Estimating population sizes of birds is of fundamental importance in species-oriented conservation and management. Worryingly, discrepancies among different population estimates are widespread in ornithological literature. A complementary review to that carried out by Murgui (2011) on the available literature on national population sizes of Spanish birds is developed, putting the accent on data quality and methods used (geographical bias in sampling effort, sample sizes, census methods, detectability problems, consideration of the cryptic fraction of population not included in breeding pairs, how the number of breeding pairs is obtained, extrapolation methodologies, etc). I conclude with a positive, conciliatory, view about large-scale population estimates, where limitations of previous works enlighten future research and sampling programs.