Shallow genetic population structure in an expanding migratory bird with high breeding site fidelity, the Western Eurasian Crane Grus grus grus

For more than half a century, the Western Eurasian Crane (Grus grus grus) has been expanding its range toward western Europe, recolonizing areas where it had been previously driven to extinction, including the UK, the Netherlands and Denmark. The Western Eurasian Crane is, on the one hand, a very mobile, migratory species, but on the other, is territorial and shows high breeding site fidelity. Hence, its genetic population structure is subject to antagonizing forces, which have different consequences. Based on the genotyping of six highly variable microsatellite loci, we inferred the population structure of the Western Eurasian Crane from samples from eight regions. We integrated classic F-statistics including analyses of molecular variance with a priori designation of structure and divisive clustering approaches, i.e. a Bayesian procedure (STRUCTURE) and discriminant analysis of principal components, which infer structure a posteriori. According to the F-statistics, populations were only weakly differentiated, and the majority of the genetic variance (> 90%) was attributed to individuals. At first glance, the divisive approaches appeared to agree in finding four clusters. Yet, there was no correspondence regarding the composition of the clusters and none of the results were biologically meaningful. However, STRUCTURE delivered an alternative interpretation, designating the highest likelihood to a scenario without subdivision, in clear agreement with the findings based on the F-statistics. In conclusion, the Western Eurasian Crane is genetically largely homogeneous, probably as a consequence of the rapid growth and range expansion of its population.