...to understand the evolutionary potential and trajectories of migratory behaviour, which in turn requires detailed knowledge of the patterns, sources, and consequences of variance in the migratory phenotype, including variance in orientation and navigation. We collected 192 tracks of migratory journeys performed by 84 common terns (Sterna hirundo). Since almost all of these birds are of known sex, age and ancestry and since information on their natal conditions, annual phenology and reproductive performance is available, assessing the causes and consequences of variance in their migratory and navigational phenotype is now within reach.