. The Atlantic Puffin has 18 Scottish colony SPAs designated as a breeding feature, of which three are marine SPAs, and it is not a non-breeding feature for any SPAs.

The three auk species are considered to be vulnerable to offshore wind developments primarily through displacement. Razorbill, Guillemot, and Puffin were scored as having vulnerabilities to displacement of 0.9, 0.9, and 0.79 respectively, placing them joint 1^st and 4^th most vulnerable to displacement of the 11 species assessed by Searle et al. (2020). For collision vulnerabilities the three species were assessed as having much lower vulnerability, at 0.2, 0.23, and 0.17 (same order as before), placing them as the three least vulnerable species at 10^th, 9^th, and 11^th of the 11 species assessed by Searle et al. (2020) based on the methodology of Certain et al. (2015) using earlier assessments by Furness et al. (2013) and Wade et al. (2016).

Table 16. Fact box for auks (sources included as table footnotes), values are means (± one standard deviation) unless otherwise stated.

This table can be viewed in Excel format.