Population estimates are an essential demographic parameter for assessing the threat status of species and the international significance of national populations. Murgui (2011) provides the most up-to-date review of the bird population estimates produced in Spain over past decades and he describes their methodological limitations, particularly those derived from data availability, sampling bias and analytical shortcomings. Two of the main procedures associated with the generation of reliable population estimates, the calculation of densities and their extrapolation over whole-species ranges are discussed here with the aim of contributing to improving comparability between estimates. In this connection, we present some new approaches developed in the Catalan Winter Bird Atlas 2006–2009, such as the stratification of density estimates using habitat suitability indices derived from species distribution modelling, and the consideration of detectability not only for common species but also for scarcer ones. Finally, applications of population estimates are discussed and we call for Europe-wide collaboration to enable between-country comparability of such estimates to be improved.