Labile characters allow individuals to flexibly adjust their phenotype to changes in environmental conditions. There is growing evidence that individuals can differ both in average expression and level of plasticity in this type of character. Both of these aspects are studied in conjunction within a reaction norm framework. Theoreticians have investigated the factors promoting variation in reaction norm intercepts (average phenotype) and slopes (level of plasticity) of a key labile character: behaviour. A general prediction from their work is that selection will favour the evolution of repeatable individual variation in level of plasticity only under certain ecological conditions. While factors promoting individual repeatability of plasticity have thus been identified, empirical estimates of this phenomenon are largely lacking for wild populations. We assayed aggressiveness of individual male great tits (Parus major) twice during their egg-laying stage and twice during their egg-incubation stage to quantify each male's level of seasonal plasticity. This procedure was applied during six consecutive years; all males breeding in our plots during those years were assayed, resulting in repeated measures of individual reaction norms for any individual breeding in multiple years. We quantified among- and within-individual variation in reaction norm components, allowing us to estimate repeatability of seasonal plasticity. Using social pedigree information, we further partitioned reaction norm components into their additive genetic and permanent environmental counterparts. Cross-year individual repeatability for the intercepts (average aggressiveness) and slopes (level of seasonal plasticity) of the aggressiveness reaction norms were 0·574 and 0·516 respectively. The mean of the posterior distributions suggested modest heritabilities (h2 = 0·260 for intercepts; h2 = 0·266 for slopes), but these estimates were relatively uncertain. Males behaved more aggressively in areas with higher breeding densities, and became less aggressive and less plastic with increasing age; plasticity thus varied within individuals and was multidimensional in nature. This empirical study quantified cross-year individual repeatability, heritability and age-related reversible plasticity in behaviour. Acknowledging such patterns of multi-level variation is important not only for testing behavioural ecology theory concerning the evolution of repeatable differences in behavioural plasticity but also for predicting how reversible plasticity may evolve in natural populations.

Males of socially monogamous species can increase their siring success via within-pair and extra-pair fertilizations. In this study, we focused on the different sources of (co)variation between these siring routes, and asked how each contributes to total siring success. We quantified the fertilization routes to siring success, as well as behaviors that have been hypothesized to affect siring success, over a five-year period for a wild population of great tits Parus major. We considered siring success and its fertilization routes as “interactive phenotypes” arising from phenotypic contributions of both members of the social pair. We show that siring success is strongly affected by the fecundity of the social (female) partner. We also demonstrate that a strong positive correlation between extra-pair fertilization success and paternity loss likely constrains the evolution of these two routes. Moreover, we show that more explorative and aggressive males had less extra-pair fertilizations, whereas more explorative females laid larger clutches. This study thus demonstrates that (co)variation in siring routes is caused by multiple factors not necessarily related to characteristics of males. We thereby highlight the importance of acknowledging the multi-level structure of male fertilization routes when studying the evolution of male mating strategies.