Population structure and gene flow in two long-distance migrant birds, the Bicknell's thrush (Catharus bicknelli) and veery (C. fuscescens)

Gene flow counteracts the effects of genetic drift and local natural selection regimes. As such, it tends to make populations genetically homogenous and promotes the retention of genetic diversity. Birds, especially migratory species, are among the most mobile and active of organisms and generally show relatively high levels of gene flow, although varying habitat requirements result in varying distributions. The Bicknell's Thrush and the Veery are two closely-related long-distance migratory birds with very different distributional patterns. The former is largely restricted to upper elevation, conifer-dominated habitats in a handful of northeastern North American mountain ranges, and the latter is nearly uniformly distributed across northeastern North America. I examined the effect of distributional pattern and migratory behavior on gene flow by comparing the population structures of these two species.; I used mitochondrial DNA control region III sequences to examine genotypic variation in Bicknell's Thrush and Veery. In addition, I obtained sequences of Gray-cheeked, Hermit, and Swainson's thrushes to examine phylogenetic relationships in the genus Catharus. Phylogenetic analysis suggested that the Gray-cheeked Thrush, Veery, and Bicknell's Thrush form a species group within Catharus and that Bicknell's Thrush and Veery may be sister species. The latter finding is counterintuitive because of the Bicknell's Thrush's long-term historical status as a subspecies of the Gray-cheeked Thrush.; Sequencing of Veery samples revealed 10 mitochondria) genotypes, and Bicknell's Thrush samples revealed seven mitochondria) lineages. Population genetic analysis with the programs AMOVA, HAPLO2, and Fluctuate showed no significant population structure in either species. Comparable results were produced with the coalescence-based gene flow program Migrate. These results indicate that there is effective panmixis among the geographically isolated populations of Bicknell's Thrush as well as in the more uniformly distributed Veery.; In spite of the larger number of mitochondria) variants in the Veery, the Bicknell's Thrush had higher overall genetic diversity and deeper lineage structure than its congener. The pattern of variation in Veery lineages appears to have arisen from a glacial population bottleneck followed by rapid population growth at the onset of the current interglacial. In contrast, Bicknell's Thrush has apparently had a more stable but much smaller population throughout its history.; The results of this study indicate that Bicknell's Thrush populations are well-connected by movement of birds among them. As such, conservation of this scarce species should concentrate on habitat preservation and maintenance of large source populations to recolonize small habitat islands rather than become occupied with concern over genetic diversity or subdivision of the species into management units.