• Calonectris diomedea
• foraging areas
• foraging behaviour
• procellariiform
• seabird

The Optimal Foraging Theory assumes that animals optimize their energy budget by maximizing energy intake and minimizing energy expenditure per unit of time. During the breeding season, seabirds are central place foragers and they breed in large groups resulting in intense resource exploitation around the colony. If prey are uniformly distributed and abundant, the foraging areas of different colonies may overlap; if prey are patchily distributed, intraspecific competition can be high resulting in inter-colony foraging area segregation. Multi-colony studies on seabirds showed that when intraspecific competition is high (that is when colonies are close and large) segregation of foraging areas often occur. I focused on the Scopoli’s shearwater (Calonectris diomedea), a large procellariform breeding in the Mediterranean Sea. The aims of this study were to characterize the foraging behaviour of Scopoli’s shearwater and to investigate whether there was segregation between foraging areas of animals breeding in colonies of different size in the Tuscan Archipelago. A total of 30 adult birds breeding on the islands of Giannutri (n=8), Cerboli (n=8), Pianosa (n=7) and Argentarola rock (n=7) were GPS tracked during the chick-rearing period (July – September 2020). The period from 22nd July to 9th August was considered to have a comparable number of birds tracked in each colony. The route segmentation analysis was performed to identify the main behavioural patterns exhibited during foraging flights from GPS data. The categories referring to foraging behaviour (“intensive search” and “extensive search”) were chosen to identify individual foraging areas. These areas were estimated by means of the Kernel Density Estimation (KDE) method, which is the commonly used method in this kind of studies on seabirds. Since Scopoli’s shearwaters nest on cliff edges, a second method suitable to identify hard boundaries within the home range was applied, that is the Local Convex Hull (LoCoH) method. The Utilization Distribution (UD) overlap among individuals was calculated and used to investigate whether there was segregation among individuals by means of hierarchical clustering. As expected, the LoCoH method estimated smaller foraging areas with less portions of the mainland compared to KDE method, and this is important to evaluate seabirds foraging areas also for management purposes. The number of tracked individuals, even if small, seemed to be quite representative of the foraging behavior of the individuals of each colony. The results showed that individuals from Argentarola rock and Giannutri Island foraged around their breeding colony and along the coast of Tuscany and Lazio; birds nesting on Pianosa Island all foraged around their breeding colony whereas individuals from Cerboli Island explored wider areas. On average, the degree of overlap was greater between individuals of the same colony than between individuals from different colonies and the UD of individuals from Pianosa Island did not overlap either individual UDs from Argentarola rock nor Giannutri Island. Birds from Pianosa and Giannutri Islands formed two relative homogeneous groups and tended to have non-overlapping foraging areas. Only individuals from Pianosa Island tended to have an exclusive trophic area, probably because they are surrounded by high productive waters. In the areas surrounding the other islands, there may be lower value of productivity or there may be a greater disturbance that forces the use of more distant sites. This multi-colonies study provides information about the habitat use in foraging behaviour of Scopoli’s shearwaters and it seems to confirm that relatively large neighbouring colonies tend to have non-overlapping foraging areas.