Daily, seasonal and annual variation in area use of Lesser Black-backed Gulls (Larus fuscus) related to offshore renewable developments

BTO scientists have deployed hundreds of GPS tags on Lesser Black-backed Gulls since 2010. In this study, long-term GPS tracking devices were fitted to 36 adult gulls breeding at South Walney, in Cumbria. During the course of data collection (2016–19), two more offshore wind farms were built in addition to those already present in the area. This allowed the researchers to examine how tagged gulls’ habitat use might or might not change before, during and after the construction of these new developments.

Overall, the study showed that tagged Lesser Black-backed Gulls did not spend a lot of their time offshore (less than 1.25% of their overall time budgets). They spent slightly more of their time offshore at night (1.82%) than during the day (1.00%). The gulls’ individual ‘home ranges’ (the total area an animal uses for foraging and breeding) varied significantly from bird to bird, and through time. For instance, home ranges were smaller at night than during daylight hours, and they shrank in late May (when birds were spending a lot of time incubating their eggs) and again in late July (at the end of the breeding season). Similarly, the distance gulls covered on their trips away from the colony varied, with the shortest trips coinciding with late May and longer trips from August onwards, as birds began to lose their association with the colony and disperse after the end of the breeding season.

Only two tagged gulls had home ranges that overlapped with offshore wind farms, and fewer than half of the birds tracked visited an offshore wind farm in any given year. The proportion of birds visiting offshore wind farms did not vary significantly from year to year, including in areas of sea where wind farms were being built or coming into operation during the course of data collection.

The results from this tracking study were perhaps surprising. Although gulls are known to use a range of terrestrial areas, traditional thinking is that they still widely use offshore areas, especially during the chick rearing period. The lack of offshore movements in this study meant that broad responses in foraging and home ranges were not detected directly in relation to the construction of new developments. 

Nonetheless, it was useful to show variation in these measures between years and to highlight the importance of a multi-year dataset. It was also useful to collect evidence specifically from a protected area located close to new offshore developments. South Walney is a nature reserve designated as a Site of Special Scientific Interest and a Special Protection Area. In the past, potential impacts on birds using protected areas close to new or existing developments were inferred from data collected at other sites. Thanks to this study we now have evidence from individuals at a protected site directly affected by the new developments.