Evolutionary theory predicts that individual behaviour is focused on maximizing the proportion of its genes in all the genes of the next generations (≈fitness). In other words each individual should produce as many offspring of high quality as possible. To achieve this, numerous decisions have to be made about the allocation of limited resources like time and food to both reproduction and survival. The aim of this thesis is to learn more about how these kind of trade-offs affect the life of free-living jackdaws. To study this, a brood size manipulation experiment was conducted to see how a change in the number of young to be raised would affect parental birds, but also how the environmental circumstances during early development affected the offspring. As shown in this thesis, telomeres (large strains of repetitive DNA at the end of chromosomes) and particularly the rate at which they shorten can be used as a biomarker of remaining lifespan. In sons, telomere shortening was increased when they were reared in enlarged broods, providing a possible link between environmental conditions during early development and fitness. Such an effect was not observed in daughters. At the same time however the growth of daughters was much more affected compared to sons. This suggests that sons and daughters have alternative coping strategies, sons investing most in growth while daughters give priority to their health.