Animal personality and fitness: a study on a migratory passerine bird

Behavioral differences among individuals, that are stable over time and consistent in different contexts, are termed as ‘personality’. Particular attention has been given to personality traits along the reactive-proactive axis. Reactive and proactive (shy and bold) individuals are expected to differently respond to risk and differ in their aggressiveness, with the latter being more risk-prone and aggressive. These differences may have consequences on individual fitness and be associated with a survival-reproduction trade-off, that can maintain contrasting personalities within-populations. However, studies on this topic have been conducted on few wild populations and literature lacks of empirical works testing this possible trade-off. Here we aimed at evaluating the relation between personality (reactive-proactive axis) and fitness (annual survival and seasonal reproductive success) in breeding barn swallows (Hirundo rustica). We recorded behavioral responses of females to a direct disturbance during incubation and to a novel object exposure on their nest (red or blue ball) in two consecutive breeding seasons. We found moderate but significant within-individual consistency and high inter-individual variability in personality along the reactive-proactive axis. Females returning faster to the nest after disturbance (bolder) showed higher seasonal reproductive success (more fledglings) than shier ones, apparently without paying significant costs in terms of annual survival. Therefore, proactive personality might benefit from a selective advantage over the reactive one. However, longitudinal analyses on lifetime reproductive success and longevity are required to corroborate these findings.