Mitochondrial control region polymorphism reveal high amount of gene flow in Fennoscandian willow tits (Parus montanus borealis). 
We studied the genetic variability and differentiation of two Fennoscandian willow tit (Parus montanus borealis) populations located ca.  1000 km apart in Finland and Sweden by using the control region sequences of the mitochondrial DNA.  Individual variation in the control region was extensive since all the 13 Finnish and the 12 Swedish individuals had unique haplotypes and the mean pairwise genetic distance resulted in 0.0052 (range 0.0008-0.0109).  In the minimum spanning network connecting the genotypes, the two populations were completely intermingled.  The mismatch distribution of the combined data set was very close to expected distribution of an expanding population.  This result was supported by a significantly negative Tajima's D value.  The sequence data indicate that (1) the long-term effective population size of the breeding willow tits has been large (122,000 and 110,000 females for the Finnish and the Swedish populations, respectively); and that (2) the gene exchange between distant localities is/has been extensive.  Dispersal area for the Finnish females was estimated to be about 19,000-30,000 km2 and for the Swedish, 22,000-28,000 km2.  Thus, the whole Fennoscandian population can be regarded as one panmictic unit, without any subdivisions to local demes.  The amount of gene flow is remarkable because the willow tit has been considered a highly sedentary species.  Provided that adult birds are site-tenacious, the gene flow must occur through juvenile summer dispersal, or irruptive autumn invasions, or both.