The supra-national species indices and the multispecies indicators are based on adding national results. Is this justified, as the borders between countries are artificial?
Answer Of course, birds have nothing to do with country boundaries and bird populations should be considered as a whole across Europe. That is exactly what we do by weighing the national species indices on base of the national population size estimates (from European Red List of Birds, BirdLife International 2015). This procedure warrants that each individual bird counted gets the same weight in the trends and indices, whether it lives in one country or another.
See more in Methods, chapter 2. Supranational species indices and trends.