Global changes in climate and land use are accelerating at a blistering pace and living organisms have to cope with the new conditions. Migratory birds are particularly sensitive to these changes resulting in steep declines of their populations. Unfortunately, we are still far from understanding the real causes of these declines (i) because we have only insufficient insights into the demographic mechanisms involved and (ii) because of the uncertainty about contributions of particular environmental drivers. To address these challenges, we urgently need to integrate different approaches to study migratory bird populations which have been until now adopted only separately. Therefore, we will combine (i) individual-based tracking methods to link breeding and non-breeding areas, (ii) remote sensing data and stable isotope analysis to assess climatic changes and habitats in these areas, and (iii) long-term monitoring data to model the underlying demographic processes. By doing so, we will gain comprehensive understanding of the key mechanisms driving the declines of migratory populations.