In nature, environments are often variable and heterogeneous influencing ecological and evolutionary processes. This thesis focus on how animals interact with their environment and how that affects the reproductive decisions they make. Using empirical data collected from wild collared flycatcher populations, experiments and molecular approaches I try to unveil some of these relationships and the evolutionary, ecological and conservation implications of these findings are discussed. Firstly, collared flycatchers were shown to use breeding densities of their own and other species using similar resources when assessing costs and benefits associated to breeding in specific habitats. However, species will vary in how informative they are, and the worst competitor – with whom you overlap most in resources needs – also provides the best source of information. Collared flycatcher parents will also benefit differentially from investments in sons and daughters due to habitat characteristics and dispersal differences between the sexes. Here, I show that they will produce more of the sex that will give the highest expected fitness return given the environment they are in. These results also provide a reciprocal scenario to Clark's (1978) classical study of sex ratio adjustment in relation to local resource competition (LRC), as more of the natal philopatric sex is produced when LRC is low. Secondly, the effect of elaborated ornaments on paternity in the socially monogamous collared flycatcher was shown to be of more importance in areas where the intensity of intra- and intersexual conflicts are expected to be elevated. Hence, ornamentation by environmental interactions determines paternity, illustrating that sexual selection through extra-pair paternity is context dependent. Finally, even though the collared flycatcher populations that this thesis is based on have been studied on their breeding grounds for more then 25 years, we know little of where they are when they are not breeding. Here, stable isotope signatures in winter-grown feathers suggests that they may spend their winter with their breeding ground neighbours and do so repeatedly over years. Differences between breeding populations at this small scale should have many impactions for evolutionary and ecological processes as it will, for example, determine with whom individuals interact throughout their life.