Effective population size (Ne) is a fundamental concept within biology and can be defined as the size of an ideal Wright-Fisher population in which the rate of genetic drift is the same as in the observed population. Natural populations are not ideal so that Ne is often < Nc. A low Ne can lead to inbreeding depression and less adaptability in a population, thus it is essential to know Ne for threatened populations. Ne can be estimated using genetic or demographic data. In this study four different genetic estimators (LDNE, ONeSAMP, MLNE and CoNe) and one demographic estimator were compared using data from a natural house sparrow metapopulation. How Ne related to Nc was also examined. All four genetic estimators seemed to be upwardly biased, however, LDNE often produced estimates in the expected range (Ne<N) and thus appeared to be less biased. To understand how characteristics of natural populations may affect the rate of genetic drift it is important to examine what influence the Ne/Nc-ratio. Thus, I investigated whether population characteristics such as population size, sex ratio, immigration rate, variance in population size and population growth rate explained the variation in the Ne/N ratio for the different genetic estimators. A general result was that the immigration rate had a positive effect on the Ne/Nc-ratio. The apparent upward bias of genetic Ne estimates and the positive effect of immigration rate on Ne/Nc-ratio suggest that gene flow between subpopulations within the study metapopulation was of significant importance for the rate of genetic drift. Genetic estimators of Ne seem like promising tools. However, if no knowledge of the ecology of the population in question exists, Ne should be interpreted cautiously. When assumptions underlying estimators are violated this can lead to erroneous conclusions about genetic processes in the population.