Background
Over the past decades, tracking technologies have become more ubiquitous and helped uncover crucial spatio-temporal relationships in nature. To extend the tracking of small animals and reduce any potential adverse impact of devices, methodologies compatible with light-weight devices are sought after. Measured by light-weight geolocators, atmospheric pressure provides an untapped opportunity for global geopositioning, as its natural temporal variation is unique to each location.
Method
In this study, we estimate the position of birds by comparing pressure data recorded by the geolocator with reference data from a global weather reanalysis database. The method produces a likelihood map of the position based on (1) a mask of the locations for which the ground level elevation matches the pressure measured by the geolocator and (2) a likelihood of the mismatch between the temporal timeseries measured by the geolocator and the reanalysis dataset. This new method is introduced step by step and applied to 16 tracks from nine long- and short-distance migrant species.
Results
Using known positions of double-tagged individuals (light and pressure data), we demonstrate that our method is almost three times more accurate than light-based positioning with an average error of 44 km in our trials. In contrast to the traditional light-based approach, pressure geolocation can provide useful information for short stationary periods (less than a day) and is not affected by the equinox-problem, nor any shading effects due to weather or animal behaviour. To facilitate the application of the method, we developed an R package (GeoPressureR), together with a user guide (GeoPressureManual) and starting code (GeoPressureTemplate).
Conclusion
The use of pressure sensors to position animals has the potential to become widespread, thanks to affordable light-weight devices (<0.4 g) with a method to precisely and accurately estimate position, identify stopover locations, ultimately map a full migratory trajectory. This method broadens the reach of application of affordable light-weight tracking devices to short-distant migrants (>200km), forest-dwelling species, nocturnal animals and altitudinal migrants.