In small and declining populations levels of genetic variability are expected to be reduced due to effects of inbreeding and random genetic drift. As a result, both individual fitness and populations’ adaptability can be compromised, and the probability of extinction increased. Therefore, maintenance of genetic variability is a crucial goal in conservation biology. Here we show that although the level of genetic variability in mtDNA of the endangered Fennoscandian lesser white-fronted goose Anser erythropus population is currently lower than in the neigbouring populations, it has increased six-fold during the past 140 years despite the precipitously declining population. The explanation for increased genetic diversity in Fennoscandia appears to be recent spontaneous increase in male immigration rate equalling 0.56 per generation. This inference is supported by data on nuclear microsatellite markers, the latter of which show that the current and the historical Fennoscandian populations are significantly differentiated (FST = 0.046, P = 0) due to changes in allele frequencies. The effect of male-mediated gene flow is potentially dichotomous. On the one hand it may rescue the Fennoscandian lesser white-fronted goose from loss of genetic variability, but on the other hand, it eradicates the original genetic characteristics of this population.