Audio for Trill 4 example illustrated in  Figure 1  .
Hall M, Kingma S, Peters A
CC BY 2.5
Audio S4 from <cite style="font-style:normal">Hall M, Kingma S, Peters A (2013). "Male Songbird Indicates Body Size with Low-Pitched Advertising Songs". <i>PLOS ONE</i>. <a href="https://en.wikipedia.org/wiki/Digital_object_identifier" class="extiw" title="w:Digital object identifier">DOI</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1371/journal.pone.0056717">10.1371/journal.pone.0056717</a>. <a href="https://en.wikipedia.org/wiki/PMID" class="extiw" title="w:PMID">PMID</a> <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pubmed/23437221">23437221</a>.</cite>