Census of Great Black-backed Gulls breeding in Caithness, Scotland

Abstract

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 Survey 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.