Song amplitude and population density in two sympatric warblers,Phylloscopus schwarziandP. fuscatus

In passerine birds, song is widely used broadcasting signal. One song trait that so far has received little attention is amplitude, i.e. the sound pressure level of a song. It has been shown that there is a considerable variation in song amplitude between species, but the causes and consequences of this variation are poorly studied. Here, we measured song amplitude in two similar-sized sympatric Phylloscopus, Radde’s Warbler P. schwarzi and Dusky Warbler P. fuscatus. We found that Radde’s Warbler males sing much more loudly than males of Dusky Warbler. The sound pressure level values of songs of Radde’s and Dusky Warblers were 78.6 ± 2.4 dB(A) (n = 16 males) and 70.9 ± 1.5 dB(A) (n = 16) at a distance of 1 m, respectively. The analysis of video-recordings revealed the larger extension of the neck region (‘throat sac’) in Radde’s Warbler than in Dusky Warbler. We hypothesised that the larger maximum ‘throat sac’ volume might be responsible for the louder song of Radde’s Warbler. We also found that Dusky Warbler males settled twice as close to each other than Radde’s Warblers. We hypothesised that the louder song of Radde’s Warbler could be useful for males of this species given their need to communicate through a larger distance.