Phenotypic plasticity is caused by the interplay of external influences and the genetic background. It is thought to allow the individual to adjust to variable environmental conditions. Conditions early in life can predict those in the future and enable the organism to flexibly adjust its development accordingly. In birds, most current research focuses on phenotypic plasticity in relation to conditions experienced prenatally or during the nestling stage, while studies on influences during the crucial transition to independence and adulthood are rare. However the juvenile phase can be very plastic and especially social influences during this phase can have strong impact on developmental trajectories and modify an individual's behaviour, physiology and morphology. To understand the function of developmental phenotypic plasticity, we need to know about the consequences of these effects in enabling the organism to cope with different conditions as an adult and whether such adjustments reduce the flexibility to adjust to future changes of the environment or restrict the ability in adjusting to a different context. Therefore zebra finches (Taeniopygia guttata) will be kept in different types of social groups (small and large unisexual and mixed-sex groups) during the juvenile phase. We will test whether these different social experience at this developmental stage affect the way adults cope with different reproductive and social challenges, and whether there are consequences for performance in a non-social context. We will also examine whether these changes are permanent or can be modified by new experience when adult. To understand the mechanistic basis of the expected phenotypic modifications, we will study how these relate to hormonal and neural changes.