Caithness, northern Scotland, has historically been an important breeding area for Great Black-backed Gulls Larus marinus, but monitoring at selected colonies shows that numbers have declined substantially over the past 20 years. To investigate this decline in more detail, a survey of the whole of the Caithness coast was undertaken during the 2023 seabird breeding season. The survey found a total of 268 Great Black-backed Gull Apparently Occupied Nests (AON) at 76 colonies. Birds were widely distributed around the coast of Caithness with the most significant numbers being on Stroma (24% of the population) and the coast south of Wick (63%). Colonies were mostly small, with 65% holding just a single breeding pair. The results confirm that a substantial population decline of 74% in the number of AON has taken place since the Seabird Colony Register in 1985–88. Larger colonies have seen a gradual decrease in numbers since the latter half of the 1990s, except for Stroma where numbers were at a minimum in c. 2005 before recovering somewhat. The cause of the Great Black-backed Gull population declines remains unclear. Breeding productivity in 2023 was towards the upper end of the range found elsewhere in Britain (0.64 to 1.45 chicks per breeding pair) and, though lower in some previous years, appears not to have been a significant contributor to the declines. Fish and discards from fisheries are important components of the diet of Caithness Great Black-backed Gulls and so reductions in fish stocks or the availability of discards (based on landings of fish at harbours around the Moray Firth) may, in part, be contributing to their declines. Disturbance by ground predators such as Red Foxes Vulpes vulpes is implicated in some recent changes in the numbers of Great Black-backed Gulls breeding on the cliffs south of Wick, and the extinction of several former colonies. We discuss other potential factors contributing to the reduction in numbers and possible methods of halting and reversing the declines of Great Black-backed Gulls.

Seabirds are among the most threatened groups of birds globally, with widespread declines being observed in a broad range of groups and species (Dias et al. 2019). Despite their generalist and opportunistic tendencies, several populations of large Larus gulls have declined in recent decades (Nager & O'Hanlon 2016; Bond et al. 2016). Within the UK, Herring Gulls L. argentatus have a Red status in the Birds of Conservation Concern 5, whilst Lesser Black-backed Gulls L. fuscus and Great Black-backed Gulls L. marinus are Red listed species (Stanbury et al. 2024). A recent assessment of Great Black-backed Gull population trends across their breeding range estimated a global decline of 43–48% between 1985 and 2021, therefore arguing that the species should be reclassified as 'Vulnerable' on the IUCN Red List (Langlois et al. 2022).

Due to the opportunistic foraging behaviour of gulls, identifying the specific causes of their population declines has been challenging, and it is likely that a range of stressors are involved across the breeding and non-breeding ranges of Great Blackbacked Gulls (Lewis, in Burnell et al. 2023). These stressors include food availability, especially changes in food associated with landfill and fishery discards (Bicknell et al. 2013; Sherley et al. 2020), predation of eggs, chicks, and adults at the breeding colonies by mammalian and avian predators (Hipfner et al. 2012; White et al. 2012), and continued persecution/culling in some locations (Hario & Rintala 2016). Within Britain, declines of Great Black-backed, Lesser Black-backed and Herring Gulls on Canna, northwest Scotland, have been linked with reductions in fish landings at Mallaig, c. 40 km away (Foster et al. 2016), although such declines could simply be a return to historical population levels following a brief period of food abundance from discards (Lewis, in Burnell et al. 2023). Another area where fish stocks and the availability of discards are likely to be important is the Moray Firth area of northern Scotland which includes fishing ports such as Fraserburgh, Macduff, Buckie and Lossiemouth on its southern side, and Wick and Scrabster on the northern side.

Nationally important numbers of seabirds breed around the Moray Firth, notably on the cliffs of Caithness, and historically it has been an important area for Great Black-backed Gulls. From 1970 until the mid 1990s, Caithness held around 5% of the British breeding population, but numbers have subsequently declined (Mitchell et al. 2004; Swann 2016, 2018). Between 2002 and 2022 counts at selected Caithness colonies revealed a decline of 66% (Oksien & Sellers 2023). Subsequently during the 2022 breeding season; it became evident that there had been a large mortality of seabirds breeding all along the east coast of Scotland following an outbreak of Highly Pathogenic Avian Influenza (HPAI) (Cunningham 2022, HPAIG 2023; Tremlett et al. 2024). In Caithness this mortality began at the end of May 2022, shortly after the completion of the 2022 seabird monitoring counts.

To evaluate whether previous counts were representative of the Caithness population, and to enable comparisons between prior counts from selected Caithness colonies and other surveys of the whole area, we carried out a full census of the Great Black-backed Gull population breeding in Caithness during the 2023 breeding season. We also sought to document the location and breeding habitat of all breeding Great Black-backed Gull nests in Caithness, partly to provide a firm baseline against which future changes can be judged, and partly as an aid for future counters. To complement the Great Black-backed Gull census, we also recorded productivity from a subset of colonies. Measuring demographic rates alongside population counts can provide useful data to understand the drivers of population change, even in long-lived species such as seabirds (Cairns 1987). The 2023 survey also provided the opportunity to undertake a preliminary assessment of the impact of the HPAI outbreak on the Caithness Great Black-backed Gull population.

We thank Anson MacAuslan and the Welbeck Estates for allowing us free access to the cliffs south of Berriedale, to Alex Jappie in Helmsdale and to Caithness Seacoast Ltd, Wick for their help in making the boat trips possible. Thanks also to Donald Omand for monitoring and providing information on the gulls breeding at Dounreay. Finally, we are indebted to the Reviewers and Editors for their helpful suggestions.

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