How does Willow Warbler background genomic differentiation contrast with *Ficedula* flycatchers?

Willow Warblers show low background differentiation, contrasting with higher overall divergence in *Ficedula*.

The genomic landscape of the Willow Warbler migration divide is unusual when compared to other diverging bird populations used as references. While the three chromosomal hotspots show extreme differentiation, the rest of the genome, even between individuals separated by thousands of kilometers, is nearly uniform. This state of low background differentiation is characteristic of a species that has recently expanded its range while maintaining large effective population sizes and high levels of gene flow across most of its territory. This stands in sharp contrast to certain other diverging avian groups, specifically mentioned as *Ficedula* flycatchers, which exhibit significantly higher overall genomic divergence across their respective populations, indicating a much longer period of overall separation or restricted gene flow.