Funding for this research was provided by:
Deutsche Forschungsgemeinschaft (GE 1253/3-1 & GE 1253/3-2)
Russian Science Foundation (14-50-00029)
Přírodovědecká Fakulta, Univerzita Karlova (Grant for research activity (PhD student))
Association for the studies of Animal Behaviour
Russian Fund for Basic Research (16-04-01721)
Text and Data Mining valid from 2019-11-01
Received: 5 May 2019
Revised: 17 October 2019
Accepted: 24 October 2019
First Online: 19 November 2019
Compliance with ethical standards
: All applicable international, national, and institutional guidelines for research on animals were followed. Samples used in this study are based on a non-invasive recording technique from a distance. Birds were not caught, but for few individuals, playback stimulation was performed to trigger an active singing behaviour. This stimulation was kept as short as possible and we are not aware of any consequences for subjects’ breeding or welfare. Passive recording did not require supervision from institutional or regulatory bodies, but authorizations were granted from legal authorities in each different sampling country when needed. In the wintering population, recordings were made within the framework of another research project (for more details see Geberzahn et al. CitationRef removed, CitationRef removed).
: The authors declare that they have no conflict of interest.
: Analyses reported in this article can be reproduced using the data provided in Souriau et al. CitationRef removed (data Tables with recordings information and spectrograms prints used for the human-based assessment of element variability).