A comparative analysis of song amplitude across and within bird species

Animals use acoustic signals to exchange information essential to their survival and reproduction. A crucial parameter in acoustic communication is the amplitude of the signal, as it plays a decisive role in signal transmission and can also encode information. However, signal amplitude has been largely neglected in animal communication studies because it is difficult to assess. We measured song amplitudes in 17 European songbird species (Passeriformes) in the field and investigated the sources of variation between and within species. We found that song amplitude increased with increasing background noise (Lombard effect), in the presence of singing rival males, and it varied with the time of day. These findings highlight that birds can adjust how loud they sing in response to changes in the biotic and abiotic environments. However, our phylogenetically informed analysis found no support for the long-standing hypotheses that song amplitude reflects body size or territory size across bird species. We suggest that the variation of song amplitude between species is related to differences in ecology, in the strength of sexual selection, and in the costs of singing loudly rather than related to body size, as suggested before.