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Abstract

**Abstract**<br>

The existence of among-individual variation in behaviour within populations is poorly understood. Recent theory suggests that fine-scale individual differences in investment into current versus future reproduction may lead to a ‘slow-fast’-pace-of-life continuum, also referred to as the ‘pace-of-life-syndrome’ (POLS) hypothesis. According to this idea, individuals are predicted to differ in their level of risk-taking, which may drive among-individual variation and covariation of behaviours. Consistent individual differences in aggression, an ecologically relevant and potentially risky behaviour, have been reported across the animal kingdom. Here we test whether such individual differences in aggression are a manifestation of underlying differences in risk-taking. In a wild blue tit (<em>Cyanistes caeruleus</em>) population, we used standard behavioural tests to investigate if male territorial aggressiveness and risk-taking during breeding are positively related. At the start of breeding, we simulated conspecific territorial intrusions to obtain repeated measures of male aggressiveness. Subsequently, we measured male risk-taking as their latency to resume brood provisioning after presenting two different predators at their nest: human and sparrowhawk, a common predator of adult songbirds. First, we found substantial repeatability for male aggressiveness (<em>R</em> = 0.56 ± 0.08 SE). Second, while males took longer to resume provisioning after presentation of a sparrowhawk mount as compared to a human observer, risk-taking was repeatable across these two predator contexts (<em>R</em> = 0.51 ± 0.13 SE). Finally, we found no evidence for a correlation between male aggressiveness and risk-taking, thereby providing little support to a main prediction of the POLS hypothesis.

**Significance statement**<br>

Consistent, and often correlated, individual differences in basal behaviours, such as aggression, exploration and sociability, are found across the animal kingdom. Why individuals consistently differ in their behaviour is poorly understood, as behavioural traits would seem inherently flexible. The ‘pace-of-life syndrome’ (POLS) hypothesis proposes observed behavioural variation to reflect differences in risk-taking associated with individual reproductive strategies. We tested this idea in a wild blue tit population by investigating whether individual males that were more aggressive toward territorial intruders also took more risk when provisioning their nestlings under a threat of predation. While we found consistent individual differences in both aggressiveness and risk-taking, these behaviours were not significantly correlated. Therefore, our study demonstrates among-individual variation in ecologically relevant behaviours in wild blue tits but provides little support for the POLS hypothesis.