The older the bolder: common goldeneye antipredator behaviour based on long-term individual data

Long-term individual-based studies of parental investment are pivotal to understanding the costs and benefits associated with defensive strategies under stressful conditions. In the context of avian research, limited literature exists on longitudinal changes in antipredator behaviour during incubation and potential selection pressures on a specific trait. We studied the response of common goldeneye, Bucephala clangula , to disturbance, mimicking a predation threat in a nestbox population from 1984 to 2021. From a total of 652 captured females, 516 were recaptured as breeders 2–15 times over the study period. During the nest-monitoring visits, female behaviour was classified as ‘shy’ (flushing before the human observer reached the nestbox, or they flew into the net placed over the nest hole), ‘average’ (leaving the nest when the observer scratched the nestbox wall, or they showed inconsistent flush patterns such as once behaving shy and once bold in the same year), or ‘bold’ (leaving the nest once physically touched or they did not leave at all). Our results provide evidence of plasticity of antipredator behaviour, since the probability of exhibiting a bold reaction to a perceived predation danger increased with age, whereas shy individuals became less common. On the other hand, we found a high level of repeatability of behavioural response across individuals. The findings reveal the coexistence of plastic and personality traits among common goldeneye females. Moreover, there was a positive linear interaction between age and clutch size on boldness, but only for clutches below the parasitism threshold (fewer than 13 eggs). Interestingly, hatching success was not related to behaviour, suggesting no direct selection pressure on antipredator behaviour and that depending on age/residual reproductive value, either shy or bold types are favoured to benefit the lifetime reproductive success of individuals. • Older females were more persistent in incubating under perceived predation danger. • Bolder individuals had larger clutches, up to the parasitism threshold. • In heavily parasitized nests, the boldness probability of females declined with age. • There was a high level of consistent behavioural difference across individuals. • Antipredator behaviour had no significant effect on hatching success probability.