Singing behind the stage: thrush nightingales produce more variable songs on their wintering grounds
The songs of migratory passerine birds have a key role in mate attraction and territory defence during the breeding season. Many species also sing on their wintering grounds, but the function of this behaviour remains unclear. One possible explanation, proposed by the song improvement hypothesis, is that the birds take advantage of this period to develop their singing skills for the next breeding season. If so, non-breeding songs should reflect features of an early phase in song development, characterized by high vocal plasticity. In our study, we tested this prediction by comparing songs of thrush nightingales (Luscinia luscinia) recorded at two different breeding areas in Europe and one wintering area in Africa. While all songs from European localities had a typical structure characteristic of the study species, 89% of the songs recorded from Africa were highly variable, lacking such typical structure. We conducted further detailed analysis of breeding and winter songs that exhibited species-specific structure. First, we explored plasticity at the syllable level using a cross-correlation analysis, to obtain similarity scores as a measure of consistency. Second, we asked multiple human observers to quantify element variability. Our results showed significant differences in syllable consistency between breeding and wintering grounds, with more consistent delivery of syllables in the breeding areas. Likewise, element variability was substantially lower in the breeding populations. While both results fit the predictions of the song improvement hypothesis, more research is needed to elucidate the roles of singing on the wintering grounds.
Many migratory songbirds sing on their wintering grounds, outside the breeding period. While the role of singing during breeding has been broadly studied, our understanding of the function of winter singing remains limited. We analysed songs of the thrush nightingale, a migratory songbird with highly complex songs, comparing song structures recorded from breeding populations in Europe and an African wintering site. We demonstrate that males recorded at wintering locations sang songs with both significantly lower syllable consistency and higher element variability. Such characteristics are comparable to those observed during the sensorimotor phase of song development, previously described in other species. This pattern supports the song improvement hypothesis, suggesting that males singing on the wintering grounds may practice songs for the next breeding season. This study contributes to the understanding of the functions of songbird vocal behaviour out of the breeding context.
KeywordsThrush nightingale Song function Song plasticity Song consistency Element variability Non-breeding singing
We want to thank all the colleagues involved in the assessment of element variability for their precious help: Ondřej Belfín, Paolo Biella, Sharina van Boheemen, Hana Kahounová, Jan Mertens, Alexandra Průchová, Lucie Jiránková and especially Adam Petrusek for his overall support and help with the manuscript. NG would like to thank Wolfgang Goymann, Sonja and Andreas Kümmerle and Liz and Neil Baker for logistic support as well as Mackubi Joseph for help with fieldwork in Tanzania. VI and IM are grateful to Vladislav Antipov for the recording of nightingales in Russia. We also thank David Hardekopf for language corrections. Additional thanks go to Tony Archer, James Bradley, Josh Engel, Niall Perrins and Derek Solomon for kindly consenting to the use of their recordings. Lastly, we are grateful to two anonymous reviewers and Jeff Podos for their valuable comments on a previous version of the manuscript.
This work was supported by the Association for the Studies of Animal Behaviour (ASAB) to NG; the Deutsche Forschungsgemeinschaft (grant numbers GE 1253/3-1 and GE 1253/3-2 to NG); the Russian Fund for Basic Research (grant number 16-04-01721 to VI) and the Russian Science Foundation (grant number 14-50-00029 to VI).
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. 2009, 2010).
Conflict of interest
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. 2019 (data Tables with recordings information and spectrograms prints used for the human-based assessment of element variability).
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