Population genetic structure of Australian magpies: evidence for regional differences in juvenile dispersal behaviour

Territorial group size in Australian magpies (Gymnorhina tibicen) ranges fr om monogamous pairs to groups of more than 20 individuals. It has been hypo thesized that large territorial groups result from the retention of juvenil es after a breeding effort. If this is true, local populations consisting o f large groups are likely to exhibit the most genetic structure, because ov er time similar genotypes will tend to be confined to limited areas if juve niles are predominantly philopatric. The objective of the present study was to test this hypothesis using allozyme and mitochondrial DNA data to provi de indirect estimates of regional gene flow (derived from hierarchical popu lation subdivision analyses). These data were used in combination with esti mates of group size to infer patterns of dispersal among magpie populations across mainland Australia. Territorial groups were significantly larger in the south-west compared to three eastern regions. Although inferred levels of gene flow were substantial for all four regions, a striking pattern eme rged from both sets of genetic data: more differentiation was evident among populations in the south-western region than in any eastern region. We con clude that levels of juvenile dispersal influence group size in G. tibicen, because in the south-western region where groups were largest, populations were most genetically differentiated. Our results suggest that contrasting population genetic structures may develop within a single species as a res ult of differences in social system.