Tracking of small Asian migratory songbirds. Bird-borne light-level geolocators record daylength along the migration route, from which location can be estimated. Geolocators are mounted with leg-loop harnesses on the back of A) Siberian Rubythroat, B) Red-rumped Swallow, C) Yellow-breasted Bunting and D) Pallas’s Grasshopper Warbler. The devices are well visible from above (A,B) but are hardly visible in more natural postures (CeD). Photographs by R. J. Heim, L. Fuhse, A. Heim and W. Heim (from A to D). From Heim et al 2020, https://www.sciencedirect.com/science/article/pii/S2351989420307563?via%3Dihub#fig1