Long distance song bird migration has strong genetic basis (Berthold 1991) but there is still no consensus on how and which genes control these massive scale airbourne movements. In my PhD project we build on an already established and very promising study system: the Willow Warbler (Phylloscopus trochilus) with its two European subspecies: (Northern acredula and Southern trochilus) that differ in migration routes and wintering grounds within Africa (Hedenström and Pettersson 1987). At our disposal we have decades of research results as well as a rich and diverse data set from entire Scandinavia (Bensch et al. 2009; Lundberg et al. 2017). Thorough genomic analysis has showed that these two subspecies have virtually no genome-wide differentiation, however there are two inversions (each on separate chromosome and total of ~200 protein coding genes) harboring nearly all fixed differences between N Swedish and S Swedish birds (Lundberg et al. 2017). In my project we particularly focus on a migratory divide in central Sweden where acredula and trochilus subspecies meet and freely interbreed (Liedvogel et al. 2014). We already have migration tracks from S Sweden (unpublished n=4) and NE Russia (n=3 Sokolovskis et al. 2018) that are accompanied with genetic data. In 2018 we deployed 343 geolocators (250 of which in the hybridzone alone) on Willow Warblers across Sweden which will result in a 50 to 100 more migratory phenotypes and a massive amount of full genome resequencing data that will help to ultimately approach mechanistic explanation to an innate behavior that coordinates individual animals to commit perilous cross continental flights twice each year.