The Baltic Sea population of the Willow Warbler (Phylloscopus trochilus) is genetically homogeneous. In spite of this, two geographical boundaries (1 latitudinal and 1 longitudinal) separating two distinct winter migration destination behaviors have recently been discovered in previous work together with our collaborators, Prof. Staffan Bensch and colleagues, Lund University. Since the migration behavior of songbirds is genetically determined, the Baltic Sea Willow Warbler population represents an excellent system for studying early events in evolutionary adaptation. Our recently completed study with Bensch et al identified two polymorphic loci that are associated with alternative winter migration destinations in Africa. It has been suggested that some classes of proteins may be preferred targets for adaptive changes and thus be intrinsically evolvable. Our studies on yeast suggest that this may be the case and that transcription factors represent one such protein class. The proposed work provides an excellent opportunity to test whether this is also the case in more complex organisms in the wild. We will use a combination of the latest high volume/ low cost DNA sequencing technologies, ecology, population genetics and bioinformatics to identify migration destination associated adaptive variation in the Baltic Sea Willow Warbler population. The approach focuses on variation affecting the sequence of proteincoding and non-coding RNAs as well as their expression level. In addition to its significance for understanding molecular mechanisms of evolutionary adaptation the study should give important information about the molecular mechanisms affecting choice of winter migration destination as well as potential applications for sensitive and timely monitoring of environmental changes in the Baltic Sea region, resulting from factors such as climate change.