SEATRACK uses three different logger technologies for year-round tracking of seabirds: archival geolocators (GLS), archival GPS and GPS transmitting through the mobile GSM network (GPS-GSM).
GPS-GSM transfers data within GSM coverage and do not need retrieval for downloading data, unlike archival loggers. Archival loggers on the other hand require much less power since they do not actively transmit data. Hence, they do not need a large battery capacity or solar panels, reducing both size and weight. Therefore, archival loggers require revisiting breeding sites for retrieving loggers, which is a feasible method for many seabird species as most individuals return to the same nesting grounds throughout their life. By 2023, average retrieval rate in SEATRACK for 11 species was 59 percent. However, retrieval rates were low for large gulls, ranging from 18 to 40 percent. Consequently, SEATRACK started tracking large seagulls using GPS-GSM mounted with harness or leg-loops in 2024.
Which technology can be used to track a species in SEATRACK depends on the weight and size of each technology, attachment methods, feasibility, and country specific regulations concerning animal instrumentation. Figure 1 gives an overview of technologies used for the different SEATRACK species.
Archival geolocators
Archival geolocators are light and small and suitable for long-term deployment with low impact on animal welfare. In SEATRACK, we attach geolocators to the bird using leg-rings and chose the geolocator model depending on the species’ size. Depending on the model, expected battery-life ranges from 1 to 5 years. In addition, geolocators record contact with saltwater and sea surface temperature, depending on the model in question, which can be important for studies of behaviour and ecology.
Geolocators produce on average two positions each date. However, light-derived positions can only be used as approximates for an individual’s true location (error >150 km). In particular, latitudes will be unreliable in weeks before and after each equinox, and positions cannot be estimated for individuals visiting Arctic or Antarctic regions during midnight sun or polar night. For more info about this, see [link: standardization].
Archival GPS
Archival GPS are light-weight devices (~3.5 g) that can be mounted to a ring on the leg of the bird. Slightly heavier and larger than geolocators, archival GPS devices are only to be mounted on individuals of more than 900 g of weight in SEATRACK. The technology is still under development with the first prototypes deployed in 2022.
The archival GPS aim to produce one positional fix each day. Successful fixes cannot be achieved when submerged in water. Hence, periods without any flight or time on land is unlikely to produce positions. The loggers are actively using their immersion sensor to avoid spending battery on attempting GPS fixes while submerged.
GPS-GSM transmitter
Solar-powered GPS-GSM allow live tracking of large seagulls in SEATRACK – potentially over several non-breeding seasons. Communication with the logger over the GSM network also allow for reprogramming loggers while deployed to e.g. save battery when solar conditions deteriorate in winter. Deployment is always done by experienced and licensed personnel trained in the chosen attachment method.
