Long-distance dispersal in migratory pied flycatchers Ficedula hypoleuca is relatively common between the UK and the Netherlands

Long-distance dispersal can potentially have important consequences for evolutionary change, but is difficult to quantify. We present quantitative estimates of natal dispersal between the UK and the Netherlands in a long-distance migratory bird, the pied flycatcher. Due to over 90 000 individual ringed adults caught and 730 000 ringed nestlings we are able to estimate that dispersal of young born in the UK to breeding in the Netherlands occurs on average 43 times yr-1, and probably even more recently. We estimated that between 2000 and 2009 about 0.70% of the Dutch adult population could be immigrants from the UK. No cases of dispersal from the Netherlands to the UK were observed. Dispersing individuals bring new genes to the Netherlands, as males in the UK have a darker plumage than Dutch males. We found a good qualitative match between the proportion of dark males breeding in two Dutch populations and the estimated immigration rate from the UK. This and potentially other genetic differences may allow for new evolutionary directions if these genes are beneficial in the new population. The causes of dispersal are unknown, but it is striking that especially birds from the northern range limit dispersed.