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.

Data files including information about experiments:

- Souriau_et_al_2019_BEAS_Source_data_Syl_Correlation.xls: 

A XLS spreadsheet containing information on the cross-correlation analysis comparing the syllable consistency in thrush nightingale (Luscinia luscinia) songs from breeding and wintering grounds, as well as additional information on the recordings.

- Souriau_et_al_2019_BEAS_Source_data_Syl_Correlation.txt: 

A tab-delimited TXT file containing information on the cross-correlation analysis comparing the syllable consistency in thrush nightingale (Luscinia luscinia) songs from breeding and wintering grounds.

- Souriau_et_al_2019_BEAS_Source_data_Elt_Variability.xls: 

A XLS spreadsheet containing information on the element variability analysis comparing songs of the thrush nightingale (Luscinia luscinia) from breeding and wintering grounds.

- Souriau_et_al_2019_BEAS_Source_data_Elt_Variability.xls: 

A tab-delimited TXT file containing information on the element variability analysis comparing songs of the thrush nightingale (Luscinia luscinia) from breeding and wintering grounds.

- Souriau_et_al_2019_BEAS_Spectrograms_Elt_Variability_analysis.pdf:

A PDF file displaying the spectrograms used for the element variability visual assessment of thrush nightingale (Luscinia luscinia) songs from breeding and wintering grounds (see Methods).