First biologging record of a foraging red-throated loon Gavia stellata shows shallow and efficient diving in freshwater environments

Recently, Red-throated Loons Gavia stellata (RTL) have been the subject of increased interest due to their negative interactions with shipping, offshore wind farms, and other marine industry activities. This has driven a desire to quantify the behaviour and ecology of this understudied species, particularly during the non-breeding season. To achieve this, Time Depth Recorder (TDR) and Global Location Sensor (GLS) tags were deployed on individuals from several European locations. Due to an incidental mortality, one set of tags was retrieved early. The single set of tags recorded activity from June to August 2018. The TDR collected records for 14 d, providing the first ever biologging data on RTL foraging in Europe. The bird was tagged 90 km from the coast; therefore, it only used freshwater lakes and was never recorded entering saltwater. The individual mostly undertook shallow dives, with maximum and mean depths of 20 m and 5.4 m, respectively. Foraging constituted 22.9 % of total activity during the sampling period. The RTL had diel foraging patterns, with dives being shallower and more frequent at times of “twilight” compared to “daylight.” These results provide novel information on an RTL's diurnal patterns of water depth usage and foraging effort during the summer, demonstrating the potential of data loggers to provide key insights into the foraging ecology of this species.