Population trends of birds across the iron curtain: Brain matters

One approach to assess human impact on species’ population dynamics is to correlate ecological traits of species with their long-term population trends. Yet, few studies investigated population trends in multiple regions that differ in human impact to reveal which traits explain population trends over larger geographic areas and which only regionally. We examined the relationship between various species traits and long-term population trends of 57 common passerine bird species from 1991 to 2007 in three adjacent regions in central Europe that experienced differences in socioeconomic history: North-Western Germany, Eastern Germany and the Czech Republic. We tested effects of habitat, dietary and climatic niche, migratory strategy and cognitive ability, measured as relative brain size. We predicted that traits reflecting socioeconomic and land-use change had stronger effects in former communist countries than in North-Western Germany due to marked changes in these countries after 1990. We found that climatic niche and migratory strategy affected bird abundances similarly in all regions suggesting their influence is invariant across central Europe. In contrast, brain size showed regionally varying effects. The effects were negligible in North-Western Germany, slightly positive in Eastern Germany and strongly positive in the Czech Republic. Increases of species with large brains suggest that species with good cognitive abilities might have been better able to adapt to rapid socioeconomic change and make use of novel opportunities after the end of communism. Regional differences in population trends among species thus appear to be driven by an interaction between cognitive abilities and socioeconomic change.