We present a method for estimating nesting population size in a passerine bird based on counts of active nests, which can provide more accurate estimates on true population size than other methods. We surveyed 5 different reed habitat types in a region of Serbia in 2009-2011 to estimate the population size of the Great Reed Warbler. The estimation was based on the counts of active nests and was limited to the first clutches laid by females. We recorded a total of 442 nesting females. To estimate the population size for the studied region, the nesting densities were multiplied by the total area of the reedbed and the mean proportion of reed cover in each reedbed. This method may be used in areas/countries where there are no regular censuses and in bird species with strong habitat affiliations. In contrast to other methods, our estimation excludes uncertainty due to the presence of floater males and double counting of singing males, but considers polygyny, overall improving the accuracy of the estimate of the nesting population size. The disadvantages are that the method is time-consuming and demands considerable effort and reliable data on habitat area and quality.