Tracking migratory songbirds: accuracy of light-level loggers (geolocators) in forest habitats

1. Tracking return migrations in songbirds has been impossible until recently when miniaturization of light-level loggers enabled observation of the first complete round trip. Although geolocators are extensively used on animals at sea, little is known about how accurate geolocators are for tracking terrestrial or forest-dwelling migrants. 2. To test the accuracy of geolocators for tracking migratory songbirds living in forested habitat, we calibrated geolocators to a source population located in central Europe and collected location estimates based on the source population calibration from stationary geolocators deployed over an 800 km NE to SW gradient in Western Europe. Additionally, we fit non-migratory songbirds (European blackbirds, Turdus merula) with geolocators for 12 months to compare known locations of individuals with locations estimated by geolocators. 3. We found an average error +/- 95% CI of 201 +/- 43 km in latitude for stationary geolocators in forest habitat. Longitude error was considerably lower (12 +/- 03 km). The most accurate geolocator was on average 23 km off target, the worst was on average 390 kmoff. 4. The winter latitude estimate error for geolocators deployed on sedentary birds was on average (+/- 95% CI) 143 +/- 62 km when geolocators were calibrated during the breeding season and 132 +/- 75 km when they were calibrated during the winter. Longitude error for geolocators deployed on birds was on average (+/- 95% CI) 50 +/- 34 km. 5. Although we found error most likely due to seasonal changes in habitat and behaviour, our results indicate that geolocators can be used to reliably track long-distance forest-dwelling migrants. We also found that the low degree of error for longitude estimates attained from geolocators makes this technology suitable for identifying relatively short-distance movements in longitude.