Range expansion and the possibility of an emerging contact zone between two subspecies of Chiffchaff Phylloscopus collybita ssp.

The Chiffchaff Phylloscopus collybita is represented in Sweden by two diffe rent subspecies; the northern well-established abietinus and the southern r ecently established collybita which has expanded its range northward during the past two decades. At present, an area approximately 500 km wide separa tes the two subspecies. In order to document differences between the northe rn and southern populations we compared morphology, vocalisation, habitat c hoice, and neutral genetic variation in mitochondrial (mt) DNA and at four microsatellite loci of 30 male Chiffchaffs from each subspecies. Our result s show significant differences in several morphological traits and in song. Playback experiments revealed a significant difference in aggressive behav iour depending on which population-specific song that was played to the bir ds. Mitochondrial DNA was geographically structured with similar to 90% of the birds carrying a mt haplotype matching their sample population. No alle lic differences at the microsatellite loci were found between populations. Our data demonstrate a substantial differentiation between the northern and southern populations despite gene flow, clearly separating them into the s ubspecies abietinus and collybita.