Light-level geolocation in polar regions with 24-hour daylight
129 – 134
15 October 18
Swiss Ornithological Institute, Department of Bird Migration, Seerose 1, CH-6204 Sempach, Switzerland
Solar geolocation has become one of the most frequently used tools in wader migration research. Geolocators provide location estimates based on recorded light intensities, and more specifically, on the changes in light during the twilight periods, allowing an increase in knowledge on how waders migrate across the globe. Yet, quite a number of species breed in polar regions where they experience 24-hour daylight. Given the lack of recorded twilight times on the geolocators during this period, analysis methods have been unable to resolve birds’ positions when they are in constant daylight. This is especially problematic in the older geolocator generations that could record light only over a narrow range of light intensities. Some newer geolocators record the full light range, allowing even small changes in light intensity during the day (under bright light conditions) to be detected. However, the common methods for estimating locations are not designed for changes in high light regimes that lack dark periods during the night. Previously, I developed and implemented a method for analysing continuous light records, which evaluates the likelihood of a measured light cycle being from a given location, leading to first estimates of breeding sites in Sanderlings Calidris alba and Great Knots C. tenuirostris. However, the final decision on the breeding site was somewhat subjective and a formal description of the method was still lacking. Here, I describe a new development for estimating high-latitude positions, which is implemented in a freely available R Package called PolarGeolocation.