A study on the effects of using different kinds and combinations of loggers on European shags (Gulosus aristotelis) was recently published in the Journal of Avian Biology. The authors conclude that loggers can have negative impacts on the shags’ foraging behaviour and have clear recommendations for designs of future logger studies.
The European shags in the study were equipped with various types and combinations of data loggers. Photo: Nina Dehnhard/NINA
Useful tools in rapid development
Loggers have become a central part of research on seabirds and other groups of animals over the past 20 years. In seabird research, loggers provide very useful documentation of diving behaviour, colony attendance, important marine areas and area use during and outside the breeding season.
Developments in technology have resulted in loggers becoming smaller but, at the same time, their use has increased. Researchers are deploying loggers on more individuals, smaller species and in more colonies, and they are being deployed over longer periods of time as a result of better batteries and built-in solar panels. It is therefore increasingly important to assess whether the use of loggers has any negative consequences for the birds.
Measured effects on adults and chicks
Nina Dehnhard at the Norwegian Institute for Nature Research has led a study on the impacts of loggers on European shags breeding at Sklinna in central Norway. She is one of six researchers collaborating on this study. Using tracking data in combination with information on body mass, chick growth and adult survival in shags, Dehnhard and her colleagues could measure how different types of loggers affected the birds.
– Loggers of different kinds can be attached to birds in different ways – some are attached to the leg ring, while others are taped to the back or tail of the bird, she explains.
– In our study, we used three different types of loggers that were mounted on breeding adult shags in different ways and combinations – some for longer and others for shorter periods of time.
The study included a control group of breeding shags that had no loggers attached.
Logger categories and attachment positions on European shags: a) plastic ring only (control group); b) GPS (Global Positioning System) with TDR (temperature and depth recorder), both attached to the tail; c) GPS attached to the tail with TDR mounted
Reduced body mass and chick growth
Looking at the recorded weights of the birds studied, the researchers found that adult shags lost body mass regardless of logger category and duration of deployment. But not only the adults were affected. Chicks of parent birds carrying the heaviest types of loggers also had reduced growth rates compared to other chicks. Deployment duration also made a difference, as well as mounting method.
– We saw that birds with long-term loggers and leg-mounted dive recorders reduced their trip duration and performed shorter dives compared to birds with shorter deployment durations and tail-attached loggers, says Dehnhard.
Considerations for future studies
When diving, shags use their feet to move forward in the water. Leg-mounted loggers may affect the birds’ locomotion, thereby reducing their ability to dive to catch prey. This likely explains the observed difference in diving depths between birds with and without leg-mounted loggers. The researchers behind the study have clear recommendations for those planning to conduct logger studies on diving seabirds:
– First and foremost, loggers that are too heavy and causing too much drag must be avoided. Researchers should also consider reducing the number of birds needed in logger studies, says Dehnhard.
As there were also negative impacts on both chick growth and adult survival of shags carrying heavy loggers in this study, the authors advise against deploying loggers on the same individuals over consecutive years.
Contact: Nina Dehnhard, Senior Research Scientist
Read article: Loggers affect the foraging behaviour and fitness of European shags