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 environment. 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 to body size, as
suggested before.