Numerous avian species are good candidates to study the emergence of complex forms of sociality, in particular the species showing advanced cognitive skills, and the one living in groups all year long, coping with the costs and benefits of group living. In my thesis, I investigated the proximate mechanisms underlying the diversity and dynamic of social relationships in captive adult rooks, a colonial and monogamous corvid species. I found that rooks have the ability to form non-reproductive social bonds and several pair bonds throughout their life. In addition, I found: that the group social structure was flexible enough to allow temporal variations of relationships, group composition and sex-ratio; and that dominance relationships do not have any long lasting effects on the stabilisation of the structure. Overall, my work demonstrates that rooks social structure extends far beyond a simple aggregation of reproductive pairs, and may be acknowledged as a multi-level social organisation.