Using ringing data to inform geolocator deployment: a case study of the Red-capped Robin-chat Cossypha natalensis in East Africa

Thanks to their light weight and low cost relative to GPS trackers, light-level geolocators are uniquely positioned to uncover bird migration patterns across less well-financed and understudied regions of the world. A main drawback of geolocators is the need to recapture equipped birds to retrieve the data. Maximizing the recapture rate is therefore critical to the success of any geolocator study. In this paper, we present a methodology drawing on historical ringing data in order to inform the deployment of geolocators, both in terms of how many birds can be equipped, and when/which birds to equip in order to maximize retrieval. We illustrate this methodology with a geolocator study of Red-capped Robin-chats (Cossypha natalensis) on the coast of Kenya and find that it accurately estimates how many geolocators to source. It also provides insights into which classes of birds (based on age, capture history, and timing within the season) are most likely to be recaptured. Finally, the analysis of recapture rates allows minimization of geolocator use and thus potential negative impacts to a population.