Migration direction in a songbird explained by two loci

Migratory routes and remote wintering quarters in birds are often species
and even population specific. It has been known for decades that songbirds
mainly migrate solitarily and that the migration direction is genetically
controlled. Yet, the underlying genetic mechanisms remain unknown. To
investigate the genetic basis of migration direction, we track genotyped
willow warblers Phylloscopus trochilus from a migratory divide in Sweden,
where South-West migrating and South-East migrating subspecies form a
hybrid swarm. We find that migration direction follows a dominant
inheritance pattern with epistatic interaction between the two loci
explaining 74% of variation. Consequently, most hybrids migrate similarly
to one of the parental subspecies and therefore do not suffer from the
cost of following an inferior, intermediate route. This has significant
implications for understanding the selection processes that maintain
narrow migratory divides.