This project aims at contributing to a general theoretical framework for the association between developmental stability and individual quality, through experimental study of the stress sensitivity of different traits and life stages. We therefore expose individual Great Tits to a ‘challenge experiment’ with limited stressfactor, so as to reduce their buffering capacity during development (reflected by increased asymmetry) but without directly affecting their fitness. This allows us to compare induced levels of asymmetry across traits under similar stress regimes, and to relate asymmetry (and, hence, the instability of development) to effective fitness differences at individual level. Presumed stress effects are validaded through a set of independent, biological data (physiological respons variables).