Meier CM, Peev SG, Liechti F. 2020. Bulgaria Sofia - Long term study on migratory movement of Alpine swifts (Apus melba)-reference-data. Movebank Data Repository. <a href="https://doi.org/10.5441/001/1.7kd62k00/5">https://doi.org/10.5441/001/1.7kd62k00/5</a>