Where is the genetic signal concentrated in Willow Warbler migratory phenotypes?

In just three highly differentiated regions on chromosomes 1, 3, and 5.

The genetic basis for the complex migratory divergence in the Willow Warbler is remarkably localized. Despite the birds undertaking vastly different, long-distance migrations, the analysis revealed that across nearly the entire genome, the two main migratory phenotypes are virtually undifferentiated. This homogeneity suggests recent range expansion and significant gene flow across most of the species range. The critical genetic structure distinguishing the migratory strategies is instead sharply focused into only three specific, highly differentiated regions. These regions are located on chromosomes 1, 3, and 5, spanning lengths between 4.0 and 13.1 Mb, and they contain hundreds of genes whose structure suggests a history of isolation potentially dating back over a million years.