Abstract
While some seabird species are specialist foragers, others are considered to have more generalist foraging strategies. However, substantial individual-level specialisation has been documented even within generalist species. Understanding the spatial and foraging ecology of seabirds is crucial for assessing the ecological drivers and fitness consequences of a specific foraging strategy, and this also enables the prediction of key information with important implications for marine spatial planning and conservation. Here, we study individual foraging behaviour and environmental variables influencing behavioural transitions using tracking data from 25 adult Great Black-backed Gulls Larus marinus during the 2024 breeding season in an archipelago in southern Norway. Twenty data loggers recorded 3,285 foraging trips during the entire breeding season, and we excluded data from five individuals whose breeding attempts failed. We found a clear bimodal pattern of short and long trips, whereby birds made frequent foraging trips to a productive shelf edge characterised by shallow waters. Birds were more likely to switch from foraging to a transit state when sea depth increased. Unlike previous studies on large gulls, we found little evidence for spatial segregation of foraging areas. Our study highlights low levels of apparent individual specialisation and the occurrence of overlapping foraging areas during breeding in this large seabird in the North Sea. In contrast to previous studies, we demonstrate that GPS deployment using leg-loop harnesses did not adversely affect nesting success and that our methods can successfully be applied to study the spatial ecology of this species.