Thanks to their light weight, multi-sensor geolocators are ideally suited to study the migration patterns of smaller birds. Indeed, while GPS trackers provide accurate data, their cost is prohibitive and their weight still restricts their use to larger birds (100grams and above). In particular, thanks to their affordability, geolocators can be deployed in under-studied contexts, bearing great potential, for instance, to uncover the migration strategies of many intra-African migrants, still largely unknown to this day.
Arguably, the major drawback of these devices is the need to recapture the same individual to retrieve the data from the device. This creates significant hours of fieldwork, largely outweighing the cost of the devices themselves. Typically, only 10-30% of the devices tend to be retrieved.
In one of the first geolocator deployments on the African continent, on the coast of Kenya, we tackled this issue by leveraging an existing ringing database. In order to equip individuals that would be most likely to recaptured the following year, we compared recapture rates of juveniles and adults, as well as individuals captured at different times of the year, to design an optimal ringing schedule (when, where, how long to ring) and define which classes of birds to equip. The presented analysis enables us to accurately estimate how many devices we would realistically be able to equip, and thus optimize resources, both by accurately estimating how many devices to order, and by minimizing the hours spent in the field while maximizing recapture.
The methodological tools presented in this study aim to be accessible to all researchers who have historical ringing data for the species studied at hand, to help plan and carry out geolocator studies.
The results of this study were published in the Journal of Field Ornithology:
Nussbaumer, R., L. Kirao, F. Liechti, and C. Jackson. 2022. Using ringing data to inform geolocator deployment: a case study of the Red-capped Robin-chat Cossypha natalensis in East Africa. Journal of Field Ornithology 93(2):8. https://doi.org/10.5751/JFO-00113-930208