Individual animals can react to the changes in their environment by exhibiting behaviours in an individual-specific way leading to individual differences in phenotypic plasticity. However, the effect of multiple environmental factors on multiple traits is rarely tested. Such a complex approach is necessary to assess the generality of plasticity and to understand how among-individual differences in the ability to adapt to changing environments evolve. This study examined whether individuals adjust different song traits to varying environmental conditions in the collared flycatcher (Ficedula albicollis), a passerine with complex song. We also aimed to reveal among-individual differences in behavioural responses by testing whether individual differences in plasticity were repeatable. The presence of general plasticity across traits and/or contexts was also tested. To assess plasticity, we documented 1) short-scale temporal changes in song traits in different social contexts (after exposition to male stimulus, female stimulus or without stimuli), and 2) changes concerning the height from where the bird sang (singing position), used as a proxy of predation risk and acoustic transmission conditions. We found population-level relationships between singing position and both song length and complexity, as well as social context-dependent temporal changes in song length and maximum frequency. We found among-individual differences in plasticity of song length and maximum frequency along both the temporal and positional gradients. These among-individual differences in plasticity were repeatable. Some of the plastic responses correlated across different song traits and environmental gradients. Overall, our results show that the plasticity of bird song 1) depends on the social context, 2) exists along different environmental gradients and 3) there is evidence for trade-offs between the responses of different traits to different environmental variables. Our results highlight the need to consider individual differences and to investigate multiple traits along multiple environmental axes when studying behavioural plasticity. 

We recorded song for 12 years in a wild population of collared flycatchers (Ficedula albicollis). We recorded songs after exposing the focal males to different focal contexts (displaying a male or female conspecific or without stimulus) and we also recorded the height of the position from where the bird sang. We also recorded the date of the measurement and the age of the focal male (1-year old or older).
From the song recordings three song traits were extracted: song lengt, maximum frequency and complexity for each song.

The continuous variables used in the statistical models were z-transformed.

There are missing values in complexity, as for the calculation of this variable the type of each syllable should be known and this could not be allways determined.