Breeding site fidelity and winter admixture in a long-distance migrant, the tufted duck (Aythya fuligula)

Long-distance migrants are, by definition, highly mobile but it is poorly understood if this leads to high rates of gene flow and an essentially panmictic global population structure. Genetic divergence in migratory species could be promoted, for example, by fidelity to distinct migratory pathways. In this study, we investigate the population genetic structure of tufted duck (Aythya fuligula), a long-distance migrant with a largely continuous breeding distribution across Eurasia. Distinct, longitudinally oriented flyways have been postulated based on geographically disjunct wintering areas and are supported by evidence from ringing data. We generated sequences of the mitochondrial control region and multi-locus microsatellite genotypes for several hundreds of samples from the European and Asian breeding and wintering grounds including some individuals infected with highly pathogenic avian influenza virus H5N1. Significant differentiation between breeding sites was observed for both marker types, but F(ST) values were approximately 10 times higher for maternally inherited mitochondrial DNA than for biparentally transmitted nuclear markers. The genetic differentiation between the postulated European and Asian flyways was similar to that observed within continents and, in general, genetic divergence was not associated with geographic distance. Neither marker type showed evidence of genetic substructure among aggregations on the European wintering grounds. Our results suggest some breeding site fidelity, especially in females, but extensive population admixture on the wintering grounds. Several scenarios may explain the observed lack of genetic divergence between Europe and Asia including non-equilibrium conditions following a recent range expansion or contemporary gene flow across the postulated migratory divides.