Experimental evidence of slight alteration of male Collared Flycatcher songs based on the female audience

Bird song is an important and variable sexual signal in many passerine species. One function of this variability may be that males show mate choice and can alter their song according to the quality of the female to which they are singing. If attractive songs are costly, we can hypothesize that males sing more attractively or invest more in singing when trying to attract a better-quality partner. We tested this hypothesis with a field experiment in a wild population of Collared Flycatchers Ficedula albicollis. We presented female stimuli differing in the amount of plumage ornamentation to males on their territory and then recorded their song. We repeated the experiment at least twice for each individual with different female stimuli. We obtained 67 recordings from 29 males, each with 10–50 song responses. Five song traits (song length, mean frequency, frequency bandwidth, tempo and complexity) extracted from the recordings were used in further analyses examining whether the identity of the focal male and the female stimuli explain variance in song traits, and whether males change their songs when exposed to different females. While among-individual variance was considerable in all investigated song traits, female stimuli explained variance only in complexity and tempo. This indicates that males alter these song traits if they sing to different females. We could not clearly identify whether the difference in the response to female stimuli was driven by female quality either at population or individual levels. Overall, our results reveal individuality in song plasticity, and suggest that male mate choice may exist in this species.