Estimating geolocator accuracy for a migratory songbird using live ground-truthing in tropical forest

Abstract

Miniaturized light-level geolocators allow year-round tracking of small migratory birds, but most studies use calibration only at breeding sites to estimate geographic positions. Ground-truthing of positions in tropical habitat is needed to determine how accurate breeding site calibrations (i.e. sun elevations) are for estimating location of winter sites. We tested the accuracy of geographic assignments using geolocator data collected from Wood Thrushes (Hylocichla mustelina) in Central America. For a given light threshold, sun elevation angle was higher in the tropics than at breeding sites and also varied significantly at tropical winter sites between wet (OctDec) and dry (Jan-Mar) seasons. However, estimation of Wood Thrush territory latitude did not differ significantly when using breeding or tropical dry season sun elevation. Average error in assignment to tropical sites was 365 ± 97 km (0.2-4.4°) in latitude. To obtain the best latitude estimates in the tropics with geolocators, we recommend using locations during the dry season where sun elevations are closer to those measured at breeding sites. We emphasize the importance of longitude in assigning forest birds to unknown sites; longitude estimates for Wood Thrushes in the tropics were, on average, within 66 ± 13 km (0-0.6°) of actual longitude. Latitude estimates were more accurate (180 ± 48 km) when assigning birds to breeding sites using deployments of geolocators in the tropics. Studies of species that are territorial in winter could collect more accurate migratory connectivity data by deploying geolocators at tropical wintering sites.