Decline of Great Black-backed Gulls breeding in Caithness, 2002–2022

M. OKSIEN

& R.M. SELLERS

This paper describes a project to monitor the numbers of Great Black-backed Gulls breeding in Caithness which shows that the population underwent a decline between 2002 and 2022, amounting to a loss of about two-thirds of the population. Reasons for the decline are discussed, the most important probably being reduced food availability impacting on survival. Other factors including poor productivity in the final years of the study, and predation or disturbance by Foxes may also be implicated at certain colonies.

Introduction

For much of the past half century Caithness has been an important breeding area for Great Blackbacked Gulls Larus marinus. Figures from the first two national seabird censuses suggest that its population here had remained stable between 1970 and at least 1985 at around 1,000 apparently occupied nests (AON) representing a little over 5% of the then British population and about 6.5% of that in Scotland (Mitchell et al. 2004). By the time the third national seabird survey, Seabird 2000, was carried out in 1998–2002, the population is reputed to have fallen to just 211 AON, a decrease of 79% from the 1985–88 figure, and then partially recovered to about 336 AON in 2015/16 according to surveys undertaken for Scottish Natural Heritage (Swann 2016, 2018). In 2002 we began a series of annual counts at selected sites around Caithness. The results show the Caithness population has undergone a progressive decline in numbers over the past 20 years.

Methods

Great Black-backed Gull breeding numbers were monitored annually in 2005–20 and in 2022 at 21 sites in Caithness, and these, together with some preliminary counts, at a smaller number of sites between 2002 and 2004, form the basis of the population monitoring we describe here. Monitoring sites were selected because: (i) they were being visited for monitoring of other species notably Cormorants Phalacrocorax carbo (see Sellers 2020), (ii) habitat type (to include both colonies on stacks and on beaches) and (iii) geographical spread (in particular to ensure that colonies on both the north and east coasts of Caithness were included). Although counting larger colonies provides the most efficient way of monitoring overall numbers, some small (1–3 AON) colonies have been included to investigate how numbers at these have varied. The distribution of the monitoring sites is shown in Figure 1, and their key features are summarised in Table 1.