Loggers affect the foraging behaviour and fitness of European shags

Biologgers enable real-time collection of detailed behavioural and
physiological data from wide-ranging animals, including seabirds
inhabiting remote regions. However, the number of studies using tracking
devices has not been matched by research exploring the behavioural and
fitness costs of deployment, and the effects on data reliability. We
assessed how GPS loggers, video loggers, and time depth recorders (TDRs)
affect the behaviour, physiology, and reproductive performance of European
shags (Gulosus aristotelis) breeding on Sklinna, Norway. The loggers
varied in mass, attachment location, and deployment duration, allowing
comparison of their relative effects. Birds without loggers served as
controls to assess logger-related changes in adult body mass, chick
growth, reproductive success, and survival. Birds with long-term
tail-mounted GPS loggers and leg-mounted TDRs showed altered foraging
behaviour, including shorter trips and dives, compared to birds with
tail-mounted GPS and TDRs of the same weight, instrumented for only two
days. A mean loss in body mass was experienced by adult birds regardless
of the logger-type used, while chick growth rate dropped to 20% of that
observed in control nests when video, TDR and GPS loggers (~4% of body
mass) were deployed together. Logger attachments did not impact
reproductive success and overall logger birds showed higher survival than
controls. However female survival was lower than males among birds fitted
with video and long-term GPS loggers. Our results demonstrate the
importance of measuring behavioural and physiological effects that can
scale over time. The advances in our understanding of animal ecology and
behaviour generated by biologging have been impressive, but there is a
need to consider the impacts on animal welfare and data quality.
Consistent reporting of logger deployment details is essential to assess
biologging impacts across species and refine protocols that account for
device weight, drag and attachment location.