Large-scale population declines have been documented across many faunal assemblages. However, there is much variation in population trends for individual species, and few indications of which specific ecological and behavioural characteristics are associated with such trends. We used the British Common Birds Census (1968–1995) to identify specific traits associated with long-term abundance trends in UK farmland birds. Two factors, resource specialization and relative brain size, were significantly associated with population trend, such that species using atypical resources and with relatively small brains were most likely to have experienced overall declines. Further analyses of specific brain components indicated that the relative size of the telencephalon, the part of the brain associated with problem solving and complex behaviours, and the brain stem might be better predictors of population trend than overall brain size. These results suggest that flexibility in resource use and behaviour are the most important characteristics for determining a species' ability to cope with large-scale habitat changes.