Fight song: variation in singing behaviour and song structure during natural agonistic interactions in a tropical songbird, Adelaide's Warbler ( Setophaga adelaidae )

Birds may use their singing behaviours and song structure as agonistic signals in territorial encounters. We conducted an observational study to test this hypothesis in male Adelaide's Warblers Setophaga adelaidae , a tropical songbird that defends a territory year‐round. We described two singing behaviours and nine song structure variables (including vocal performance measures) around the time of natural territorial encounters. We found that birds decreased their song rate and song type switching around the time of encounters. Our findings allow us to reject the hypotheses that male Adelaide's Warblers use high song type diversity or high song rates as agonistic signals. They are, however, consistent with the hypothesis that repetitive singing may be an agonistic signal. Our results also suggest that song may not be an important agonistic signal in close range encounters and low song rates may provoke aggression. This study demonstrates how an observational approach grounds our understanding of aggressive signalling in the reality of natural agonistic encounters. Interestingly, our findings suggest that male Adelaide's Warblers mediate aggressive encounters with repetitive songs rather than high vocal performance or song diversity.