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.