The work presented here aims at understanding the nature, epigenesis and function of personality types (here called behavioral profiles) in birds, focusing on a wild bird species, the great tit (Parus major). Lines bidirectionally selected for exploration show a wide array of social and non-social behavioral differences, and also some differences in physiological parameters. Line differences in these characteristics and their relationships show significant temporal consistency. The results show a surprising similarity between the great tit and a rodent model, suggesting a fundamental principle in the organization of behavioral profiles. The nature of this principle and whether or not it is multi-dimensional is discussed. However, the similarity with a chicken model is less clear, which points to some caution for generalization. The epigenesis of great tit behavioral profiles is discussed. Selection experiments with replication and backcrosses reveal a strong genetic basis, and suggest an influence of maternal effects. Ontogenetic manipulations indicate strong developmental plasticity, suggesting adaptive adjustment to prevailing environmental circumstances. They also show that behavioral characteristics belonging to the same profile can become uncoupled. Finally, field data on several fitness parameters of the different personalities in wild great tits are summarized. These data suggest that variation in selection pressure in time and space and assortative mating are plausible mechanisms accounting for the maintenance of different behavioral profiles within the same population.