No reliable evidence for immediate noise-induced song flexibility in a suboscine

Vermilion flycatchers, a suboscine, sing songs with more elements in territories with higher urban noise levels. We tested the hypothesis that this pattern is achieved through vocal flexibility, by which individuals add elements to their songs when noise increases; we also tested whether males modulate other song attributes and song output with noise. To this end, we did a playback experiment with free-living males where we recorded their songs during three noise treatments: first ambient noise (FAN), high urban noise (HUN), and second ambient noise (SAN) treatments. We counted the number of song elements and measured acoustic attributes both in the whole song and in the song terminal element (T). Males did not modify the number of song elements, nor song minimum frequency, with noise. The T minimum frequency slightly increased during SAN when compared to the FAN and HUN treatments, but it did not differ between the HUN and FAN treatments. Thus, we interpret these results as a lack of reliable evidence of immediate noise-induced song flexibility in frequency parameters. Song entropy decayed during the trials, but this seemed to be an effect of time and not a noise-induced change. Vermilion flycatchers appear to be less capable of modulating spectral song attributes to cope with noise than many oscines. We discuss other potential strategies that this species may use to deal with noise and a possible mechanism by which males end up singing longer songs in noisier territories (natural selection).