The use of light-level geolocators to study wader movements

Light-level geolocators have been deployed on a number of wader species to track migratory movements, and identify breeding, stopover, and wintering areas. These devices, which have only become available for small to medium-sized waders in the past few years, measure ambient light levels and store them in a time series in their internal memory. Such data allow estimates of the time of sunrise and sunset, and by conversion, latitude and longitude, on a daily basis. Geolocator use is limited to situations where there are periods of night and day, and when the need for location accuracy is rather low (hundreds of kilometres). Inaccuracies in the locations are primarily in latitude (as opposed to longitude) and are strongly positively correlated with proximity to the equinoxes and to the equator. Deployment is currently limited to situations where individuals can be recaptured at some later date. Breeding grounds can be ideal for species that are site-faithful although low densities and remote working conditions limit this approach. Migratory stopover and wintering grounds can be fruitful deployment locations although recaptures may be more difficult. Geolocators are typically attached using leg rings or flags, leg harnesses, or body harnesses. In principle implanting a geolocator in the coelomic cavity is possible but this has not yet been attempted. More studies are needed to adequately assess the  impact of attaching geolocators to birds. We recommend studies on birds fitted with geolocators both in captivity and in the field, and the measurement of return rates of birds with and without geolocators. Future developments will likely see further miniaturization, remote downloading of data, and refinement of data analysis techniques.