Sexual selection has been largely assumed to be static and to always favour the most exaggerated trait. However, changes in the ecological or social environment can modify the strength and direction of sexual selection (e.g. favouring flexibility in female preferences). Unfortunately, how sexual selection and environmental variability interact is still poorly understood as evidence is scarce and incomplete. The first objective of the present project is to assess the direction, strength and outcome of sexual selection under different environmental conditions in terms of parasite prevalence, population density and weather by analysing long-term databases (including breeding success and male sexual traits and condition of thousands of individuals from nine British populations over more than 15 years) in the Red Grouse Lagopus lagopus scoticus. Secondly, I will experimentally prove for the first time in a wild species at a population level that changes in the ecological environment can modify sexual selection. In two different grouse populations, three sub-areas will be selected and increased nematode load, nematode removal or control treatments will be applied to all individuals occurring in those areas. If sexual selection increases under restrictive conditions, I predict stronger selection for male traits in populations with larger parasite loads (at correlative and manipulative levels), in populations living at high densities and in populations subjected to harsher weather conditions (low winter-spring temperatures and high rainfall). Thirdly, grouse females will be challenged with increased loads of nematodes in order to experimentally study the effect of health status on mate choice. Theory predicts that females in good condition should have more attractive mates compared to poor quality females. Thus, I expect to find lower strength of preference for male traits in challenged females as compared to control or nematode-removed females.