The effect of social environment on bird song: listener-specific expression of a sexual signal

Animal signals should consistently differ among individuals to convey distinguishable information about the signalers. However, behavioral display signals, such as bird song are also loaded with considerable within-individual variance with mostly unknown function. We hypothesized that the immediate social environment may play a role in mediating such variance component, and investigated in the collared flycatcher (Ficedula albicollis) if the identity and quality of listeners could affect song production in signalers. After presenting territorial males with either a female or male social stimulus, we found in the subsequent song recordings that the among-stimulus effects corresponded to non-zero variance components in several acoustic traits indicating that singing males are able to plastically adjust their songs according to stimulus identity. Male and female stimuli elicited different responses as the identity of the female stimuli affected song complexity only, while the identity of male stimuli altered also song length, maximum frequency and song rate. The stimulus-specific effect on song in some cases decreased with time, being particularly detectable right after the removal of the stimulus and ceasing later, but this pattern varied across the sex of the stimulus and the song traits. We were able to identify factors that can explain the among-stimulus effects (e.g. size and quality of the stimuli) with roles that also varied among song traits. Our results confirm that the variable social environment can raise considerable variation in song performance, highlighting that within-individual plasticity of bird song can play important roles in sexual signaling.

Methods
After presenting territorial collared flycatcher males with either a female or male social stimulus, we recorded their song and extracted several song traits from the recordings to investigate whether the song is influenced by the immediate social environment. We generally calculated song traits from 20 songs, but we also repeated the analysis calculating song traits for consecutive 5 songs in order to investigate the temporal change of the effect of the social stimuli.

We provide four tables, two containing song variables calculated for 20 songs from the recordings, including also morphological data for the listener birds among others, and two with song variables calculated for the subsequent bins of 5 songs, separately for the two datasets that corresponds to the experiments using male or female listener birds.