Boldness, defined as the behavioural response of individuals when facing a risky situation, is a major personality trait and often phenotypically correlates with other behavioural traits, in particular aggres-siveness, exploration behaviour and neophobia. Yet, whether such links result from among-individual correlations, i.e. form behavioural syndromes sensu stricto, is often ignored and whether such syn-dromes may yield individual fitness benefits, and thus be selected for, remains poorly explored. We measured boldness as the nest defence behaviour against a dummy nest predator in a natural population of a small passerine bird, the collared flycatcher, Ficedula albicollis, and investigated the existence of a behavioural syndrome between boldness and two other behavioural traits, aggressiveness (measured as the agonistic response to competitors) and neophobia (measured as the behavioural response to a novel object in a known environment). Then we assessed the links between this potential syndrome and reproductive success, measured as fledging and recruitment success. Boldness score differed between the sexes and depended on whether the partner was present during the test. Nevertheless, it was repeatable, showing that boldness can be considered as a personality trait in our population. We found a positive among-individual correlation between boldness and aggressiveness, showing the existence of a behav-ioural syndrome between both personality traits. This syndrome was related to reproductive success: the number of fledged young (but not recruitment probability) increased with one integrative value of the boldness-aggressiveness syndrome. Conversely, boldness score was not correlated with neophobia. Our results thus clearly reveal a behavioural syndrome between boldness and aggressiveness with possible consequences for reproductive success in the study population, and therefore raise the question of the evolutionary implications of such a behavioural syndrome.
