Phylogeography of bar-tailed godwits: pre-LGM structure in Beringia and westward colonization of post-glacial Europe

In migratory birds, high mobility may reduce population structure through increased dispersal and enable adaptive responses to environmental change, whereas rigid migratory routines predict low dispersal, increased geographic structure, and limited flexibility to respond to change. We used nextRAD sequencing of 14,318 single-nucleotide polymorphisms to explore the population genetics and phylogeographic history of the bar-tailed godwit, Limosa lapponica , a migratory shorebird with six recognized subspecies and known for making the longest non-stop flights of any landbird. Using scenario-testing in an Approximate Bayesian Computation framework, we infer that bar-tailed godwits existed in three main lineages at the Last Glacial Maximum (LGM), when much of their present-day Arctic and sub-Arctic breeding range persisted in a large, unglaciated Siberian-Beringian refugium. Subsequently, population structure developed at both longitudinal extremes: in the east, a genetic cline exists across latitude in the Alaska breeding range of subspecies L. l. baueri ; in the west, one lineage diversified into three extant subspecies L. l. lapponica , taymyrensis , and yamalensis , the former two of which migrate through previously glaciated western Europe. We also detected unrecognized population structure among bar-tailed godwits wintering in Europe, wherein a significant proportion of purported lapponica individuals were in fact taymyrensis , necessitating a re-assessment of the migrations, ecology, and population estimates for these subspecies. In the global range of this long-distance migrant, we found evidence of both (1) fidelity to rigid behavioral routines promoting fine-scale geographic population structure (in the east), and (2) flexibility to colonize recently available migratory flyways and non-breeding areas (in the west).